what is life biology essay

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1 Definition of Life

Study guide, learn objectives.

• Identify the core characteristics that define “life.”
• Identify the commonalities and differences among various definitions of “life.”
• Discuss the challenges of settling on a single definition of life.

Key Concepts and Terms

• Sensitivity to stimuli

Reproduction

• Homeostasis
• Growth and development
• Energy processing

Biology is the science that studies life. What exactly is life? This may sound like a silly question with an obvious answer, but it is not easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. Although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.

From its earliest beginnings, biology has wrestled with four questions: What are the shared properties that make something “alive”? How do those various living things function? When faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? Finally, what biologists ultimately seek to understand is how this diversity arises and how it continues. As new organisms are discovered every day, biologists continue to seek answers to these and other questions.

Properties of Life

All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.

Order (Cells)

Cells are considered the most basic units of life because they represent discrete units that can independently embody all of the other characteristics listed below. All life living things consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, atoms make up molecules. These in turn make up cell components or organelles. Multicellular organisms, which may consist of millions of individual cells, have an advantage over single-celled organisms in that their cells can be specialized to perform specific functions and even sacrificed in certain situations for the good of the organism as a whole. How these specialized cells come together to form organs such as the heart, lung, or skin in organisms like the toad shown in Figure 1  will be discussed later.

Sensitivity or Response to Stimuli

Organisms respond to diverse stimuli. For example, plants can grow toward a source of light or respond to touch ( Figure 2 ). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). The movement toward a stimulus is considered a positive response, while the movement away from a stimulus is considered a negative response.

CONCEPTS IN ACTION

Watch this video to see how the sensitive plant responds to a touch stimulus .

Single-celled organisms reproduce by first duplicating their DNA, which is the genetic material, and then dividing it equally as the cell prepares to divide to form two new cells. Many multicellular organisms (those made up of more than one cell) produce specialized reproductive cells that will form new individuals. When reproduction occurs, DNA, which encodes genes, is passed along to an organism’s offspring. These genes are the reason that the offspring will belong to the same species and will have characteristics similar to the parent, such as fur color and blood type.

Growth and Development

All organisms grow and develop according to specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species’ young ( Figur e 3 ) will grow up to exhibit many of the same characteristics as its parents.

Regulation/Homeostasis

Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, such as the transport of nutrients, response to stimuli, and coping with environmental stresses. For example, organ systems such as the digestive or circulatory systems perform specific functions like carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.

To function properly, cells require appropriate conditions such as proper temperature, pH, and concentrations of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through a process called homeostasis or “steady state”—the ability of an organism to maintain constant internal conditions. For example, many organisms regulate their body temperature in a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear ( Figure 4 ), have body structures that help them withstand low temperatures and conserve body heat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.

Energy Processing

All organisms (such as the California condor shown in Figure 5 ) use a source of energy for their metabolic activities. Some organisms capture energy from the Sun and convert it into chemical energy in food; others use chemical energy from molecules they take in.

The diversity of life on Earth is a result of mutations or random changes in hereditary material over time. These mutations allow the possibility for organisms to adapt to a changing environment. An organism that evolves characteristics fit for the environment will have greater reproductive success, subject to the forces of natural selection.

Evolution by natural selection results in adaptations . All living organisms exhibit a “fit” to their environment. Biologists refer to this fit as adaptation and it is a consequence of evolution by natural selection, which operates in every lineage of reproducing organisms. Examples of adaptations are as diverse as unique heat-resistant Archaea that live in boiling hot springs to the tongue length of a nectar-feeding moth that matches the size of the flower from which it feeds. All adaptations enhance the reproductive potential of the individual exhibiting them, including their ability to survive to reproduce. Adaptations are not constant. As an environment changes, natural selection causes the characteristics of the individuals in a population to track those changes.

Watch the following video to review the characteristics of life. Do the characteristics listed in the video differ from those listed above?

the study of living organisms and their interactions with one another and their environments

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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• The Biology of Sex and Death
• 1.01 Scientific Methodology

What is life?

• Life on earth
• Tree Thinking
• What is evolution and why do biologists think it’s important?
• Population & Community Ecology
• Life interacts
• Reproduction without sex (Asexual Reproduction)
• What is sex?
• Trait Inheritance & Genetic Variation
• Human Reproductive Cycle
• Plant Growth and Reproduction
• Sexual Dimorphism and Selection Selection
• Animal Mating Systems
• Chromosomes, genes, and DNA
• Gene expression and development
• In Vitro Fertilization and Gene Editing
• Genetically Modified (Transgenic) Organisms
• Senescence, Aging, and Death
• Heritable disease and Complex traits
• Infectious disease spread
• Innate and Adaptive Immune Responses
• Immunization and Allergies, or How the immune system can help or hurt us
• Cancer Biology
• Extinction & Conservation Biology

Learning Objectives

• Define life and identify the common features of life on earth that distinguish living organisms from non-living entities
• Explain biological evolution as an emergent property of life
• Know that organisms can be grouped by their evolutionary relatedness from domains (largest) to species (smallest), that every species on earth has a unique “scientific name,” and be able to recognize scientific names when you encounter them.
• Compare and contrast the defining characteristics of bacteria, archaea, and eukarya, the three domains of life. Contrast these to viruses.
• Explain the endosymbiotic theory of the origin of eukaryotes, and how endosymbiosis theory explains the unique characteristics of eukaryotes, relative to bacteria and archaea.

Life on Earth

We are intimately familiar with many of the living organisms around us because we’ve seen and interacted with “macroorganisms” all our lives. But some forms of life—”microorganisms”—went unrecognized for most of human history because they were impossible to see until microscopes were invented. Other living organisms may be difficult to recognize, even when seen, as in this picture below.

Lithops is a genus of succulent plants (like cacti) native to South African deserts. Lithops are often called ‘living stones’ because they look like surrounding rocks, excellent camouflage if you are a delicious desert plant. Can you spot them in the image above? Image from http://harrywoolner.wordpress.com/plants/lithops/

Defining “life” leