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A journal of undergraduate writing and research, from wip at uga, the pleistocene overkill hypothesis, the pleistocene overkill hypothesis: an optimal foraging perspective.

by Claire Brandes

overkill hypothesis wikipedia

The Quaternary Period is infamous for overseeing the extinction of some of prehistory’s most charismatic species, including the wooly mammoth and American mastodon. Two primary, yet conflicting, hypotheses aim to explain the cause of this biodiversity loss, leading many scholars to ask: was climate change or human overhunting responsible for the demise of North American megafauna? Support for the latter theory, or the overkill hypothesis, comes from archaeological evidence suggesting that the arrival of the first humans in the Americas, the Paleoindians, and the first megafaunal extinctions occurred roughly in tandem. This piece further explores this hypothesis through an economic lens, considering the advantages and disadvantages to hunting large game with regards to tradeoffs in energy expenditure. Considering optimal foraging theory the overkill hypothesis is improbable. Total energy expended exceeded the proposed net caloric return rate for consumption of a mammoth, suggesting that a variety of factors, and not human hunting alone, ultimately caused the Quaternary extinction event. This research carries serious implications regarding the dangers of climate change as well as human overexploitation of natural resources as risk factors for biodiversity loss and should be considered in conversations surrounding modern day conservation efforts.  

Quaternary period, megafauna, extinction, Paleoindians, optimal foraging

The plains of prehistoric North America were once habitat to the largest mammals to ever walk the planet. These large mammals, or megafauna, are defined with respect to their taxonomic group, and typically denote species that weigh over 1000 kg (Lupe & Schmitt, 2016). Megafauna went extinct almost systematically during the latter part of the Pleistocene Epoch, the period of Earth’s history from 2.6 million years ago to 11,700 years ago (Johnson, 2018). The Quaternary extinction event, which began around 12,000 years ago, saw the demise of fifty-seven species of megafauna, representing 35 genera, in the following 2,000 years, including the popularly known wooly mammoth and two other proboscideans, the taxonomic order to which mammoths and elephants belong (Bulte et al., 2005; Surovell & Waguespack, 2009). The average rate of extinction for these animals is typically one per every 40,000 years (Bulte et al., 2005). Compared to this standard, the loss of life during this time period was tremendous (Bulte et al., 2005).    

Two leading theories aim to explain the Quaternary extinction event. First, a significant degree of climate change occurred during the Pleistocene, which some scholars blame for megafaunal extinction (Bulte et al., 2005). Changes in vegetation occurred in response to the formation and subsequent movement of ice sheets across the North American landscape. As the theory goes, the foraging efficiency of large herbivores decreased as they struggled to adapt to glacial activity, which proved to be detrimental to their survival (Bulte et al., 2005). The second theory, referred to as the overkill hypothesis, suggests it was not changes in climate but early humans who hunted large game species to extinction. Archaeological evidence suggests that the arrival of the first humans in the Americas, the Paleoindians, and the first megafaunal extinctions occurred roughly in tandem (Bulte et al., 2005). Known for their distinctive spear points that were thought to have been used to hunt large game, the aptly named Clovis people are among the most well-known of these early hunter-gatherers (Waguespack & Surovell, 2003). Thus emerged the idea of humans as super-predators who took out these prehistoric beasts.

Due to the limitations of knowledge revealed by the archaeological record, the true cause of the Quaternary extinction remains highly contentious within the scientific community. Like many facets of early human behavior, the subsistence strategies of Paleoindians are open to multiple, often conflicting interpretations that further complicate this debate. Yet the use of behavioral models developed through years of research and ethnographic observation may work to elucidate these matters. An analysis of optimal foraging theory predicts a generalized foraging strategy for Paleoindian hunter-gatherers, as opposed to large-game specialization or a reliance on large-game as a primary food source, discounting the plausibility of the overkill hypothesis.

As a theoretical framework, optimal foraging theory aims to predict how a forager will behave in pursuit of a food source. Within this theory are several decision sets that are used to determine how prey will be valued, pursued, and handled (Winterhalder, 1987). Each dimension is meant to illustrate a step in the process of obtaining food from the time an individual starts their search until the food is processed and ready to be consumed. An assessment of diet in its entirety must then consider the difficulties experienced by a forager as they progress through each set of decisions. This framework will be used to assess the plausibility of the overkill hypothesis by examining the likelihood of megafauna as a staple of the Paleoindian diet. In particular, the inclusion of mammoths will be analyzed in a greater depth, as they are the only megafaunal species with a clear association to Paleoindian kill sites (Byers & Ugan, 2005).

Due to their size and subsequent caloric value, mammoths and other megafauna such as mastodons and wooly rhinos have long been hypothesized to have been highly ranked prey by early humans. Evidence present at Paleoindian kill sites has proven that mammoths were once successfully hunted by these people. In a 2003 study examining 33 known Clovis archaeological sites, proboscidean remains, a taxonomic group of megafauna, were present in 79% of assemblages, the most commonly occurring genera of all faunal discoveries (Waguespack & Surovell, 2003). In zooarchaeological theory, body size is considered to be a strong indicator of prey rank (Lupo & Schmitt, 2016). A wooly mammoth is estimated to provide roughly 7,000 pounds of edible tissue (Byers & Ugan, 2005). In the framework of optimal foraging theory, this measurement characterizes mammoths as highly desired prey in the eyes of a hungry Paleoindian.

The diet-breadth model predicts which and how many types of food resources a forager will pursue based on what they could encounter. The simple, yet fundamental assumption of this model goes as follows: a forager’s goal is to maximize their net rate of energy intake (Winterhalder, 1987). Prey is categorized as high value if they yield significant post-encounter return rates, measured in amount of kilocalories worth of meat obtained per hour of handling time (Lupo & Schmitt, 2016). Handling time is the average time spent, once prey is encountered, pursuing, dispatching, and preparing the animal for consumption (Smith et al., 1983). Byers and Ugan (2005) assess post-encounter return rates would need to equal upwards of 30,000 kilocalories per hour to maintain the appeal of a specialized diet. Significant constraints placed on the diet-breadth model complicate how large game may be ranked as prey handling times increase (Winterhalder, 1987). Megafaunal specialization could therefore be maintained successfully only under conditions in which mammoths existed in abundance, were easily killed, and took little time to process (Byers & Ugan, 2005). 

Mammoths likely occurred in low density rates across the Pleistocene landscape. A negative correlation exists between animal size and density, which is observable in present day environments (Byers & Ugan, 2005). Modern elephants will be used to establish surrogate data unobtainable from actual mammoth remains; elephants and mammoths, who share a common ancestor (a recent relationship when considered in evolutionary terms) are comparable in size.

African elephants today exist at an average rate of 1.3 animals per square kilometers, and Indian elephants at an even lower density of 0.6 animals per square kilometer (Waguespack & Surovell, 2003). The likely population density of mammoths may be extrapolated from this data to predict an equally low rate of occurrence for these animals. Larger animals also tend to be organized patchily, meaning groups of mammoths most likely occurred in clumps on the landscape, as opposed to a uniformly distributed species that are more easily discovered and isolated by predators (Byers & Ugan, 2006). Due to predeation by a multitude of carnivores, large-bodied herbivores of the late Pleistocene were likely to maintain a small population. Several studies theorize that populations of megafauna were receding even before the Clovis people entered the Americas (Byers & Ugan, 2005). 

The predicted rate of encounter for mammoths likely does not support human specialization in hunting megafauna (Byers & Ugan, 2005). Even given that mammoths were highly ranked prey, their presumed scarcity would predict a broader and less specialized Paleoindian diet. Most decision sets within optimal foraging theory assume foragers will behave in a manner that maximizes their rate net of energy return per unit of foraging time. Byers and Ugan (2005) estimate hunters would need to slay a mammoth once per hour of search time in order to maintain a sustainable balance of energy expended and gained. Long search times can be particularly detrimental to overall return rates for highly ranked prey (Smith et al., 1983). Consequently, opportunity cost increases with longer search times. Foraging behavior can be modeled on a marginal value curve, where, at a certain point, searching for a specific prey animal loses its value as it goes on for an extended period of time (Winterhalder, 1987). Simply put, the longer a forager spends hunting only a specific type of prey, the greater the potential gain lost had they pursued an alternative food resource. This phenomenon results in a conservation effect where a higher payoff is achieved by foragers who do not exhaust resources and therefore maintain tolerable search times. Pleistocene foragers would have presumably moved between resource patches at a rate that would not cause local prey extinctions (Smith et al., 1983).

Assuming mammoths were abundant enough to support Paleoindian hunting rates, subsequent costs to hunting the animal emerge, beginning with pursuit costs. Pursuit time refers to the time spent stalking, chasing, and dispatching prey, including time lost to failed hunts. Pursuit time, contingent on encounter rates, tends to increase with prey body size (Lupo & Schmitt, 2016). Mammoths are estimated to weigh over six tons, almost 100 times the weight of an average human. In the natural world, a relationship in size between predator and prey to this degree is unheard of. Wolves who target adult moose, a considerably larger animal, about eight times its size, do not even reach this standard. For prey that are disproportionately small or large compared to the size of the hunter, difficulty during capture often ensues and that decreases the net caloric return rates (Surovell & Waguespack, 2009).

The aforementioned failed hunt, a key element of pursuit costs, occurs when prey is pursued but hunters are unable to successfully capture or kill the animal. Today’s African elephants are known to have thick skin that allows the animal to persevere through multiple attacks and escape when hunted (Lupo & Schmitt, 2016). The thick fur coat once adorned by mammoths provides further evidence of these animals’ naturally occurring protection. Modern ethnographic accounts of pursuit times associated with large-game document low success rates. This phenomenon is observable across continents from the indigenous Martu people of Australia to the Hadza hunter-gatherers that are native to Tanzania. The latter group suffers a 97% failure rate for any individual on any given day pursuing an African elephant (Lupo & Schmitt, 2016). Any foraging groups sensitive to these risks are more likely to adopt a wider breadth of diet (Surovell & Waguespack, 2009).

Preparation time for consumption of megafauna also increases the cost of hunting these animals. Larger animals take longer to butcher, running the risk of meat spoilage if this is not done in a timely manner. In order to consume meat, hunters must either transport the carcass or move their entire camp and community to the kill site, both options exhausting a considerable amount of time and energy. Ethnographic accounts of elephant hunts establish an average of 40–52 men required to carry flesh and bones, or around half that number to transport dried meat back to camp (Lupo & Schmitt, 2016). Total handling time for an animal the size of a mammoth has been estimated to range between 75 to 187.5 hours, an extremely taxing time frame (Byers & Ugan, 2005). Depending on their fat content, Byers and Ugan (2005) grant a 105–175 hour window of processing time where post-encounter return rates justify megafaunal specialization. Given an estimated mean value of around 131 hours of handling time, including mammoth in the diet is unlikely to yield more kilocalories than expended in many cases. To compensate for these restraints, Paleoindians may have exploited only a partial amount of a carcass once killed. While cutting processing costs, this practice nonetheless results in waste. Partial utilization of a kill reflects a drop in overall return rates in kilocalories per unit of pursuit and processing times (Byers & Ugan, 2005). Thus, the processing costs of procuring a mammoth were likely not worth the effort compared to the amount of meat possibly obtained by exploiting certain mid-sized game, including bovids and deer.

Hunting megafauna like elephants is rare among present day African hunter-gatherer tribes (Byers & Ugan, 2005). Modern elephant hunters are a group composed of select individuals who have the training and knowledge to successfully dispatch an elephant. Those who attempt to do so are often motivated more so by the social prestige that accompanies a large kill rather than sustenance, as is the case of Hadza elephant hunters known as tûmas. Even then, elephants were reportedly not procured often (Lupo & Schmitt, 2016). Furthermore, there is little evidence to demonstrate specific group dynamics of this nature within small, early human societies. Thus the prestige of a mammoth kill remains entirely speculative.

As far as mammoth remains found in association with Paleoindian kill sites, scholars argue that these sites are subject to discovery bias that exaggerates how often these animals were actually taken for food. Mammoth-bearing archaeological sites are more easily discovered due to the considerable size of proboscidean bones and are also afforded considerable research attention given the implications of these finds (Surovell & Waguespack, 2009). Many archaeological sites dated to the Pleistocene epoch include additional fossil evidence of many small and medium-bodied prey types such as camel and bison, often overlooked in the discourse surrounding the overkill hypothesis (Byers & Ugan, 2005; Waguespack & Surovell, 2003).

Although Paleoindians were known to hunt mammoths, through the lens of optimal foraging theory the proposed search and handling times for procuring megafauna does not support a specialized diet. It is considered efficient within the diet-breadth model to include prey in the diet only if it produces overall return rates higher than the amount of energy expended obtaining the kill. The proposed search and handling times associated with procuring a mammoth decrease overall return rates to an unsustainable amount, with Paleoindians expending a greater amount of energy than gained by exploiting megafauna (Lupo & Schmitt, 2016). When assessing the roles of both climate change and human overkill in the Quaternary extinction, it is likely that the late Pleistocene climate exacerbated the effects of Paleoindian hunting practices in a deadly fashion. This combination of fatal circumstances was not limited to North America; climate change is considered to have played a large role in South American megafaunal extinctions as well (Metcalf et al., 2016). This region faced an even greater decline in biodiversity with an astounding loss of 83% of megafaunal genera. The archaeological record suggests that despite instances of hunting these species, early humans coexisted with megafauna for a minimum of 1,000 years. Palaeoclimatological reconstructions of the Pleistocene show that climate change was prevalent in South America as well, directly contributing to extinctions (Metcalf et al., 2016). When considering the circumstances surrounding loss of megafauna in both the American continents, it is likely that one factor alone, like overhunting, did not cause the extinction events.

Ultimately, optimal foraging theory does not support the hypothesis that human hunting alone caused the extinction of the vast amount of North American megafauna. The likely interaction that occurred was between a species already in decline due to a variety of factors and foreign predators, both technologically and cognitively advanced in an unprecedented way. No matter the true cause, the Quaternary extinction event carries serious implications regarding the dangers of climate change as well as human overexploitation of natural resources.

Bulte, E., Horan, R. D., & Shogren, J. F. (2005). Megafauna extinction: A paleoeconomic theory of human overkill in the Pleistocene. Journal of Economic Behavior & Organization , 59 (3), 297-323.

Byers, D. A., & Ugan, A. (2005). Should we expect large game specialization in the late

Pleistocene? An optimal foraging perspective on early Paleoindian prey choice. Journal of Archaeological Science, 32 (11) , 1624-1640.

Johnson, W. Hilton (2018).  Pleistocene Epoch .  Encyclopedia Britannica . https://www.britannica.com/science/Pleistocene-Epoch

Lupo, K. D., & Schmitt, D. N. (2016). When bigger is not better: The economics of hunting megafauna and its implications for Plio-Pleistocene hunter-gatherers. Journal of Anthropological Archaeology, 44 , 185-197.

Metcalf, J. L., Turney, C., Barnett, R., Martin, F., Bray, S. C., Vilstrup, J. T., … & Cooper, A. (2016). Synergistic roles of climate warming and human occupation in Patagonian megafaunal extinctions during the Last Deglaciation.  Science Advances ,  2 (6), e1501682.

Smith, E. A., Bettinger, R. L., Bishop, C. A., Blundell, V., Cashdan, E., Casimir, M. J., . . . Stini, W. A. (1983). Anthropological Applications of Optimal Foraging Theory: A Critical Review [and Comments and Reply]. Current Anthropology, 24 (5), 625-651.

Surovell, T. A., & Waguespack, N. M. (2009). Human Prey Choice in the Late Pleistocene and Its Relation to Megafaunal Extinctions. In G. Haynes (Ed.), American Megafaunal Extinctions at the End of the Pleistocene (pp. 77-105). Dordrecht: Springer.

Waguespack, N. M., & Surovell, T. A. (2003). Clovis hunting Strategies, or How to Make out on Plentiful Resources. American Antiquity, 68 (2), 333-352.

Winterhalder, B. (1987). The Analysis of Hunter-Gatherer Diets: Stalking an Optimal Foraging Model. In M. Harris and E. Ross (Eds.), Food and Evolution: Toward a Theory of Human Food Habits (pp. 311-339). Philadelphia: Temple University Press.

Acknowledgements: I thank Dr. Bram Tucker for providing valuable feedback over the course of producing this piece.

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  • v.8(19); 2018 Oct

The overkill model and its impact on environmental research

Lisa nagaoka.

1 Department of Geography and the Environment, University of North Texas, Denton, Texas

Torben Rick

2 Department of Anthropology, Smithsonian Institution, National Museum of Natural History, Washington, District of Columbia

Steve Wolverton

Associated data.

Citation and survey data will be uploaded to FigShare.com.

Research on human‐environment interactions that informs ecological practices and guides conservation and restoration has become increasingly interdisciplinary over the last few decades. Fueled in part by the debate over defining a start date for the Anthropocene, historical disciplines like archeology, paleontology, geology, and history are playing an important role in understanding long‐term anthropogenic impacts on the planet. Pleistocene overkill, the notion that humans overhunted megafauna near the end of the Pleistocene in the Americas, Australia, and beyond, is used as prime example of the impact that humans can have on the planet. However, the importance of the overkill model for explaining human–environment interactions and anthropogenic impacts appears to differ across disciplines. There is still considerable debate, particularly within archeology, about the extent to which people may have been the cause of these extinctions. To evaluate how different disciplines interpret and use the overkill model, we conducted a citation analysis of selected works of the main proponent of the overkill model, Paul Martin. We examined the ideas and arguments for which Martin's overkill publications were cited and how they differed between archeologists and ecologists. Archeologists cite overkill as one in a combination of causal mechanisms for the extinctions. In contrast, ecologists are more likely to accept that humans caused the extinctions. Aspects of the overkill argument are also treated as established ecological processes. For some ecologists, overkill provides an analog for modern‐day human impacts and supports the argument that humans have “always” been somewhat selfish overconsumers. The Pleistocene rewilding and de‐extinction movements are built upon these perspectives. The use of overkill in ecological publications suggests that despite increasing interdisciplinarity, communication with disciplines outside of ecology is not always reciprocal or even.

1. INTRODUCTION

Research on human impacts on the environment, whether studying greenhouse gas emissions, overharvesting fisheries, or deforestation of rain forests, has grown significantly over the last 40 years. The growing number of new journals focusing on the Anthropocene (e.g., Anthropocene , Anthropocene Review , Elementa ) reflects this increase in interest. An important area of discussion revolves around how far back humans have been having a significant impact on the environment (Boivin et al., 2016 ; Braje & Erlandson, 2013a ; Smith & Zeder, 2013 ). It is the subject of not only defining the boundaries of the Anthropocene and other concepts such as the Sixth Extinction, but understanding the nature of the relationship between humans and the environment. It is into this discussion of how long people have been having a significant impact on the environment that the overkill explanation for Pleistocene megafaunal extinctions plays a role.

In the 1960s, Paul Martin ( 1958 , 1967a ), a geoscientist and paleobiologist, developed the overkill hypothesis, in which human hunting was proposed to have caused the extinction of the megafauna that roamed North America during the Pleistocene. During the last 50 years, the hypothesis has been extended to include all anthropogenic factors and has been applied to human colonization virtually anywhere in the world at any period (Burney & Flannery, 2005 ; Martin, 1984 ). Recently, overkill (Pleistocene or otherwise) has been used as a prime example in ecological or conservation studies stating that humans have profound impacts on the environment and have been doing so for millennia (Donlan, 2007 ; Donlan et al., 2006 ; Sherkow & Greely, 2013 ; Svenning et al., 2016 ). However, among researchers studying the extinction of Pleistocene megafauna (many archeologists and paleobiologists), the cause of the extinctions and the validity of overkill as an explanation are still being debated. Thus, what is a subject of debate (the cause of the terminal Pleistocene megafauna extinctions) to some is being used as a prime example of anthropogenic environmental destruction by others. So, how is it that these different communities of researchers view the role of overkill in megafaunal extinctions so differently?

In this paper, we summarize the overkill hypothesis and the debate about the cause of the late Pleistocene megafaunal extinctions. We then delve into problems of cross‐disciplinary communication by conducting a citation analysis of cited works of Paul Martin, the author of the overkill hypothesis. We document how overkill is interpreted and used differently by archeologists and ecologists. For many ecologists, overkill holds significant meaning for the relationship between humans and the environment that has consequences for conservation. While a number of important studies have been conducted by archeologists, ecologists, and Quaternary scientists since Martin's research, including recent studies by ecologists that support climate/multidisciplinary models (e.g., Di Febbraro et al., 2017 ; Lima‐Ribeiro & Diniz‐Filho, 2013 , 2017 ; Nogues‐Bravo, Rodiguez, Hortal, Batra, & Araujo, 2008 ) and others that support human impacts (Bartlett et al., 2016 ; Sandom, Faurby, Sandel, & Svenning, 2014 ), our focus here is not to review that extensive literature, but instead is to focus on interdisciplinary communication, particularly through citation of the seminal works on overkill by Martin. If environmental and anthropological scientists are to study the Anthropocene together, researchers face a challenge to improve interdisciplinary communication.

2. THE OVERKILL HYPOTHESIS DISSECTED

By the end of the Pleistocene, a suite of 37 genera of large‐bodied mammals became extinct in North America (Grayson, 2015 ; Meltzer, 2015 ). There are two main competing hypotheses to explain the extinction of these megafauna that are based on the timing of the extinctions and either the arrival of people to the Americas or climate change at the end of the Pleistocene. Research evaluating these hypotheses has involved investigators from archeology to the geosciences to evolutionary biology and ecology (Koch & Barnosky, 2006 ; Meltzer, 2015 ). Until the 1960s, the extinctions were primarily a paleontological subject of research, believed to be caused by warming climate that occurred during deglaciation in the transition from the late Pleistocene to early Holocene. There was little evidence that people interacted with megafauna let alone that they lived in the same places at the same time. However, with the advent of radiocarbon dating, the arrival of people in North America was documented back to the Late Pleistocene (Haynes, 1964 ). Thus, a temporal association was established between people and megafauna. Martin argued that if people were present in the Americas alongside the megafauna, then they could have been a factor in their extinction. As an alternative explanation to climate change, Martin ( 1958 , 1967a , 1973 ) proposed the overkill hypothesis in which humans hunted the megafauna to extinction.

While the timing of both climate change and human colonization overlaps with megafaunal extinction, the mechanisms for how climate change was able to cause extinction in this context were unclear. In particular, Martin ( 1967a ) questioned why megafauna had survived multiple interglacial periods during the Pleistocene only to go extinct at the end of the last glacial period. On the other hand, with the rise of environmentalism in the 1960s, the mechanism for Martin's overkill model was intuitive and self‐evident (Grayson, 2001 :41; Grayson & Meltzer, 2003 :590). It was easy to conceptualize how people could have caused an extinction event because the impacts of (and protests against) human‐caused environmental degradation were on the nightly news. By the 1980s, Martin ( 1984 ) had expanded the overkill model beyond North American Pleistocene extinctions to explain mass extinctions globally as a function of human colonization: wherever people go, species go extinct. Since then, the model has grown considerably outside of archeology and paleontology and is often used as evidence for the harm that people can perpetrate on the environment.

The mechanism for how people were able to cause the extinctions through hunting makes several key assumptions. When the argument is teased apart, it is easier to evaluate whether or not overkill adequately explains the extinctions. The first two assumptions use the “island analogy” (Nagaoka, 2012 ). First, the mechanisms for extinction of continental megafauna are similar to those that impact island fauna . In his explanation for overkill, Martin ( 1967a , 1984 , 1990 ) described many prehistoric and historic examples of extinction of island species following human colonization in places such as Madagascar, New Zealand, Hawaii, and other Pacific islands as support for the idea of overkill (see also Steadman & Martin, 2003 ). Island fauna often evolves in the context of low predation pressure resulting in traits such as flightlessness and ground‐nesting in birds, and naïve behavior in general. These traits along with high endemism and small populations make island species more vulnerable to predation and environmental perturbations, and thus extinction. While it is widely recognized that the circumstances for island extinctions can differ from those on continents, these island examples demonstrated that people could and did cause extinctions, which planted the seeds for the process of anthropogenic extinctions in other contexts.

To bolster the analogy between naïve island fauna and continental Pleistocene megafauna, Martin developed the second assumption: continental megafauna were vulnerable to extinction like island fauna because humans are superpredators . Continental megafauna coexisted with a large predator guild and thus had evolved a suite of predator defenses. However, if humans were hyper‐efficient predators, then megafauna could be naïve to their specific type of predation. People were so efficient at hunting that the megafauna went extinct before they could develop an appropriate predator response (Martin, 1973 ). Indeed, the Blitzkrieg version of overkill has people hunting megafauna in a wave across North America (Mosimann & Martin, 1975 ). This assumption is often accepted as fact. Neither the degree of human hunting efficiency nor the absence of predator response has yet to be evaluated or demonstrated.

A third assumption relates to the empirical requirements of the model. Archeological data are particularly important for evaluating the overkill hypothesis because the test implications are not just that people coexisted with the megafauna, but that they directly interacted with the megafauna in such a way as to cause extinction. Thus, stone tools embedded in megafauna bones reflect hunting, cut marks reflect butchering, and (potentially) burnt bone suggests cooking. Empirically, however, there is little archeological evidence for these types of direct association between people and megafauna, let alone that human predation had a significant impact on megafaunal populations. The megafauna that humans are directly associated with are limited to five (mammoths, mastodons, gomphotheres, camels, horses) rather than all 37 genera, with mammoth as the most common taxon (Grayson & Meltzer, 2002 ; Meltzer, 2015 ). And only a small number of sites, 15–26 (the veracity of the association is debated among archeologists), have been identified as showing evidence for a direct association between stone artifacts and remains of extinct megafauna (e.g., Meltzer, 2015 ; Surovell & Waguespack, 2008 ).

The paucity of archeological evidence for interaction between people and megafauna has been called the “associational critique” (Grayson, 1984a ; Meltzer, 1986 ). But it has been deftly handled by Martin ( 1973 , 1984 ) and others (Fiedel & Haynes, 2004 ; Surovell & Grund, 2012 ), who assume that there is a small sample of sites with evidence of association only because the extinction process was so rapid that the remains were not buried and thus did not preserve . The absence of evidence, specifically the absence of association, is used as evidence for overkill. Requiring evidence of association is considered too stringent a criterion to expect for ancient deposits (Surovell & Grund, 2012 ).

Critics have countered this explanation in several ways. Arguing that the “absence of evidence is evidence” is not a scientific means to evaluate a hypothesis. If there is a paucity of data, then other alternative means should be found to test the hypothesis. Even if the absence of association between people and megafauna was a valid measure, it could be used to both support both the overkill and climate hypotheses. If climate change was the major cause of megafaunal extinction, then a paucity of sites with association would not contradict expectations. However, it is more important for overkill to demonstrate that the lack of sites is a result of poor preservation of sites and remains. Interestingly, there are many paleontological sites from the Late Pleistocene with mammoth (Agenbroad, 2005 ; Widga et al., 2017 ) and other extinct megafauna (Meltzer, 2015 ). The higher proportion of remains in paleontological contexts compared to archeological ones suggests that megafaunal mortality may be better explained by natural rather than anthropogenic causes. The alternative argument is that preservation in archeological contexts is less likely than in paleontological ones. But this has not been demonstrated.

What is particularly startling about advocating that association should not be a requirement for evaluating overkill is that this is a foundational concept for historical disciplines, such as archeology, geology, and paleontology. Association is used to argue that spatial relationships between fossils and artifacts within deposits reflect past events and behaviors. Thus, to argue that demonstrating association is not necessary or that it is too onerous of a requirement is to argue that these disciplines are not necessary for understanding these extinctions. This is unfortunate given that archeology is the only one of these three historical disciplines that can provide evidence of direct interaction between humans and megafauna.

When overkill was introduced, the model appeared to have a clear mechanism for how megafaunal extinction occurred. However, the reality is that the argument uses a series of untested assertions about human–environment interactions. Thus, the best evidence for overkill is the temporal association between megafaunal extinctions and human colonization. Unfortunately, the extinctions also co‐occur with climate change at the end of the Pleistocene. Further compounding the problem is that archeology over the last few decades has continued to demonstrate that many of the earliest peoples in the Americas had broad spectrum diets focused on small game, aquatic resources, and a variety of foods that were far more abundant than megafauna (Cannon & Meltzer, 2004 , 2008 ; Dillehay et al., 2017 ; Erlandson et al., 2011 ). Similarly, other studies demonstrate that many megafauna species were extinct prior to human arrival (Boulanger & Lyman, 2014 ; Lima‐Ribeiro & Diniz‐Filho, 2013 ). Given that causes for the Pleistocene extinctions are unresolved, it is interesting to see that the overkill model features prominently in the ecological and conservation literature.

3. DIFFERENT DISCIPLINES, DIFFERENT INTERPRETATIONS

The use and relevance of overkill as the cause of the Pleistocene extinctions varies within and between disciplines. Within archeology, the literature on overkill has become polarized between perspectives of proponents and critics of overkill (e.g., Fiedel & Haynes, 2004 ; Grayson & Meltzer, 2003 , 2004 ). Thus, it may appear that there is a debate for or against overkill. However, the average archeologist is not represented in such black and white terms. We surveyed archeologists about what killed the megafauna during a poster session at the annual meeting of the Society for American Archaeology in 2012 ( n  = 91). Eighty‐two percent believed that the extinctions were caused by multiple variables with climate change as the only single cause identified (Figure  1 ). In a separate but similar survey of 112 archeologists, 63% of archeologists identified a combination of factors caused the extinctions (Wheat, 2012 ). In our survey, respondents who believed there were multiple causes for the extinctions were asked to identify which causes were involved in the extinctions. Most respondents identified climate change more often as one of the causes, with human impacts either directly through hunting or indirectly through landscape change as the other factor (Figure  2 ). To archeologists, overkill is not the dominant explanation for the extinctions.

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Archeologists’ responses to the prompt, “The main cause of megafaunal extinctions in North America is” ( n  = 91)

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The causes for megafaunal extinction identified as playing a role by archeologists who believe that the extinctions were multicausal

Yet, outside of archeology, overkill as the prime mover for the extinctions seems to have taken on a different trajectory. As archeologists, it is surprising to encounter publications in which overkill explains megafaunal extinctions and is used as an example of human impacts in general. For example, a recent popular science book and New York Times 10 Best Books of 2014 use the overkill model as follows:

If…people were to blame [for the extinctions] – and it seems increasingly likely that they were – then the import is almost disturbing. It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer – to use the term of art an “overkiller” – pretty much right from the start Kolbert ( 2014 : 229–230)

While this excerpt was written by a science writer, the section after this quote describes Paul Martin's overkill model. The lack of direct archeological evidence is never discussed.

To evaluate how the overkill literature is being used differently by different communities of researchers, we conducted a citation analysis of journal articles that cite four of Paul Martin's publications on overkill. His 1967 book chapter, “Prehistoric overkill” and his 1973 Science article “The discovery of America,” are the first full descriptions of the overkill model. In 1984, he coedited the book, Quaternary Extinctions , with Richard Klein, which reviews extinctions across different regions. Martin also authored a chapter in the book entitled “Pleistocene overkill: The global model” that brings together his perspective on overkill as the cause for megafaunal extinctions worldwide.

We used the cited reference search in Thompson Reuters’ Web of Science database to find articles that cited these four publications. The articles spanned from 2015 when the analysis was originally done and 1995, the earliest extent of the Web of Science database at the time. We then categorized the publications into groups—archeology, Quaternary, ecology, and other. The other category consisted of publications in fields such as philosophy, law, or sociology. For this study, we focus on archeological, Quaternary, and ecological publications. They differ in the subject matter and time depth. Archeological publications were those written by archeologists on human prehistory or paleoecology. Quaternary publications represent paleontological, historical biogeography, or paleoecological publications that generally focus on evolutionary processes related to a specific taxon. Ecological publications are neoecological studies that study taxa in contemporary contexts or that presented research related to conservation.

We found a difference in how Martin's publications were cited in these three areas of research. Authors in each research area tended to cite different publications when referring to overkill (Table  1 ). Of the three groups, Martin's publications tend to be cited the most in Quaternary publications. Archeologists cite Martin's earlier publications, particularly his 1973 article, probably because it discusses the relationship between human colonization and overkill. In contrast, within ecological publications, Martin's later works are cited.

The percentage of citations for four of Paul Martin's publications by publication type

To understand how Martin's publications were being cited in the different types of publications, we analyzed the text associated with each citation. We evaluated the citations only for the 1984 publications because they represent more recent thought and also have more even coverage across all three categories of research. Of the 388 references, copies of 378 articles were obtained. Of those, 363 were categorized as archeological, Quaternary, or ecological. The remaining fifteen articles were in social sciences and humanities publications. For each publication, the text associated with the Martin citation was recorded. Each use of the citation was then categorized based on the claim it was used to support. Citation examples are presented below.

All three research areas cite Martin's work as evidence that either a large number of species went extinct at the end of the Pleistocene or that the cause of the extinction is being debated (Table  2 ). However, about one‐third of the ecological publications, or five or six times the archeological or Quaternary publications, use Martin's work as evidence to support the claim that humans are directly responsible for the extinctions (i.e., human predation) or that humans are capable of causing great damage to the environment, including extinctions. For example:

The percentage of times Martin ( 1984 ) and Martin & Klein ( 1984 ) were cited within the three types of publications, and the claim for which the publications were cited

“There may be a variety of situations in nature, of course, in which consumers or consumer populations are not controlled by predation. For instance, before the late Pleistocene overkill of large mammals in Australia and North and South America (Martin & Klein, 1984 ), most of the earth's ecosystems contained megaherbivore species whose adult members, like today's elephants, were too large to be killed by the largest predators. Soulé and Terborgh ( 1999 : 811–812)

In addition, a number of articles claim that there is “growing consensus” or increasing or mounting evidence that humans caused the extinctions (e.g., Blondel, 2008 ; Kodric‐Brown & Brown, 2007 ). Interestingly, many of these articles cite Martin & Klein's ( 1984 ) book (Table  3 ), which is a general compendium about extinctions during the Quaternary, including papers that suggest alternatives to overkill (Grayson, 1984b ; Kiltie, 1984 ). Authors are thus incorrectly citing the book, possibly confusing it with Martin's chapter in the book. Thus, while many of the publications in the archeological and Quaternary categories suggest overkill as one potential explanation for megafaunal extinctions, a greater proportion of the ecological sample promotes it as a likely cause and/or a well‐founded process and cites either of Martin's ( 1984 ) publications as justification.

Reference cited when supporting overkill in the ecological literature

Because overkill is more commonly used within the neoecological literature as an example of anthropogenic impacts, the assumptions of the model are also being used as if they are confirmed ecological processes. For example, a small percentage (7%) of the ecological articles cite the publications to support the idea that humans are hyper‐efficient predators rendering continental fauna in effect, naïve . But only one of the archeological or Quaternary articles uses the publications for the same purpose. The following quotes illustrate how the assumption about humans as superpredators has been used:

The late Pleistocene invasion of the Americas by humans might be the most recent case of an introduced predator exerting large impacts on continental prey (Barnosky, Koch, Feranec, Wing, & Shabel, 2004 ); once again, however, it is likely that human impact was magnified by the naivete′ of New World prey toward this novel predator archetype (Wilson 1992 ; Martin, 1984 ). Cox and Lima ( 2006 )
Rather than having a continent of fearless animals waiting to be killed by an advancing wave of hunters (e.g., Flannery 2001), it is more likely that human hunters posed unique threats, and that while not entirely predator naïve, the hunted animals did not have a sufficient antipredator behavior to cope with these unique threats. Blumstein ( 2006 )

Interestingly, in both of these cases, the authors have brought up the naivete’ of Pleistocene megafauna because it is an exception not seen elsewhere that they need to explain. Alternatively, the authors could have argued that the role of naivete’ in megafaunal extinction is an untested assumption.

The assumption that fauna were naïve to human hunting at initial contact has also had an impact on studies on the historical biogeography of Africa. A few of the ecological articles cite Martin's publications to argue that Africa maintained a high diversity of large‐bodied mammals following the Pleistocene because the fauna had evolved with humans and thus were not naïve to them. Martin himself made this same claim in his 1967 article, Africa and Pleistocene overkill.

We believe that Africa's uniqueness in range contraction is the result of a fundamental difference in the spatial dynamics of the extinction forces in Africa. Human evolution in Africa allowed species there to adapt to coexist with humans (Martin, 1984 ). However, as humans expanded their range out of Africa and into the other regions of the world they encountered animals that were naive to their abilities and suffered extinctions (Diamond, 1984; Martin, 1984 ). Channell & Lomolino ( 2000 )

These authors are arguing that the difference in biodiversity across continents is partly a result of the distribution of humans. Specifically, it is argued that post‐Pleistocene species diversity in Africa is greater because the fauna coevolved with humans and thus was adapted to their superpredatory skills (Faith, 2014 ; Wroe, Field, Fullagar, & Jermin, 2004 ). Thus, the biodiversity of Africa is due to the fact that hunter–gatherers have not been able to hunt species to extinction as they have elsewhere. As discussed above, the superpredatory skills of humans are linked to the analogy between islands and continents. Continental extinctions are similar to island extinctions if people are superpredators and megafauna were naïve to their predatory skills. But if the extinctions involved any other predator–prey relationship, would extinctions of small, endemic, island populations be studied together with extinctions of widespread continental populations? Indeed, if we exclude all island examples, the causes for continental megafaunal extinctions are diverse, often multicausal, and have limited evidence for overkill (Barnosky et al., 2004 ).

Another ecological process that overkill seems to demonstrate is that human colonization of new lands leads to faunal extinction . Remember that island extinctions following human colonization were used to provide a mechanism for extinction. Martin extended human colonization as a causal factor from islands to virgin lands in general in this 1984 book chapter, such that the coincident timing of people and extinction is proof that humans had a negative impact on fauna.

Colonization and hunting by aboriginal humans played a major role in the extinction of the Pleistocene megafauna in North America and other parts of the world (Martin & Klein, 1984 ; Owen‐Smith 1987). Brown and McDonald ( 1997 )

The idea that human colonization had detrimental impacts on fauna even shows up as an important fact in guidelines proposed to promote conservation literacy.

Impacts of human colonization in ancient times: Human societies have a long history of causing extinctions and major changes in ecosystems. (1) In the prehistoric (Martin & Klein, 1984 ) and historic (Crosby 1993) past, arrival of humans to new areas led to extinctions of other species and large‐scale changes in natural communities. Trombulak et al. ( 2004 : 1185)

Unfortunately, the underlying idea behind this process is not just that human colonization causes extinctions but that humans as a species are inherently destructive. In the conservation and neoecology literature, this idea has been used two ways. First is to use the relationship to set up the argument that humans have been detrimental to the environment; thus, ecological reparations are required . This thinking has led to proposals such as Pleistocene rewilding and de‐extinction (Donlan, 2007 ; Donlan et al., 2006 ; Sherkow & Greely, 2013 ; Svenning et al., 2016 ). Some proponents of both proposals use overkill to argue that North American fauna is depauperate because humans caused the mass extinctions at the end of the Pleistocene. Thus, it is argued that it is our moral and ethical responsibility to repopulate the landscape with descendants, close relatives, or clones of the megafauna. We note that many ecologists have been critical of the overkill model and particularly of Pleistocene Rewilding (Fernández, Navarro, & Pereira, 2017 ; Lima‐Ribeiro & Diniz‐Filho, 2013 , 2014 , 2017 ; McCauley, Hardesty‐Moore, Halpern, Young, & Seddon, 2017 ; Nogués‐Bravo, Simberloff, Rahbek, & Sanders, 2016 ; Richmond, McEntee, Hijmans, & Brashares, 2010 ; Rubenstein & Rubenstein, 2016 ). However, the overall trend in the citation analysis and literature we reviewed is that overkill is more likely to be treated as the explanation for the extinctions such that support for overkill can be found even in the argument of some critics of rewilding (Oliveira‐Santos & Fernandez, 2010 ; Perring et al., 2015 ).

4. COMMUNICATION BREAKDOWN

So why do these research communities differ in their perspectives on overkill and megafaunal extinctions? One explanation is that the archeological literature, which discusses the empirical research on the role of people in the extinctions, is less likely to be accessed by researchers publishing in the ecological literature. Recent bibliometric and citation analyses appear to support limited interaction between these two groups. For example, Rosvall and Bergstrom ( 2011 ) analyzed citations from over 9 million articles across nearly 8,000 journals to understand connectivity and information networks among academics. They identified four major clusters of research. The physical sciences and the life sciences form the two largest clusters of research. The third cluster of ecology and earth sciences includes ecology, conservation biology, and Quaternary research. Social sciences, into which archeology falls, form the fourth cluster. Thus, ecologists and Quaternary scientists may be more likely to read and cite one another's research than they are to read and cite archeological journal articles.

If archeologists are publishing about megafaunal extinctions only in archeological journals, then ecologists may be less likely to encounter these articles. Donald Grayson and David Meltzer are prominent critics of overkill whose work is published predominantly in archeological journals (Grayson, 1984a , 1984b , 2001 , 2007 ; Grayson & Meltzer, 2002 , 2003 , 2004 , 2015 ; Meltzer, 1986 , 2015 ; see also Wroe, Field, & Grayson, 2006 ). While they are commonly cited in publications about the Pleistocene extinctions in the archeological and Quaternary literature, they are rarely cited in the neoecological literature (Table  4 ). Thus, researchers outside of the social sciences may have been less likely to encounter information on the importance of association and the paucity of evidence for association between megafauna and humans. However, none of the publications by archeologists that support overkill that are published in broader scientific journals (e.g., Faith & Surovell, 2009 ; Haynes, 2007 , 2013 ; Surovell & Waguespack, 2008 ; Surovell, Waguespack, & Brantingham, 2005 ) tend to be cited either. This suggests that cross‐disciplinary communication, particularly from archeology to ecology, is limited. Researchers publishing in the neoecological literature may not recognize archeology's role in evaluating overkill and may not look to the archeological literature as an important source of information.

Percentage of publications citing Martin ( 1984 ) that also cite publications by Grayson

One solution would be for archeologists to publish their findings in ecological journals. Currently, the peer‐reviewed ecological literature in which archeologists assert that there are issues with the overkill hypothesis is limited (e.g., Wolverton, 2010 ). But this is not from a lack of effort. In our experience, claims made by archeologists about the data and underlying assumptions of overkill are downplayed by some ecologists. Unfortunately, these issues are not being debated in the ecological literature, but occur in discussions at conferences and in reviews of grant proposals and publications (see Grayson and Alroy, 2001 for a rare exception). Colleagues who are ecologists typically will cite papers by ecologists to question our claims. During the peer review process for past research articles on this topic, we have been often pointed toward ecological research that examines the strength of the correlation between the extinctions, climate change, and human colonization at the regional or global scale (e.g., Bartlett et al., 2016 ; Prescott, Williams, Balmford, Green, & Manica, 2012 ; Sandom et al., 2014 ). These publications are cited during peer review as evidence that ecologists do not support overkill or the idea that humans alone were responsible for the extinctions.

Our rebuttal to these assertions is twofold. First, the nature and source of the human impact data in these studies is rarely questioned. Reviewers may be surprised to learn that the data on the magnitude of human impact used in many of these models is directly derived from overkill. They often rely on the assumption that human colonization causes extinctions to model human impact rather than on empirical data. Thus, the evidence for climate change is contrasted against assertions about human impacts to evaluate the strength of the correlation between each factor and extinctions. These models further strengthen rather than detract from our argument that the tenets of overkill are deeply embedded in ecology and conservation biology.

Second, we find it curious that some researchers appear reticent to accept arguments and data from archaeologists, particularly given that our field of expertise is studying the interaction and impact of human actions on species and ecosystems across time and space. Summary studies by paleo‐ or neoecologists are not equivalent to archeological studies that assess the quality of evidence for association between megafauna and humans during the terminal Pleistocene. As archeologists, we take for granted that archeological data are necessary for evaluating the role that humans may have played in the extinction of the megafauna. The presence of megafaunal remains in archeological sites is required to demonstrate that people interacted with megafauna, not just that they coexisted on the continent at the same time. Archeological data are also needed to demonstrate the nature of those interactions that people hunted, butchered, and ate megafauna. These data are necessary to understand the magnitude of impact that humans had on megafaunal populations. As was mentioned earlier, these types of data are in short supply. Only five of the 37 genera of extinct megafauna in North America have direct evidence of association with very few archeological sites that exhibit convincing evidence that people commonly and effectively hunted the five genera. In contrast, over 95% of the sites dating to the early period of human occupation and containing remains of those five extinct taxa are paleontological (Meltzer, 2015 ; Table  1 ). Thus, there is an extensive record of nonassociation between the extinct megafauna and people. In addition, the record on the timing of the extinctions is variable indicating that some megafauna went extinct before human arrival in North America and some persisted for a period after human colonization (Faith & Surovell, 2009 ; Grayson, 2007 ; Grayson & Meltzer, 2002 , 2003 , 2004 ; Meltzer, 2015 ). We assume that if these data were incorporated into the models comparing the impact of climate change versus human colonization, the results would be substantially different. Because of the variability in the timing of extinctions across taxa and in the evidence for interactions between humans and megafauna species, archeologists have argued that unraveling the mechanisms for the extinctions will require a “Gleasonian” approach, in which the extinction process is studied species by species (Grayson, 2007 ; Meltzer, 2015 ; for species examples, see Hill, Hill, & Widga, 2008 ; Widga et al., 2017 ).

Thus, in terms of interdisciplinary communication, researchers publishing in the ecological and archeological literature seem to have knowingly or unwittingly settled into a status quo. Martin's work serves different purposes for those publishing in the different research areas. Those using it to support claims about human–environment interactions and ecological processes that the overkill model promotes may not recognize the shortcomings of the model because information flow between the various groups is limited. Archeologists have the necessary datasets to evaluate the human role in the extinctions and bring to the table a different but relevant perspective. But bringing this perspective into the neoecological literature has been limited and challenging.

5. CHARACTERIZING HUMAN–ENVIRONMENT INTERACTIONS WITH OVERKILL

While communication about overkill between archeological and neoecological research areas is limited, there is greater information flow between Quaternary and neoecological publications. However, like archeological publications, the Quaternary literature does not promote overkill as the dominant explanation for the extinctions, but generally suggests that more research is still needed (Barnosky et al., 2004 ; Koch & Barnosky, 2006 ). Thus, favoring the Quaternary literature over the archeological still does not explain why the use of overkill to characterize human–environment interactions is still more prevalent in the ecological literature. In addition, our study only focuses on publications citing Martin's publications and overkill specifically. But the ideas promoted by overkill can also be found in articles that do not cite Martin (e.g., Smith, Elliott Smith, Lyons, & Payne, 2018 ). Over the years, Grayson and Meltzer ( 2003 , 2004 ) have argued that overkill persists because it supports a particular philosophical perspective on anthropogenic environmental impacts. Over several articles, they have evaluated the history of the overkill model, particularly the logic of the argumentation, as well as the empirical evidence for the model. Within the last 15 years, they have argued more vehemently that,

The overkill position has also, despite a clear lack of empirical archaeological support, been adopted on faith by an influential subset of ecologists and used to support what are essentially political arguments. (Grayson & Meltzer, 2004 : 135)
…the overkill argument captured the popular imagination during a time of intense concern over our species’ destructive behavior toward life on earth… [it] is inextricably linked to modern times and to the homily of ecological ruin. (Grayson & Meltzer, 2003 : 590)

Thus, they assert that overkill is used as evidence of the damage that humans can do to the environment. If humans have been causing mass extinctions for thousands of years, then they are and will always be a destructive force and a significant threat to biodiversity.

While it is clear that people are having a significant impact on the environment today, it is another thing to extend this behavior back into deep time, especially when there is considerable debate on the topic (see Bartlett et al., 2016 ; Di Febbraro et al., 2017 ; Lima‐Ribeiro & Diniz‐Filho, 2013 , 2017 ; Nogues‐Bravo et al., 2008 ; Sandom et al., 2014 ). However, this monolithic view of human–environment interactions is not uncommon in neoecological publications. It is linked to a viewpoint of humans as outside of nature, in which dominion over nature is a pan‐human trait. The recent debate about the old versus new conservation has highlighted these philosophical differences in how we view human's place in nature (Doak, Bakker, Goldstein, & Hale, 2014a , 2014b ; Kareiva, 2014 ; Kareiva & Marvier, 2012 ; Marvier & Kareiva, 2014 ; Miller, Soulé, & Terborgh, 2014 ; Soulé, 1985 ). When humans are inherently separate from nature, then the relationship between humans and the environment is fixed, and the outcome is inevitable (ecological ruin). As such, nature must be preserved and kept separate from humans if biodiversity is to be maintained and the extinction threat minimized. Overkill provides justification for this preservationist perspective. However, overkill can be found even in publications advocating for a more pluralistic view of the human–nature dynamics (e.g., Kareiva & Marvier, 2011 ), suggesting that the notion of humans as a destructive force is deeply embedded. If the use of overkill is motivated by a humans‐as‐separate‐from‐nature worldview, what impact does it have on how the public and the scientific community conceptualize human–environment interactions in general? We use a recent discussion about the transition to the Anthropocene as an example of how overkill influences views about anthropogenic impacts on the environment. Specifically, overkill is used to support the perspective that human actions are monolithic in impact. This perspective leaves little room for research that examines variability, sustainability or resilience.

The Anthropocene is both a potential new geologic period of time and a perception about humans’ role within the environment (see Crutzen, 2002 ). While geoscientists are empirically evaluating the Anthropocene as a potential new geologic epoch, a broader definition of the Anthropocene used beyond the geosciences has become synonymous with the age when anthropogenic activities came to dominate the Earth's ecosystems (Autin, 2016 ; Braje & Erlandson, 2013a ). The concept has so widely captured the imagination and interest of scholars that it has led to a plethora of recent articles and several new journals focusing on the Anthropocene as a period of anthropogenic environmental impacts. It has become a powerful interdisciplinary rallying point around which scholars from diverse disciplines weigh in on human–environment issues like never before (Ellis, 2018 ).

For both the geosciences’ and the broader version of the Anthropocene, the start date is very important. But each approach uses different criteria. For the geologic epoch, defining the lower boundary has focused on empirically identifying the markers that can be used to differentiate the Anthropocene from the Holocene in geologic deposits (Lewis & Maslin, 2015 ; Zalasiewicz et al., 2008 ). When did global human impacts become significantly different from what is seen in the Holocene? The consensus seems to be that 1950 will likely be the start date (Zalasiewicz et al., 2015 ). Thus, the geologic Anthropocene is recent and represents modern anthropogenic impacts.

In contrast, with the broader usage of the term Anthropocene, the start date varies widely. But each is linked to historic turning points such as industrialization and Western exploration and expansion, or major cultural developments such as the rise of civilizations or the beginnings of agriculture (Braje & Erlandson, 2013a , 2013b ; Glikson, 2013 ; Ruddiman, 2013 ; Smith & Zeder, 2013 ; Steffen, Grinevald, Crutzen, & McNeill, 2011 ). Unlike the geologic epoch, however, the broader use of the Anthropocene tends to focus on similarities between the past and present rather than when the impacts become markedly different. Thus, the farther back in time the period extends, the more the issues of the present may be projected onto the past. While it may appear that the Anthropocene represents the history of processes that led to modern‐day environmental impacts, it is often treated as a monolithic period of human behavior and environmental impacts by humans before which existed a potentially pristine nature. The conceptual implications of an Anthropocene with significant time depth are clearly illustrated when overkill and megafaunal extinctions are used to define the beginning of the period.

Human‐caused megafaunal extinctions are used by some to argue that initial human occupation of a place marks the beginning for the Anthropocene (Doughty, Wolf, & Field, 2010 ). It is built off of Martin's idea that human arrival has had a significant, impact on biodiversity everywhere people migrate (Boivin et al., 2016 ). For example, given overkill in North America, the impact of humans has been significant and severe since the late Pleistocene when people arrived to the continent and overkilled the megafauna. Unfortunately, the logical extension of this argument is that humans are inherently destructive as a species. Thus, it could also be argued that the Anthropocene should extend back to the beginning of Homo sapiens as a species. This may seem like an extreme or marginal view, but it is a relatively common, implicit perception of humans when discussing environmental issues. For example, E. O. Wilson presented just such a scenario when discussing threats to biodiversity and human‐caused extinctions.

‘Human hunters help no species.’ That is a general truth and the key to the whole melancholy situation. As the human wave rolled over the last of the virgin lands like a smothering blanket…., they were constrained by neither knowledge of endemicity nor any ethic of conservation. Wilson, E.O. (1992) The Diversity of Life , p. 253

When overkill is used as a cautionary tale and a means to rally support for environmentalism, it portrays humans as a destructive species.

There are several important contradictory consequences for this line of thought. Using overkill to establish an older benchmark implies that prehistoric communities significantly altered the environment such that they should be classified as similar to that of modern societies. While the popular literature may highlight prehistoric examples of societal collapse due to environmental degradation (e.g., Diamond, 2011 ), much of the archeological record is characterized by persistence rather than extirpation, with the magnitude of anthropogenic impacts varying significantly across time and context.

When the Anthropocene is extended back to the evolutionary beginning of humans, then humans are a destructive or invasive species. Indeed, the blitzkrieg version of the overkill model portrays people as locusts killing megafauna and eating their way across North America (Mosimann & Martin, 1975 ). The invasive species analogy suggests that humans do not belong in any environmental context and that they are separate from nature. By extension, at no time in the evolutionary history of humans are they under the purview of ecological or evolutionary processes. Unfortunately, the extreme, yet logical solution for healing the environment would be to rid the planet of humans.

Extending the Anthropocene into deep time also ignores the real factors that make modern anthropogenic impacts particularly damaging—the combination of an exponentially increasing population, efficient and destructive extraction techniques, massive consumption, and rapid technological innovation and knowledge transmission. If humans have always been destructive, then studying the historic or prehistoric past also provides no understanding of how certain cultures were able to mitigate their impacts or in which contexts the impacts were exacerbated or what tipping points might have looked like.

In addition to unchanging human impacts across time, cross‐cultural diversity in human interactions with the environment is also ignored. Diversity in local ecological knowledge of people in all areas of the world and across all times must be considered uniform. Anthropogenic impacts are often structured as a choice between being inherently destructive and “noble savages” who are completely in tune with nature (e.g., Penn & Mysterud, 2017 :3).

The well‐documented occurrence of prehistorical overkill in the Americas, Australia, New Zealand, Madagascar, Oceania, and elsewhere should put us on notice that premodern indigenous people have not always been exemplary stewards of biotic resources. Terborgh ( 2000 )

When human–environment interactions are viewed as fixed and unchanging, studying resilience and sustainable practices of modern peoples can offer no solutions. However, like any other organism, humans can destroy, modify, enhance, or preserve depending on context. And there is an extensive continuum of human–environment interactions that range from extinctions to sustainable coexistence (Anderson, 2010 ; Turner & Berkes, 2006 ; Rick et al., 2016 ; Wolverton, Nolan and Ahmed 2014 ). Archeological research on long‐term relationships between humans and the environment and modern studies of local ecological knowledge (LEK) examine how cultural practices and institutions can mitigate environmental impact and result in sustainability and resilience. In essence, human (phenotypic) diversity is devalued when overkill is used to support a human–nature dichtomy, resulting in the view that the past is a clone of the present. As such, it is easy to deny a role in environmental and conservation discussions to any research areas that study human–environment interactions across time and space.

Thus, there are many reasons why overkill is problematic as a source for ecological explanations. Overkill remains hotly contested, and its use highlights two problems for conservation and management. First, conservation research is not maximizing its interdisciplinary potential even though it has been touted as multidisciplinary from its inception (Soulé, 1985 ). The citation patterns in this study suggest that communication between researchers publishing on environmental research in the ecological and social sciences literature may be limited. However, archeology and other social science disciplines provide the source data to the human side of human–environment relationships (Briggs et al., 2006 ; Erlandson & Braje, 2013 ; Lane, 2015 ; Rick, Kirch, Erlandson, & Fitzpatrick, 2013 ). In addition, archeology also contributes to paleoecology in similar ways as paleontology (e.g., Grayson, 1993 , 2015 ; Lyman, 2012 ), but this area of research may be less well known simply because archeology is classified as a social science or because of methodological differences between the disciplines.

Second, when overkill is used to extend large‐scale anthropogenic impacts back into the deep past, it homogenizes these impacts across time and space. Human impacts become monolithic and always catastrophic. However, even if overkill is demonstrated to have been the cause of Pleistocene megafauna extinctions, there are alternative ways of using this information. These extinctions could be used as one data point in millennia of different “experiments” of humans interacting with the environment. Thus, the focus would be on documenting the variability of anthropogenic impacts to understand when human actions are more sustainable versus more destructive. That some researchers default to treating human actions as inherently destructive indicates a core belief that humans are beyond nature and that nature, thus, needs to be protected (Callicott, Crowder, & Mumford, 1999 ). This is an interesting conundrum for environmental researchers. The logical extension is that if human–environment interactions are uniform, then not only were human impacts similar in the past, but future restoration and management are futile. If this belief is deeply embedded and if overkill as an explanation for extinctions is disproven, then the likelihood is that these researchers may look for another similar example to bolster the perception of humans’ role in the environment rather than shift the focus to understanding how people in their diverse cultural, social, political, and historical contexts impact biodiversity.

Understanding Late Quaternary extinctions has long been an important, but often polarizing area of study and considerable debate remains. While our focus has been on issues with the overkill model, particularly as they relate to interdisciplinary scientific communication, there have also been important critiques levied against the climate change model (see Bartlett et al., 2016 ). Some researchers in archeology, Quaternary sciences, and ecology are focused on multicausal explanations for Late Quaternary extinctions, with humans often seen as the final tipping point on already dwindling megafauna populations (see Barnosky et al., 2004 ; Boulanger & Lyman, 2014 ; Braje & Erlandson, 2013b ; Lima‐Ribeiro & Diniz‐Filho, 2013 ). An important step for future research on Late Quaternary extinctions, particularly as applied to conservation, as well as for researchers working on other highly interdisciplinary topics will be for scholars to read, critically evaluate, and cite material on the topic across the varied fields that are investigating this important area of interdisciplinary study.

CONFLICT OF INTEREST

None declared.

DATA ACCESSIBILITY

Authors’ contributions.

L.N. collected and analyzed the data. All authors contributed to the manuscript.

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October 5, 2018

The Problem with Ice Age Overkill

A new study highlights a communication breakdown in sciences concerned with ice age extinction

By Riley Black

Extinction

Did humans drive Ice Age animals like  Eremotherium  to extinction? The debate has important implications for our modern extinction crisis.

Brian Switek

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Where’s my mastodon?

That’s the question I’ve been asking ever since I learned that the beasts existed and perished not so very long ago. Exactly why they disappeared depends on who you ask. Some experts point to dramatic climate shifts at the end of the Pleistocene that pared back the elephant’s favored habitat. Dissenting opinion convicts human depredation, invoking waves of voracious people who ate the world’s megafauna out of existence as Homo sapiens moved out of Africa and beyond. And while there is sometimes compromise between these views - climate change destabilizing ecosystems, for example, which may have made human activity more dramatic in its effects - the fact that we’re ramping up to a sixth mass extinction crisis has often been used as part of an ecological morality tale in which humanity has been a blight on the world’s biodiversity from the end of the Ice Age until today.

This isn’t isolated or obscure academic debate. Our opinions about what killed Ice Age megafauna have played a key part in discussions over Pleistocene rewilding - bringing Asian elephants to North America to stand in for mammoths, for example - and ballyhooed reports of cloning or other forms of de-extinction. If humans were responsible for the disappearance of these animals and the ecological connections those species fostered, the argument goes, then we have a responsibility to bring them back. And perhaps that is so. But it’s also worth investigating how even the idea of overkill - whether it’s a good fit for the pattern or not - has influenced scientific realms which in turn suggest policy and ethical obligations to global ecology. That’s exactly what archaeologists Lisa Nagaoka, Torben Rick, and Steve Wolverton consider in an analysis of “ The overkill model and its impact on environmental research .”

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The question of what happened to our Ice Age megafauna does not fall under the purview of a single discipline. It’s a mystery at the intersection of various sciences, as disparate as archaeology, anthropology, ecology, zoology, paleontology, climatology, botany, and more. And given that facts do not stand alone but are interpreted through theory, it’s little wonder that practitioners of varying sciences will have different views. So, to track how various sciences have responded to the idea of Pleistocene overkill, Nagaoka and colleagues followed the citation record of Paul Martin - up until his death in 2010, the principal promoter of the idea that humans drove Pleistocene megafauna to extinction.

Nagaoka and coauthors primarily focused on two fields of study that, despite their connection, often have little communication and collaboration with each other - archaeology and ecology. What the researchers found was that the two disciplines have very different views of what happened as the Ice Age closed, which in turn colors the way the extinction of the mammoth and mastodon is used as a rhetorical tool in modern arguments about extinction. This is important because, despite its seemingly widespread acceptance, the evidence for hungry hungry humans depleting large Pleistocene animals is not only contentious, but often lacking. “The reality is,” Nagaoka and coauthors write, “that the argument uses a series of untested assertions about human-environment interactions” and direct evidence of definitive hunting by Pleistocene humans is very rare despite a rich Ice Age fossil record. 

So what does the comparison of the sciences show? Within archaeology, the role humans played in the end Pleistocene extinctions is an open question. Drawing from a survey of 91 archaeologists, as well as the citation search, Nagaoka and colleagues found that the majority of archaeologists sampled did not think that humans were the only, or even the primary, cause for the extinctions. Climate change was mentioned most often, with humans providing additional or secondary pressure in the form of hunting or altering the landscape. Among most archaeologists, who focus on the habits of people through time, the blame for the loss of Megatherium  and Smilodon does not rest on humans alone. And even though there are problems with the climate change hypothesis and others, research written and cited by archaeologists is much more likely to recognize that there is a debate to be had and that investigations are ongoing.

The picture is very different in ecology, and has gotten far more media play through books like The Sixth Extinction and highly-publicized events concerning de-extinction. The citation record is some help here. While archaeologists are more likely to cite Paul Martin’s earlier works of overkill - which focused primarily on North America and human movements through the continent - ecologists are more likely to cite his later works in which the model is global. On top of that, the researchers found, ecological papers were more likely to use Martin’s hypothetical scenario as  evidence for  the argument that humans wiped out megafauna rather than as a reference to the idea.

What’s wrong with this paper trail is that many of Martin’s untested assumptions - that megafauna were naive to invading humans, and that human dispersal across the world explains the distribution of modern megafauna - are often stated as facts. This isn’t helped by an interdisciplinary communication breakdown, as Nagaoka and colleagues call it out. Some of this is as simple as where experts publish. Critics of the overkill hypothesis, or those who see humans as one of several pressures leading to Pleistocene extinction, often publish in archaeological journals or those concerned with the latter part of the Cenozoic. Papers supporting the overkill hypothesis, however, are often published in broader scientific journals and have gotten plenty of additional publicity as being citations for debates over Pleistocene rewilding and de-extinction, thus more likely to be taken as a sign of consensus by ecologists when no such consensus exists.

One would hope that the process of science would help correct this. Archaeologists or paleobiologists could publish their investigations and critiques in ecological journals, Nagaoka and colleagues suggest, but the peer review process is uneven such that ecologists are more likely to listen to other ecologists - who are already predisposed to the overkill idea - than experts from outside fields. This is strange, Nagaoka and coauthors write, given that archaeology is the science concerned with people through time and what they were doing. Wouldn’t that insight and information be useful in determining whether or not humans were responsible for driving sabercats and ground sloths into extinction? For example, that despite a very rich Pleistocene fossil record there are only a handful of associations between humans and megafauna that can be taken as evidence of hunting? Some of the strongest proponents of overkill aren’t actually reading or citing the literature bearing directly on the subject.

This situation is hard to change, particularly because we see the awful influence of human activity on biodiversity today. That humans started this pattern in the Ice Age thus becomes a political position, and to question is sometimes treated as if the critic were denying the modern extinction crisis. Still, the fact of the matter is that overkill is an untested, unverified hypothesis that has nevertheless gained sway, with guilt over humanity's appetite for destruction driving the case for ecological penance. Whether or not humans actually sparked a global extinction crisis in the Pleistocene has become almost irrelevant in conservation communication because of the argument’s rhetorical value. “When overkill is used as a cautionary tale and a means to rally support for environmentalism, it portrays humans as a destructive species,” Nagaoka and colleagues write, apparently not through what we choose to do but because it’s inherent to our nature. It’s a dark, deterministic view of our species. More than that, this view ignores cultural diversity across time and space, treating humans as homogeneously voracious and destructive, an insult justified through flimsy correlation.

Even if overkill were eventually found to be a real and significant global phenomenon during the Pleistocene, Nagaoka and colleagues write, there is more to the story than a cautionary tale or ecological guilt trip. One of the options, they write, is that overkill offers information about different ways human cultures have interacted with the environment - in what times and places were people more destructive as opposed to more concerned with sustainability? - and that such understanding helps us better appreciate how we are intertwined with nature instead of separating ourselves as a destructive force outside of it. It’s not simply that Pleistocene overkill is not supported by evidence. It’s that the concept divides us from nature and makes us villains, perhaps irredeemable ones. We can do better.

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Paul S. Martin (1928–2010): Luminary, Natural Historian, and Innovator

* E-mail: [email protected] (CJD); [email protected] (HWG)

Affiliations Advanced Conservation Strategies, Midway, Utah, United States of America, Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, United States of America

Affiliation Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, United States of America

  • C. Josh Donlan, 
  • Harry W. Greene

PLOS

Published: February 8, 2011

  • https://doi.org/10.1371/journal.pbio.1001016
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Citation: Donlan CJ, Greene HW (2011) Paul S. Martin (1928–2010): Luminary, Natural Historian, and Innovator. PLoS Biol 9(2): e1001016. https://doi.org/10.1371/journal.pbio.1001016

Copyright: © 2011 Donlan, Greene. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Competing interests: The authors have declared that no competing interests exist.

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https://doi.org/10.1371/journal.pbio.1001016.g001

To behold the Grand Canyon without thoughts of its ancient condors, sloths, and goats is to be half blind —Paul S. Martin 1992

Paul S. Martin, a monumental figure in paleontology and ecology, died on September 13, 2010, at the age of 82. He wanted others to know, conveyed in a message from his wife, that when the time came, he would “die” and then be “dead.” He would not pass on, go gently, meet his maker, or go over to the other side. Paul combined an intense love for life, adventure, and natural history with innovative thinking, to an extent that is increasingly rare in today’s age of specialization. His impact on science was deep and transformative, cutting across many disciplines including ecology, paleontology, anthropology, and biodiversity conservation.

Paul is best known for his work on the loss of America’s charismatic megafauna, from mastodons and mammoths to saber-toothed cats and sloths. He first published on the subject in 1958, and later authored seminal works on the subject [1] – [5] . Early in his career, Paul framed what has become one of the most enduring scientific whodunits of all time: what drove Late Pleistocene extinction—and what role did humans play? In 1966, writing about his “overkill hypothesis,” in which he indicted early humans for Pleistocene extinctions, Paul commented that it “is likely to continue to provoke serious and perhaps unanswerable objections” [5] . That turned out to be an understatement: the role of humans in Pleistocene extinction has been debated, often heatedly and almost continuously, for the last 50 years. While Paul relentlessly promoted the idea humans played the significant role in the demise of the Pleistocene megafauna, he embraced even his most critical dissenters—indeed, he invited criticism. His colleague Karl Flessa commented, “Paul really wanted to see things the way his opponents saw them, in order to understand even more about his favorite topic, Pleistocene extinctions." Paul’s 50-year journey with Pleistocene extinction is documented in his memoir Twilight of the Mammoths [6] .

But Paul’s impact on science goes far beyond Pleistocene extinction. Paul Martin was a true Renaissance man—leveraging natural history to innovate new perspectives time and time again. In his monograph The Last 10,000 Years: A Fossil Pollen Record of the American Southwest , Paul took palynology (the study of pollen grains and other spores) to a far from obvious place: the dry lakes of Arizona. Edward Deevey, Paul’s post-doctoral supervisor, declared, “His pioneering work is exciting, not only for what it tells, but even more for what it promises: history, free of peat-bound preconceptions, in a land of little mud” [7] .

Left wanting to unravel more tales of the ecological past, Paul then turned to packrat middens, helping to develop a new subdiscipline of paleontology that is among the most important tools of historical ecology [8] . His classic publication, Neotropical Anachronisms with Dan Janzen, changed the way ecologists view species interactions by elucidating the role of extinct vertebrates on contemporary ecology and life history traits [9] . Paul had a life-long love affair with Mexico and the country’s diverse flora and fauna. He spearheaded the Rio Mayo Project, updating Howard Scott Gentry’s Rio Mayo Plants: A Study of the Flora and Vegetation of the Valley of the Rio Mayo, Sonora [10] , a project that involved 20+ years of field trips and the collection of over 15,000 specimens. From remote mangroves to ridge top conifer forests in the Sierra Madre Occidental, Paul traveled across one of Mexico’s most precious and inaccessible biomes—by bush plane, vehicle, burro, kayak, and foot. This was a remarkable feat for any biologist, even more extraordinary for a man that suffered from polio early in life and later relied on crutches.

Born in Allentown, Pennsylvania, Paul earned his bachelor’s degree in zoology from Cornell University. From his teens through his eighth decade, Paul was all about fieldwork. By the time he graduated from Cornell in 1951, he had published papers on the natural history of bicolored hawks and black robins in western Mexico. Paul told us of grand times shooting birds for museum specimens with a shotgun out of the top floor of Cornell’s Fernow Hall, and of long road trips to the cloud forests of Tamaulipas, Mexico that involved an old jeep, binoculars, bad roads, and lots of flat tires. In those days, the 2000+-mile trip to Tamaulipas was likely worth a degree in itself. He went on to earn a Master’s and doctorate from the University of Michigan, studying the biogeography of amphibians and reptiles in those forests, an exercise in historical ecology that heavily influenced his life. Paul spent four years at Yale University and the University of Montreal as a post-doctoral researcher before moving to the Geochronology Laboratory at the University of Arizona in 1957. He would enjoy the rest of his career in Tucson, much of it as a professor of geosciences at the University’s Desert Laboratory, perched on Tumamoc Hill.

Aside from his contributions to paleontology and ecology, Paul also influenced how we think about biodiversity conservation. For decades, almost single-handedly, he pondered the impact of megafaunal loss during the end of the Pleistocene on contemporary ecosystems: “Perhaps the long-lauded home where buffalo roam,” he wrote in 1969, “is also the land where camel and eland should play” [11] . His insights on the role of ecological history in biodiversity conservation culminated with the recent proposal of Pleistocene Rewilding, a call for science-based restoration of missing ecological functions and evolutionary potential of lost megafauna using extant conspecifics and related taxa [12] , [13] . Paul was keenly aware that nativeness, place, and history are central to the science, strategies, and aesthetics of biodiversity conservation. His work was central in elucidating the all-to-common post-Columbian bias that blinds a paleoecological view of biodiversity and ecosystems.

Another influential ecologist, Larry Slobodkin, died a year and a day before Paul Martin. Late in life Slobodkin was asked, “What do ecologists do?” He replied that it consists of choosing one of three paths and doing it well: the first was to become an expert on some group of organisms that excites you; the second was to master the most cutting-edge techniques; and the third, the most perilous path, was to “strenuously avoid doing what everyone else is doing and search for new ideas and new tests for old ideas” [14] . Paul Martin remarkably did all three brilliantly. The mystery of the Pleistocene extinction fascinated the naturalists of the 18th and 19th century, including Darwin, Wallace, Lyell, Owen, and Cuvier. Paul contributed more to solving this grand enigma than anyone living or dead. He was a pioneer in interdisciplinary research long before it was popular on academic campuses, and he sought new perspectives in natural history, often seizing on novel technologies to solve problems. But, Paul was more than that: he loved life and loved people. While our interactions with Paul were largely restricted to his later years, his influence on us—like many—was transformative. He even loved chiggers, harbingers of summer rain. One morning over coffee in the Arizona hill country, Paul declared with a big grin, “May the gods smile, the chiggers are rampant in the green hills of Sonoita these days.”

  • 1. Martin P. S (1958) Pleistocene ecology and biogeography of North America. In: Hubbs C. L, editor. Zoogeography. Washington (D.C.): American Association for the Advancement of Science. pp. 375–420.
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  • 3. Martin P. S, Klein R. G (1984) Quaternary extinctions : a prehistoric revolution. x. Tucson: University of Arizona Press.. 892p p.
  • 4. Martin P. S, Wright H. E, editors. (1967) Pleistocene extinctions: the search for a cause. New Haven: Yale University Press. 440p p.
  • 6. Martin P. S (2005) Twilight of the mammoths: ice age extinctions and rewilding America. Berkeley: University of California. 274p p.
  • 7. Martin P. S (1963) The last 10,000 years: a fossil pollen record of the American Southwest. Tucson: University of Arizona Press. 87p p.
  • 10. Martin P. S, Yetman D. A, Fishbein M. E, Jenkins P. D, Devender T. R. V (1998) Gentry's Rio Mayo plants: the tropical deciduous forest and environs of northwest Mexico. Tucson: University of Arizona Press.

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  • Published: 12 April 2021

Late Pleistocene South American megafaunal extinctions associated with rise of Fishtail points and human population

  • Luciano Prates   ORCID: orcid.org/0000-0001-6858-3837 1 , 2   na1 &
  • S. Ivan Perez   ORCID: orcid.org/0000-0002-6543-5545 1 , 3   na1  

Nature Communications volume  12 , Article number:  2175 ( 2021 ) Cite this article

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  • Archaeology
  • Macroecology
  • Palaeoecology
  • Population dynamics

In the 1970s, Paul Martin proposed that big game hunters armed with fluted projectile points colonized the Americas and drove the extinction of megafauna. Around fifty years later, the central role of humans in the extinctions is still strongly debated in North American archaeology, but little considered in South America. Here we analyze the temporal dynamic and spatial distribution of South American megafauna and fluted (Fishtail) projectile points to evaluate the role of humans in Pleistocene extinctions. We observe a strong relationship between the temporal density and spatial distribution of megafaunal species stratigraphically associated with humans and Fishtail projectile points, as well as with the fluctuations in human demography. On this basis we propose that the direct effect of human predation was the main factor driving the megafaunal decline, with other secondary, but necessary, co-occurring factors for the collapse of the megafaunal community.

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Introduction

A great number of megafaunal species became extinct all over the planet—except for Africa—during the end of Pleistocene as their ecological niches experienced significant changes. The impact of diverse factors that may have potentially triggered these extinctions is debated, with the primary causes varying according to the continent under consideration and the scientific discipline involved in the research. Some argue that only human hunting mattered, others argue for the role of climate change, hyperdisease, habitat modification, or even extraterrestrial impact 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . In the Americas, most extinctions occurred towards the end of the late Pleistocene, after the Last Glacial Maximum and near the time of the first widespread dispersal of humans from northeast Asia 11 . Because the loss of biodiversity in the Americas was severe and happened nearly synchronous with significant climatic changes and the initial influx of humans, the debate regarding the main factor(s) driving extinctions has been more controversial and persistent than for any other continent 3 , 9 , 12 .

In recent years, it has become increasingly clear that extinctions were not homogeneous in the Americas 2 , 6 , 8 , and the debate around this process was quite different in North America and South America. In North America, 70% (37 genera) of mammals with an average body mass over 44 kg (megafauna sensu Martin 13 or large mammals sensu Cione et al. 7 ) disappeared mainly between 13 and 12 k cal BP 2 . This period corresponds with an abrupt cooling episode, the Younger Dryas, and the dispersal of Clovis culture, the earliest recognized widespread archeological techno-complex in the continent 11 , 14 . Clovis is characterized by a unique technology of fluted projectile points, strongly associated with the hunting of large mammals. In the span of a single millennium, Clovis spread rapidly over most of North America. Based on Clovis and Fishtail projectile point evidence, Paul Martin 13 formulated the challenging hypothesis of the “Pleistocene overkill”, which postulated that the appearance of humans in the Americas was principally responsible for megafaunal collapse. At present, although this model has been strongly criticized 9 , and the combined effect of climatic change and human action has been the most widely argued cause of extinctions, many scholars maintain that humans could have been the principal or necessary driver of the extinctions 3 , 15 .

The situation is somewhat different in South America. There, late Pleistocene extinctions were more acute than in North America 1 , 6 , 7 , with the loss of 82% (over 40 genera, sensu Cione et al. 7 ) of megafaunal species, but less importance has been attributed by archeologist to humans as driving the process. The end of the Antarctic Cold Reversal, a South American post glacial cooling period earlier and less marked than the North American Younger Dryas 16 , is synchronous with the beginning of the spread of the South American Fell or Fishtail projectile points (henceforth, referred to as FPP), which occurred during a time span that is similar to the Clovis period but followed a few centuries later 17 , 18 , 19 . FPP, usually but not always fluted, present a fainter and more irregular spatial distribution than Clovis in North America 17 , 20 , 21 , 22 and, although they are usually claimed to be linked with local megafauna, South American sites with remains of large mammals and clear evidence of direct exploitation by humans is elusive and restricted to a few species 6 , 8 , 23 . Based on such limited evidence, and assuming humans reached South America in “pre-Clovis” times 24 , South American archeologists (1) maintain that FPP played an important but non-central role during the colonization process—unlike Clovis in North America—, and (2) do not place humans squarely within the debate on the Pleistocene megafaunal extinction. Nevertheless, because climatic changes do not fully explain extinctions by themselves 12 , 25 , a number of paleoecological and paleontological studies have increasingly credited humans as having a major role in the extinction process 1 , 7 . In this sense, it is remarkable that the megafaunal extinctions occurred just after the spread of FPP 24 and almost simultaneously with a significant slowdown of the human population growth rate. Although the new perspectives are seriously calling for the abandonment of the strict dichotomy between climate and humans as premier extinction drivers, the archeological and paleontological data have not been fully integrated using a quantitative approach. An updated study integrating both lines of evidence is necessary to rigorously evaluate the relationship between megafaunal and human dynamics, as well as to better define the actual role of humans in South America’s late Pleistocene extinctions.

In this paper, we explore the temporal changes in the density of megafaunal species, including large and mega quaternary mammals, and FPP, as well as the variation in the potential distribution in geographical space. Although other projectile heads could have been used for hunting megafauna, FPP are the most abundant and widely distributed in early South America 17 , 18 , 19 , and seem to have been a specialized weapon for that purpose 26 . So, we consider this artifact as a good empirical proxy for exploring the interaction between extinct large mammals and humans. On this basis, we evaluate with updated evidence and, from a continuous temporal and spatial perspective, the direct impact of human agency on the late Pleistocene extinctions in South America. If humans were the main drivers of the extinctions, we expect to see an inverse association between human and megafaunal population densities in time and a positive association in space.

Here, we investigate the temporal changes in density using Sum Probability Distribution of radiocarbon dates of archeological and paleontological samples (SCPD method 27 , 28 ), whereas we explore the differences in potential for distribution in geographical space by employing Species Distribution Models (SDM) 29 , 30 and Stack Species Distribution Models (SSDM) 31 . For the last analyses, we explore the distribution of the FPP and ten species that present records of physical (stratigraphic) association with humans in the archeological record ( Hippidion saldiasi , Milodon darwini, Lama gracilis, Equus neogeus , Doedicurus clavicaudatus , Megatherium americanum , Glossotherium robustum , Notiomastodon platensis, Notiomastodon waringi , and Cuvieronius hyodon ) during the end of the late Pleistocene. Except for G. robustum and D. clavicaudatus , evidence of having been subject to human consumption or processing 6 , 23 is associated with all species. We compare megafaunal dynamics with the changes in human density and distribution using a screened database of archeological radiocarbon dates 24 . If megafauna were a central resource in human economy, we not only expect that humans impact megafaunal population dynamics; we also expect that changes in megafaunal density and distribution impact the human population. We also discuss changes in the archeological record in the framework of environmental and climate change inferred by previous work in South America 18 , 32 , 33 . We explore changes in density for the whole South America 34 , but we focus our analyses on three relatively independent areas where evidence of coexistence between humans and megafauna is strongest: Pampa (including the Argentinean Pampas, South Brazil, and Uruguay), Southern Patagonia, and Andes (Supplementary Fig.  1 ). On this basis, we re-examine the main hypotheses regarding the late Pleistocene extinctions in South America and the importance of direct human impact on this process.

Summed probabilities distributions of radiocarbon dates of megafauna, FPP, and all archeological sites

The temporal changes in the density of megafauna and FPP were explored using the Sum Probability Distribution method (SCPD method) 27 , 28 . These SCPDs were compared against the curve of human density in South America based on dates of archeological sites compiled by Prates et al. 24 . Our results (Fig.  1 ), show that the radiocarbon signal of large mammals around 18 k cal BP is extremely low in South America, but clearly increases from 17,5 k cal BP, and grows rapidly and steadily between 15,3 and 12,9 k cal BP. After 12,9 k cal BP, the SCPD curve shows a dramatic decline until 11,6 k cal BP. From this date onwards, only a few genera of extinct large mammals have been recorded 35 and most of the alleged early Holocene dates have recently been called into question 36 (Supplementary Fig.  2 ). Fishtail projectile point technology appeared in South America at ca. 13 k cal BP and shows a rapid amplification of density until reaching the distribution peak between 12,4 and 12,2 k cal BP (Fig.  1 ). From this time onward, a deep decline continues until the technology virtually disappears at ca. 10,9 k cal BP. Figure  1 also shows that the archeological—human—signal is low from the earliest appearance, ca. 15 k cal BP, to ca. 13 k cal BP, when it clearly increases. Around 12,5 k cal BP, the SPD curve for all archeological sites shows a peak followed by a slight decline that extends to 11,6 k cal BP before rising again (Fig.  1 ).

figure 1

The temporal change in the density of large mammals (or megafauna) (light green shading), FPP (light-blue shading), and archeological sites (beige shading) reflected in the SCPD curves for all of South America. X axis represents Calibrated years BP and Y axis the standardized summed probability.

If we consider South America’s main geographical regions separately (Supplementary Fig.  1 and Fig.  2 ), we observe significantly similar changes in density of megafauna (Fig.  2a ) and FPP (Fig.  2b ) when compared with the permutation test proposed by Crema et al. 28 . The density of megafaunal species displays differences among regions only in the earliest and latest dates, but a general agreement in the main density peaks is observed between ca. 13,5 and 12,5 k cal BP. The FPP’s densities are almost identical among the regions (Fig.  2b ). The changes in density for all archeological dates display some correlations with the megafaunal densities, with an apparent impact of the large mammal’s density decline stronger in Patagonia than in the other regions, and a lower impact in Andes (Fig.  2c ). Figure  2c also shows that the density of human signal after 11,5 k cal BP was significantly lower in Patagonia than in the other regions in relation to what can be observed in the previous millennium (12,5–11,5 k cal BP), with the highest values of density observed in the Andes.

figure 2

The temporal change in the density of large mammals ( a ), Fishtail projectile points ( b ), and all archeological sites ( c ) described using SCPD curves. Different regions are indicated in shading colors: Andes (light blues), Pampa (light red), Patagonia (light green). X axis represents Calibrated years BP and Y axis the standardized summed probability.

Spatial distribution of megafauna, FPP, and all archeological sites

The potential spatial distribution of megafaunal species and FPP was explored using a Species Distribution Modeling approach as implemented in MaxEnt 37 , considering species and FFP occurrences between 18 k and 9 k cal BP, and the bioclimatic variables available in PaleoClim 38 , 39 . Figures  3 – 5 show the estimated distribution of the extinct megafaunal species with strong evidence of direct stratigraphic association with humans in archeological sites. The predictive capacity of the distribution models was high for all species, FPP, and archeological sites, with AUC values above 0.9 ( M. americanum : 0.99; G. robustum : 0.98; D. clavicaudatus : 0.97; E. neogeus : 0.98; L. gracilis : 0.99; M. darwinii : 0.99; H. saldiasi : 0.99; N. platensis : 0.99; N. waringi : 0.97; C. hyodon : 0.91; FPP: 0.92; all archeological sites: 0.92). The modeled potential distribution maps for almost all megafaunal species display high values for Pampa and Patagonia. Based on the values for the species, we can group them into three patterns: M. darwini , H. saldiasi , and L. gracilis with a predominant distribution in Patagonia and L. gracilis also in Pampa (Fig.  3 ); E. neogeus , D. clavicaudatus , M. americanum , and G. robustum , with a predominant distribution in Pampa (Fig.  4 ); and N. waringi and C. hyodon with a wider distribution than the other species, reaching most of Andes and northern South America, whereas N. platensis is distributed in Pampa and central Andes (Fig.  5 ).

figure 3

Potential distribution maps of the Mylodon darwinii , Hippidion saldiasi, and Lama gracilis during the late Pleistocene–early Holocene (18 k–9 k cal BP). Values of potential distribution vary from 0 (light beige) to 1 (dark green). Empty dots represent species occurrence sites. The taxonomy of Lama gracilis is under discussion. It has been mainly considered a vicariant of Vicugna vicugna in the low plains, but some morphological and molecular studies suggest that L. gracilis and V. vicugna may be the same species 65 , 66 , 67 .

figure 4

Potential distribution maps of Equus neogeus , Doedicurus clavicaudatus , Megatherium americanum , and Glossotherium robustum during the late Pleistocene–early Holocene (18 k–9 k cal BP). Values of potential distribution vary from 0 (light beige) to 1 (dark green). Empty dots represent species occurrence sites.

figure 5

Potential distribution maps of Notiomastodon platensis , Notiomastodon waringi , and Cuvieronius hyodon during the late Pleistocene–early Holocene (18 k–9 k cal BP). Values of potential distribution vary from 0 (light beige) to 1 (dark green). Empty dots represent species occurrence sites. Mothé et al. 68 has proposed that N. platensis and N. waringi are the same species, but we follow Prado and Alberdi 69 who suggest they represent two different species or two geographic variants.

Considering the modeled potential distributions of the megafauna, the local richness for all species together was estimated employing Stack Species Distribution Modeling 31 as implemented in SSDM. As Fig.  6a shows, higher values of species richness (>4) were estimated for Pampa, with intermediate values (2–4) in Southern Patagonia and Northeast Brazil, as well as lower values in Northern Patagonia, South-Central Andes, and North-Central Andes. The potential distribution of FPP also shows the highest values (between 1 and 0.6) in Pampa, with intermediate values in Patagonia, South, and Central Andes (Fig.  6b ). The distribution of all archeological sites shows a similar pattern with FPP and species richness, but the values of potential distribution are lower in Pampa (Fig.  6c ). The spatial density estimated from a kernel method in QGIS 3.14 40 displays a very similar pattern with relation to FPP and all archeological sites distribution (Supplementary Figs.  3 and 4 ). Particularly, in the 13–11 k cal BP period, the higher date density is observed for Pampa, Southern Patagonia, and South-Central Andes, where we describe the maximum values for species richness (Fig.  6 and Supplementary Fig.  4 ).

figure 6

Maps describing the local species richness of extinct large mammals (18 k–9 k cal BP) ( a ), the potential for distribution of Fishtail projectile points (13 k–10,9 k cal BP) ( b ), and all archeological sites (13 k–11 k cal BP) ( c ). Local species richness varies from 1 (light green) to >5 (dark green), and the potential distribution of both FPP and all archeological sites vary from 0 (light blue) to 1 (dark blue).

Niche overlapping analysis: megafauna vs. FPP and all archeological sites

We estimated niche overlapping using the I similarity index 41 , which varies between 0 (no overlap) and 1 (identical distributions). The similarity index of all archeological sites and FPP with megafaunal species (Supplementary Table  1 ), as well as among them, is plotted in Fig.  7 . Moreover, a non-metric multidimensional scaling analysis (nm-MDS) was performed to summarize the similarity matrix. The boxplot of I similarity index (Fig.  7a ) shows on average larger similarity values between FPP and each large mammal species than observed among all species. M. americanum , G. robustum , D. clavicaudatus, N. platensis, L. gracilis, and E. neogeus , with high values of potential distribution in Pampa, display the highest/similarity values with the FPP distribution (Supplementary Table  1 ). The nm-MDS plot shows that the FPP distribution is intermediate among all species, being closer to the species with high values of potential distribution in the Pampas (Fig.  7b ). The nm-MDS scores also show a clear separation between three clusters of species grouped according to their main distribution areas: Patagonia ( M. darwinii and H. saldiasi ), Pampa ( M. americanum , G. robustum, D. clavicaudatus, and N. platensis ), and northern South America ( N. waringi and C. hyodon ; Figs.  6 and 7b ). E. neogeus and L. gracilis are close to the Pampa and Patagonia, respectively. All archeological sites are closer to the FPP position, but they are closer overall to the N. waringi and C. hyodon species distributions (Figs.  6 and 7b ).

figure 7

Boxplot ( a ) of the values for I similarity index describing niche overlap between FPP and extinct large mammals ( n  = 10), as well as all extinct large mammal species ( n  = 45). This graph shows the minimum and maximum values (whiskers), the first and third quartiles (bounds of box), and the median (centerline). nm-MDS ordination ( b ) describing the similarity in niche among all archeological sites, FPP, and all extinct large mammal species. The minimum spanning tree describing the I similarity matrix is shown.

The SCPD curves of the density of megafauna through time (Figs.  1 , 2 and Supplementary Fig.  2 ) reflect a significant increase of the fossil record all over South America shortly after the Late Glacial Maximum (ca. 17,5 k cal BP). Although this pattern is probably linked to the expansion of large herbivorous mammals and their predators during the end of the late Pleistocene 42 , it could be also influenced by taphonomic biases or the loss of fossil specimens over time. If the expansion was real, it could be related to the favorable environmental condition for herbivores after the end of the Last Glacial Maximum 42 , 43 , and especially in the Pampas and Patagonia regions. According to our results, this expansion continued ~4,5 k years until a sudden and deep decline of the radiocarbon dated fossil record at ca. 12,9 k cal BP. Interestingly, when we compare the FPP and megafaunal SCPD curves, we observe that the sudden decline of the megafauna’s density started right after the appearance of FPP technology in South America, ca. 13 k cal BP. Moreover, as shown by Figs.  1 and 2 , from that moment an explosive increase of the FPP occurs closely followed by a steep decline of the megafauna. The frequency of FPP increases for ~600 years (until ca. 12,4 k cal BP) before declining rapidly, similar to what is observed for the megafaunal curve. Fishtail projectile points and megafauna virtually disappear together from South America ~10,9 k cal BP, supporting the hypothesis that FPP technology was directly linked to megafauna extinction.

The changes in the density of FPP over time not only seem to be related to the density of megafauna, but also to important demographic changes in South American human populations 24 . As shown in Fig.  1 , during the initial peopling of the continent, between 15 and 13 k cal BP, the archeological signal (and probably human population density 24 ) stayed extremely low, until an irruptive growth dynamic occurred just after the FPP spread over southern South America. The tight fit between the behavior of both late Pleistocene phenomena, FPP and human population expansion (Fig.  1 ), may indicate that rapid and successful dispersal of FPP technology drove the high rate of population growth of the earliest hunter-gatherers. Or, from a more conservative perspective, that users of FPP technology were the first colonizers of South America 14 . Either way, the explosive population growth of the human population stopped suddenly at ca. 12,5 k cal BP, just before FPP reached the peak of the distribution curve and started to decrease, a few centuries after the initial decline of megafauna. These temporal patterns resemble what is observed in North America for Clovis technology 15 although FPP technology is somewhat younger and overlaps with the end of Clovis, as recently observed by Waters et al. 19 . However, whether FPP technology is a southern cultural expression of Clovis or a result of independent innovation is an open question. What is clear in our temporal density results is that human demography in South America during the end of the late Pleistocene was related to the changes in the density of several megafaunal species and to the expansion of FPP technology.

The potential distribution models generated for the different species of megafauna and for the FPP (Figs.  3 – 6 ) show large spatial overlapping in South America, except for some gomphotheres ( C. hydon and N. waringi ) whose distribution does not coincide with FPP. This may suggest these species were not the principal prey of FPP hunters. The largest values of potential overlap are in the Pampas (including Argentina, Uruguay, and Southeast Brazil) and Southern Patagonia, but with smaller values in the latter. The Andes appear as a marginal area for both FPP and the megafauna species analyzed here. This correspondence is clearly observed in Fig.  6 , which shows that the largest potential distribution values for FPP (Fig.  6b and Supplementary Fig.  3 ) coincides with the areas with the largest local richness of megafauna species (Fig.  6a ), as well as the areas where high density of human occupation is observed (Fig.  6c and Supplementary Fig.  4 ). However, the human populations are distributed more widely than FPP technology in South America. Although distributional overlap is only an indirect estimation of trophic interaction 44 , this measurement strongly supports the importance of FPP technology for hunting species of large mammals during the late Pleistocene.

The areas with higher values of potential for FPP distribution and species richness are characterized by the predominance of open environments: grassland steppes in the Pampas and grassland cold steppes in Patagonia, to which the large mammal species analyzed here were better adapted and which they occupied with greater concentration at the end of the Pleistocene 7 , 42 , 43 . Moreover, the results of the niche overlap analysis show that the distribution of FPP is focused in a central geographic sector with respect to the spatial distributions of the large mammal taxa. They show greater proximity to the Pampean species as well as to Patagonian ones. This implies that the FPP, in addition to expanding rapidly and with a distribution similar to that observed for megafauna, did so mainly in the open environments. This reinforces the hypothesis that FPP were designed and used for hunting megafauna.

On this basis, we suggest that: (1) if South America were colonized ca. 15 k cal BP, when the abundance of megafauna was relatively high, their populations were unaffected because humans must have been generalist hunter-gatherers; (2) when megafauna was at maximum density in the open grassland steppe environments, mainly Pampa and Patagonia, the hunter-gatherers started to prey on them using FPP; and (3) a little later, ca. 12,9 k BP, the growth trend in number and density of megafaunal species stopped abruptly and began to decline. This drop in the density of megafauna had effects a few centuries later, ca. 12,4 k BP, in the adaptive pattern of the hunter-gatherers who used the FPP technology, which diminished rapidly until it completely disappeared when the megafauna became almost extinct. Other types of projectile heads (e.g., projectile points made of bone and El Jobo projectile points) 45 , 46 , which were probably used at the same time in preying on some species of gomphotheres, also disappeared after megafaunal extinction. It is remarkable that when megafauna and FPP disappear, an abrupt deceleration and subsequent decline occurred in the growth of the human population throughout the continent. Although this process is observed in all regions of South America, in Patagonia it seems to have been more drastic, with a stronger and longer decline over time (Fig.  2 ). This process could be related to the occurrence of a population overshoot, in which humans exceeded carrying capacity in the late Pleistocene ecological community. This could have generated a population collapse of the megafaunal prey, with the consequent drop in the carrying capacity of the environment for big-game hunters and the subsequent demographic decline in the populations of the main predator species in the trophic network, the humans.

Several important changes in climatic and environmental conditions occurred at about the same time as the extinction of large mammals and the widespread dispersal of hunter-gatherers carrying FPP technology in South America. An increase in temperatures began after the Last Glacial Maximum, ca. 18–17 k cal BP 16 , 32 , 47 , and ended with the Antarctic Cold Reversal (ACR; Fig.  8 ; ca. 14,7–13 k cal BP) 16 , 47 . This post glacial cooling period occurred when the megafauna reached their maximum growth in density (Fig.  8 ) and coincided with the North American Bølling–Allerød warm stage 48 . After the ACR, a warming period began at ca. 13 k BP, which coincided with the Younger Dryas 16 cooling period in North America. Terrestrial paleovegetation proxies do not suggest an abrupt environmental change in southern South America at the end of the ACR, but cold/cool and wet conditions persisted until 11,8 BP 49 , 50 . Open grasslands, the preferred habitat of the megafauna 7 , 42 , 43 , seem to have retracted and reached their current distribution after 12,4–11,5 k cal BP in different areas of Patagonia 7 , 32 , 33 , 42 , 51 , and probably later in the Pampas 52 . In addition, Pampa and Patagonia suffered a significant reduction of territory due to the rise in sea level 42 , 43 .

figure 8

Local species richness map of extinct large mammals and the calculated potential distribution of FPP, together with the temporal change in the density of both variables for all of South America and the climatic changes following Pedro et al. 16 . Striped areas in the map represent the potential distribution of FPP. Local species richness of extinct large mammals varies on the map from 1 (light green) to >5 (dark green). Relative climatic changes (temporal temperature curves) are indicated in beige (0–30° S) and gray (>40° S), and the ACR period in dark beige. The temporal change in the density of large mammals is represented by light green shading area (lower and upper bounds of the simulated envelope) and dashed green line (observed summed probability). The temporal change in the density of FPP is represented by light-blue shading area (lower and upper bounds of the simulated envelope) and blue line (observed summed probability). In the inset graph X axis represents Calibrated years BP and Y axis the standardized summed probability.

All these climate and environmental changes could have played the central role in Pleistocene extinctions together with humans, but there are several aspects to consider. First, the gradual effects of the end of the ACR climatic event in Pampa and Patagonia (Fig.  8 ) 16 , 32 , 49 , 50 do not seem congruent with a sudden decline of megafauna ca. 12,9 k cal BP (Fig.  8 ). Second, the shrinking of the grasslands steppes—the most favorable environmental conditions for large mammals—from southern South America occurred between 12,4 and 11,5 k cal BP 32 , 33 , 51 , 1400–500 years after the megafaunal decline (12,9 k cal BP) in South Patagonia; in the Pampas the change seems to have occurred even later. Finally, whereas in North America the initial decline of the megafauna at 13 k cal BP is contemporaneous with the beginning of the Younger Dryas post glacial cooling period at 13–11,7 k cal BP, in South America the initial decline of megafauna (12,9 k cal BP) is contemporaneous with the beginning of the warming period starting after the ACR, at ca. 13 k cal BP. Although this change could have affected the megafauna by exerting under high ecological stress, the animals did not become extinct until humans using Fishtail projectile points appeared, suggesting again that human agency could have been a determining factor driving their extinction.

Our results support the hypothesis that early hunter-gatherers were an important driver of the megafauna’s extinctions in South America. Nevertheless, there are some still unanswered questions: (1) if human arrived in South America ca. 15.5 cal BP 24 , why did they not have an impact on the megafaunal expansion until 12,9 k cal BP, despite having coexisted with them for ~2500 years; (2) why is direct archeological evidence of megafaunal exploitation so rare 6 , 9 , 23 , 53 ; and (3) why would so many megafaunal species become extinct during the end of the Pleistocene when they do not seem to have been exploited by humans, and why guanaco, probably the most heavily preyed-upon species, did not suffer extinction. Regarding the first questions, our analysis suggests that humans rapidly and strongly affected the megafaunal population, not from the time of the first arrival, but after the FPP big-game hunters came onto the scene. A simpler explanation for why archeological sites are so rare and why megafaunal populations are unaffected until 12,9 k cal BP could be that pre-Clovis in South America is not real. But it seems more reasonable to us that the food preferences of the earliest-dispersing humans have involved a broad spectrum foraging behavior, and their initial population density may have been too low to affect the large mammals. Regarding the second and third questions, recognizing that humans played a principal role in the extinction process requires neither high archeological visibility of hunting nor massive predation on all the extinct species. Large mammal populations were already adaptively vulnerable because of climatic and environmental changes, partly because most of these large-bodied species had low reproductive rates 7 , factors that were compounded by deleterious anthropogenic effects on the environments 1 , 54 . Relatively moderate or even low levels of human predation on a few species could have strongly impacted trophic networks. Thus, the environmental imbalance could have contributed to the extinctions of hunted species and others impacted by the compounded changes. As Pires et al. 55 , 56 have shown, the arrival of human and their predation of a few species of large mammals in Southern Patagonia, or even on a single one (e.g. Lama guanicoe ) would have created multiple indirect effects on species, such as M. darwini and L. gracilis , probably driving them to extinction. Specialized hunting of only a few species, employing FPP technology, could have set the baseline for the massive collapse of the megafaunal community and almost all large mammals. This process, together with the low likelihood that large prey would have been transported by early humans to remote camps 6 , 57 , and the extremely low probability of finding evidence of predation by humans on megafaunal species due to sampling/taphonomic issues, including diffusion due to the processing of carcasses 4 , 58 , could partially explain the apparent contradiction between our results and the lack of larger and stronger archeological evidence of megafaunal exploitation in South America. Although we are unable to explain why the guanaco has not gone extinct, this species was more geographically dispersed during the final Pleistocene 59 than most of the other large mammal species. This probably made the animal a better survivor than others. Furthermore, some have suggested that the endemic Patagonian guanaco did indeed become extinct but the region was re-populated by a subspecies from the Central Andes 60 , similar to what might have happened if Lama gracilis is really a subspecies of Vicugna vicugna .

In conclusion, since Martin’s proposal 13 regarding the rapid spread of big-game hunters in the Americas armed with fluted points (Clovis in the Northern Hemisphere and Fishtail in the South), the central role of humans as agent of megafaunal extinction had been little considered by South American archeology. The strong evidence of pre-FPP occupation and the weak direct evidence of interaction between humans and extinct large mammals in the archeological record has led to a consensus among archeologists that humans played a secondary role or none at all in the process. Here, however, we have shown that the available archeological and paleontological evidence is compatible with a significant impact of human hunting on megafaunal depopulation, not necessarily an overkilling effect. Particularly, we have demonstrated there is a strong spatial and temporal relationship between FPP technology, which appears to be directly related to large mammal hunting, with the density and distribution of large mammal species, as well as with the distribution and fluctuations in human demography during the critical period. Moreover, the decline in megafaunal density seems not to fit well with the timing of significant climatic and environmental changes in southern South America.

On this basis, we propose (1) that human predatory behavior was the main factor driving the megafaunal decline in South America, and (2) that the late Pleistocene environmental changes and the indirect effect of humans on the ecological web probably generated the conditions resulting in the massive collapse of the megafaunal community. Future studies are necessary to explore in-depth the individual demographic trajectories of all extinct megafaunal species and their relationships with climatic, technological and human demographic changes, as well as the relative importance of direct and indirect effects of human hunting.

The temporal changes in density and the spatial variation in the potential distribution of megafaunal species and FPP during the late Pleistocene and early Holocene were explored employing radiocarbon dates compiled by Prates et al. 24 . These radiocarbon dates were complemented with dates for paleontological megafauna recently published 51 , 60 , 61 , 62 (Supplementary Data  1 and 2 ). The radiocarbon dataset was built in agreement with standard validation criteria used in archeology, such as it was described in Prates et al. 24 . In addition, we compiled a dataset of megafauna and FPP without absolute dates, but with relative date (end of late Pleistocene) and precise geographic location using as a reference several recent works 8 , 20 , 21 , 22 , 60 , 61 , 62 , 63 and Paleobiology Database ( https://paleobiodb.org/ ), but checking and expanding the data in the original publications (Supplementary Data  3 and 4 ).

The temporal changes in the density of FPP and megafauna were explored by using the SCPD method 27 , 28 . The SCPD was reconstructed using calibrated dates binned by site in intervals of 200 years, as well as 500 years for the window size of the moving average for smoothing. The FPP and megafauna SCPDs were compared against the curve of human density in South America based in the complete dataset for archeological sites compiled by Prates et al. 24 . Moreover, the differences in SCPD among the three geographical regions studied in human density, FPP and megafauna were explored with the permutation test proposed by Crema et al. 28 , using 1000 permutations. The radiocarbon dates were calibrated using the Southern Hemisphere SHCal 20 curve and all SPD estimated with the package rcarbon for the R software 4.0 64 . Because almost all megafaunal records occur between 18 and 9 k cal BP, we restricted the plots to this period.

The spatial variation in the FPP and megafauna was explored by using an SDM approach 30 . These models are widely used in paleoecology 60 , 63 . For comparative purposes we also explored the potential distribution for human archeological sites dated between 13 and 11 k cal BP, as well as the spatial kernel density of dates using QGIS 3.14 40 . We used a maximum entropy approach to species and projectile point distribution modeling based on presence-only data, as implemented in MaxEnt 29 , 37 . We consider as predictor the bioclimatic variables available in PaleoClim for the 14,7–12,9 k cal BP interval ( http://www.paleoclim.org/ ) 38 , 39 . The performance of each model was evaluated using the AUC statistic (i.e., area under curve) 30 , which measures the explicative power of the model varying between 0 and 1 (the better predictive value). We also explored the combined distribution of megafaunal species to produce a community-level model by using an SSDM approach 44 . The species distribution and stacked species distribution models were generated using the MaxEnt algorithm implemented in the packages dismo and SSDM for the R software 4.0 44 , 64 . The local map of species richness was estimated using the method of summing continuous habitat suitability maps (pSSDM) 44 .

Finally, we estimated the niche overlapping using the I similarity index proposed by Warren et al. 41 . This statistic is based in the Hellinger distance that quantifies the similarity between two probability distributions, being related to the Euclidean distance for discrete distributions. The I similarity index is a modification of this distance to compare Hellinger-based results to more conventional ecological measures of niche overlap, varying between 0 (no overlap) and 1 (identical distributions). The values of similarity indexes for potential species and FPP distributions were plotted and a no-metric Multidimensional Scaling (nm-MDS) was performed to summarize the similarity matrix. The I niche overlap similarity index was estimated using the package dismo for the R software 4.0 44 , 64 . The nm-MDS analysis was performed in the software PAST 4.0.

Reporting summary

Further information on research design is available in the  Nature Research Reporting Summary linked to this article.

Data availability

All relevant data are within the paper and its Supplementary Data files.

Code availability

R scripts to analyze the radiocarbon and occurrence species data are available as Supplementary Code  1 .

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Acknowledgements

We thank Gustavo Politis, Valeria Bernal, Gustavo Martínez, Diego Rindel, Matías Medina, Laura Miotti, Gary Haynes, and Alfred Rosenberger for their helpful suggestions and comments on preliminary versions of the manuscript. This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Universidad Nacional de La Plata (UNLP), and Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) through projects PIP-244/2015, PIP-729/2015, PI-11-N932/2020, and PICT-3645/2015.

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Prates, L., Perez, S.I. Late Pleistocene South American megafaunal extinctions associated with rise of Fishtail points and human population. Nat Commun 12 , 2175 (2021). https://doi.org/10.1038/s41467-021-22506-4

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overkill hypothesis wikipedia

New Research

Are Humans to Blame for the Disappearance of Earth’s Fantastic Beasts?

100,000 years ago, giant sloths, wombats and cave hyenas roamed the world. What drove them all extinct?

Lorraine Boissoneault

Lorraine Boissoneault

Ice_age_fauna_of_northern_Spain_-_Mauricio_Antón.jpg

Turn the clock back 1.8 million years, and the world was full of fantastic beasts: In North America, lions, dire wolves and giant sloths prowled the land. South America boasted camel-like creatures and giant 4,500-pound bears. Eurasia had rhinoceroses and cave hyenas, while Australia teemed with giant wombats and 7-foot-tall flightless birds. Across all those continents and many islands were massive, long-nosed creatures that included the notorious woolly mammoths and mastodons.

Today we have less than half of the species known as megafauna—an exclusive club whose members weigh at least 97 pounds when fully grown—on all continents but Africa. Where did these giants all go? In the past 50 years, archaeologists have started to come to a damning conclusion: Perhaps they would still be here if humans hadn’t arrived on the scene.

First proposed in 1966 by paleontologist Paul Martin, this “overkill hypothesis” stated that the arrival of modern humans in each new part of the world brought with it the extinction of all those huge animals, whether through hunting them or outcompeting them. The hypothesis paints humans as a potent force of destruction and was highly controversial when Martin first proposed it. But over time it gained traction—though never full consensus—in the archaeological community.

Today, some archaeologists continue to fight back. For many of these Pleistocene extinctions, humans probably aren’t to blame, says archaeologist Ben Marwick. The key to his argument is timing: Marwick and other researchers recently found human artifacts in the Madjedbebe rock shelter in northern Australian that indicate humans came to the island 65,000 years ago , 10,000 years earlier than previously believed. That’s critical, because Australian megafauna didn’t start going extinct until sometime between 60,000 and 40,000 years ago.

The new data “puts people on the landscape well before megafauna started suffering population stress and showing signs of extinction,” Markwick says. “It’s like the alibi for humans: It absolves them of central responsibility.”

Are Humans to Blame for the Disappearance of Earth’s Fantastic Beasts?

Marwick believes his team’s find in Australia may be close to a deathblow for the hypothesis. “In archaeology we rarely have such decisive finds that shift the argument from an ongoing debate to a fact, but my feeling is that this could be one of those moments,” he says. Indeed, recent finds have threatened to upend the conventional timeline of human migration. Marwick pointed to a controversial study from April that suggests humans arrived in North America 100,000 years earlier than previously believed. “It looks as if the whole global debate of megafauna extinction is getting a simultaneous revision,” he says.

If that’s true, the consequences would go beyond the scientific. “If it turns out we didn’t kill the megafauna,” says Marwick, “that might suggest our relationship is more one of being just another species on the landscape, rather than a total domination and inflicting environmental violence.”

But for two scientists, the same data can tell vastly different stories. Geologist and paleontologist Gifford Miller of University of Colorado at Boulder thinks Marwick’s study proves the exact opposite of what Marwick claims. “One of the previous arguments against a human role in the Australian megafaunal extinction was that humans first appeared there 50,000 years ago and animals were gone almost immediately after, which doesn’t given them enough time to build a population size sufficient to have any kind of impact," he says.

Marwick’s data, he says, helps solve this discrepancy. The earlier arrival date gives humans time to grow in number over generations, spreading across the landscape, eating whatever they came across and transforming the environment. “It’s undeniable that humans are preying on some of these large animals,” says Miller, “and undeniably something happens to the ecosystem structure and function at about the same time.”

Miller knows the signs of human hunting better than most. He has spent years studying the burnt remains of eggs laid by Australian thunder birds ( Genyornis newtoni ), giant flightless avians that went extinct approximately 47,000 years ago. In 2015 Miller and others published a paper in Nature Communications arguing the burn patterns on these eggshells, which have been found in more than 200 hearth sites across Australia, were different than what would be seen from natural wildfires.

“You can’t make a firm statement, but I think the smart money is [the megafauna] would still be around if humans hadn’t arrived,” he says.

Are Humans to Blame for the Disappearance of Earth’s Fantastic Beasts?

One counterargument to the overkill hypothesis is that ancient climate change killed off all those huge beasts. Scientists on this side of the fence argue that, depending on the continent, fluctuations in temperature and humidity spelled doom for the megafauna. Yet Miller, who has also studied climate change in Australia from the Quaternary period beginning 2.5 million years ago to today, finds that argument lacking.

“These are animals that have been living through the Quaternary, and it’s a rough time. In Australia, you go from extreme cold and dry during Ice Age periods and warmer and wetter conditions in interglacial times. And they’ve made it through 20 of these cycles.”

Archaeologist Todd Surovell tends to agree with Miller’s assessment, though he didn’t always. Surovell was a student under Martin, the father of the overkill hypothesis. Initially, he was skeptical. “The biggest hang up is the paucity of physical evidence. For me that was the case for a long time,” Surovell says. But when he started studying megafaunal extinction on a global scale, not just in North America, the patterns he saw astounded him. “Hunting these large mammals tracks global colonization perfectly,” he says. “Humans move to a new place, [megafauna] suffer extinction.”

Of course, the ecological and archaeological landscapes are vastly different between Australia and North America, where Surovell does most of his research. In Australia, there’s relatively little evidence of humans hunting or eating megafauna other than the eggshells Miller studied; scientists have found no definitive proof that humans caused the doom of dozens of other deceased species, including 25-foot-long lizards and 1,000-pound kangaroos. But in North America, there are dozens of sites that show human exploitation of mammoths for their meat and tusks, though those sites can sometimes be contentious in their own right.

“There are more than 70 mammoths that have been argued to have some cultural association [human markings or evidence of butchery],” Surovell says. “Of those, the archaeological community would accept 16 as definite.” Some argue that just because a spearhead was found in the same location as a mammoth, it doesn’t mean humans killed it; they could’ve just been scavenging its meat. And then there’s the fact that plenty of other species—from short-faced bears to Glyptodons , which were essentially 1-ton armadillos—but we have no evidence of humans hunting them.

That means humans didn’t play any role in driving these other giants extinct, Surovell clarifies. “I don’t know anything about hunting ground sloths, but I imagine a 16-year-old kid with a spear coming across one, is he going to ignore that? No,” he speculates. 

Are Humans to Blame for the Disappearance of Earth’s Fantastic Beasts?

While he’s considered the impacts of large carnivores like lions and saber-toothed cats, Surovell believes that the pressure humans put on the landscape forced those apex hunters to extinction. After all, humans aren’t just hunters; we’re ecosystem shapers. By changing the landscape and driving out predators, we may have had a far larger impact than even the deadliest non-human predators. For Miller, all the evidence of humankind’s ability to alter ecosystems provides irrefutable evidence of the overkill hypothesis.

But that doesn’t mean the debate is anywhere near settled.

Like climate change or human evolution, Miller says, the debate over whether humans are responsible for extinctions can be about values just as much as it is about data. “I’m not sure there’s any evidence that will convince people who don’t want humans to be responsible for such a big change,” Miller says. “They want to put it in terms of good and bad. There’s nothing to do with good and bad. It’s not that humans set out to exterminate things. They’re trying to feed their families the most efficient way they can.”

The idea that ancient hunter-gatherers dramatically altered their ecosystems doesn’t necessarily line up with the stereotypes people have, Surovell adds—which is all the more reason to find evidence for the hypothesis. “People like to think we don’t see major human environmental impacts until the Neolithic, with farming, but I think that’s absolutely not the case,” he says. “We see it from the very beginning of human existence on the planet. I think it speaks to our nature as animals, as ecological agents, as shapers of the environment.”

What all of them agree on is that the debate is far from being over, and people will continue hunting for evidence to prove and disprove the overkill hypothesis. Surovell believes it would take faunal evidence from a dozen early North American human sites to really solidify the hypothesis among North American archaeologists. But he does say this: “The best way to falsify overkill is to show animals went extinct before human arrival. For 50 years now we’ve been trying to falsify overkill and we’ve failed. That, to me, suggests it’s a pretty strong hypothesis.” 

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Lorraine Boissoneault

Lorraine Boissoneault | | READ MORE

Lorraine Boissoneault is a contributing writer to SmithsonianMag.com covering history and archaeology. She has previously written for The Atlantic, Salon, Nautilus and others. She is also the author of The Last Voyageurs: Retracing La Salle's Journey Across America. Website: http://www.lboissoneault.com/

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  1. The Good Genes Hypothesis

  2. The OVERKILL hypothesis ! #history #lessons #ancient #discovery #viral

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  5. Extinction of Ice Age Megafauna (Younger Dryas)

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COMMENTS

  1. Late Pleistocene extinctions

    It compares the overkill hypothesis (predator hunting = 0) with second-order predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that second-order predation is more consistent with extinction than is overkill (results graph at left). The Pleistocene extinction model is the only test of multiple ...

  2. Quaternary extinction event

    It compares the overkill hypothesis (predator hunting = 0) with second-order predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that second-order predation is more consistent with extinction than is overkill (results graph at left). The Pleistocene extinction model is the only test of multiple ...

  3. Paul Schultz Martin

    Paul Martin at Rampart Cave, home of the Shasta ground sloth in Grand Canyon, ca. 1975. Paul Schultz Martin (born in Allentown, Pennsylvania in 1928, died in Tucson, Arizona September 13, 2010) was an American geoscientist at the University of Arizona who developed the theory that the Pleistocene extinction of large mammals worldwide was caused by overhunting by humans.

  4. The Pleistocene Overkill Hypothesis

    The second theory, referred to as the overkill hypothesis, suggests it was not changes in climate but early humans who hunted large game species to extinction. Archaeological evidence suggests that the arrival of the first humans in the Americas, the Paleoindians, and the first megafaunal extinctions occurred roughly in tandem (Bulte et al., 2005).

  5. The overkill model and its impact on environmental research

    2. THE OVERKILL HYPOTHESIS DISSECTED. By the end of the Pleistocene, a suite of 37 genera of large‐bodied mammals became extinct in North America (Grayson, 2015; Meltzer, 2015).There are two main competing hypotheses to explain the extinction of these megafauna that are based on the timing of the extinctions and either the arrival of people to the Americas or climate change at the end of the ...

  6. The Problem with Ice Age Overkill

    Still, the fact of the matter is that overkill is an untested, unverified hypothesis that has nevertheless gained sway, with guilt over humanity's appetite for destruction driving the case for ...

  7. What Killed the Great Beasts of North America?

    The idea that humans wiped out North America's giant mammals, or megafauna, is known as the "overkill hypothesis." First proposed by geoscientist Paul Martin more than 40 years ago, it was inspired in part by advances in radiocarbon dating, which seemed to indicate an overlap between the arrival of the first humans in North America and the ...

  8. Test of Martin's overkill hypothesis using radiocarbon dates ...

    Abstract. Following Martin [Martin PS (1973) Science 179:969-974], we propose the hypothesis that the timing of human arrival to the New World can be assessed by examining the ecological impacts of a small population of people on extinct Pleistocene megafauna. To that end, we compiled lists of direct radiocarbon dates on paleontological ...

  9. Paul S. Martin (1928-2010): Luminary, Natural Historian, and Innovator

    In 1966, writing about his "overkill hypothesis," in which he indicted early humans for Pleistocene extinctions, ... Martin P. S (1966) Africa and Pleistocene overkill. Nature 212: 339-342. View Article Google Scholar 6. Martin P. S (2005) Twilight of the mammoths: ice age extinctions and rewilding America. ...

  10. PDF Late Quaternary Extinctions: State of the Debate

    Overkill hypothesis: hypothesis that extinction results because human hunting causes death rates to exceed birth rates in prey species Megafauna: animals with a body mass greater than 44 kg kyr BP: 1000 years before present INTRODUCTION Fifty thousand years ago, ecosystems around the globe were populated with large animals that are now extinct.

  11. PDF A requiem for North American overkill

    overkill hypothesis is due almost entirely to Martin, who has developed the hypothesis in sufficient detail to make it convincing to many (e.g. Refs. [3,4,8,12,19-22,77, 79,88]), although its most vocal adherents are primarily those whose expertise lies outside the place and time period involved. Martin's hypothesis has changed some-

  12. Late Pleistocene South American megafaunal extinctions ...

    Based on Clovis and Fishtail projectile point evidence, Paul Martin 13 formulated the challenging hypothesis of the "Pleistocene overkill", which postulated that the appearance of humans in ...

  13. The overkill model and its impact on environmental research

    Pleistocene overkill, the notion that humans overhunted megafauna near the end of the Pleistocene in the Americas, Australia, and beyond, is used as prime example of the impact that humans can have on the planet. However, the importance of the overkill model for explaining human-environment interactions and anthropogenic impacts appears to ...

  14. Pleistocene Epoch

    Pleistocene Epoch - Megafaunal Extinctions: The end of the Pleistocene was marked by the extinction of many genera of large mammals, including mammoths, mastodons, ground sloths, and giant beavers. The extinction event is most distinct in North America, where 32 genera of large mammals vanished during an interval of about 2,000 years, centred on 11,000 bp. On other continents, fewer genera ...

  15. Are Humans to Blame for the Disappearance of Earth's Fantastic Beasts

    First proposed in 1966 by paleontologist Paul Martin, this "overkill hypothesis" stated that the arrival of modern humans in each new part of the world brought with it the extinction of all ...

  16. The overkill model and its impact on environmental research

    We then delve into problems of cross-disciplinary communication by conducting a citation analysis of cited works of Paul Martin, the author of the overkill hypothesis. We document how overkill is interpreted and used differently by archeologists and ecologists. For many ecologists, overkill holds significant meaning for the relationship between ...

  17. What caused extinction of the Pleistocene megafauna of Sahul?

    The overkill hypothesis proposes that human hunting drove these animals extinct. Conceivably, this resulted from selective killing of big animals [17,18]. It is also possible that non-selective hunting differentially removed large species because of their low population growth rates and consequent sensitivity to small increases in mortality ...

  18. The overkill hypothesis as a plausible explanation for the extinctions

    The overkill hypothesis is among the more radical, and probably the most controversial, of the proposed explanations of Late Wisconsin megafauna extinctions. It is also the only one that approaches the requirements of a valid, testable hypothesis. It specifies the mechanism, timing, and specific effects of a discrete phenomenon (Martin, 1973 ...

  19. A review of some attacks on the overkill hypothesis, with special

    This paper addresses misrepresentations and errors in attacks directed against the Overkill hypothesis that was proposed by Paul Martin to explain selective late Pleistocene extinctions. The opposing Climate-Change hypothesis to explain extinctions is driven by ideology as much as by objective reasoning because it is repeated so frequently ...

  20. Overkill hypothesis

    Other articles where overkill hypothesis is discussed: Pleistocene Epoch: Megafaunal extinctions: The first theory, the so-called overkill hypothesis, receives support from the coincidence in the timing of the mass extinction and the appearance of large numbers of human hunters, as evidenced by the Clovis complex, an ancient culture centred in North America.

  21. Overkill hypothesis

    Language links are at the top of the page across from the title.

  22. Overkill

    Overkill (term), the use of excessive force or action to achieve a goal. Surplus killing, when a predator kills more prey than it can eat. Overexploitation, depletion of a natural resource through overharvesting. Overfishing. Overkill hypothesis, a proposed cause of Quaternary extinction.

  23. Paul Martin (disambiguation)

    Paul Martin (wrestler) (born 1949), American wrestler better known as Paul Orndorff or Mr. Wonderful. Paul Martin (amputee athlete) (born 1967), American world-record holding Paralympian and triathlete. Paul Martin (rugby league) (born 1967), rugby league player for Canberra Raiders. Paul Martin (water polo) (born 1982), South African water ...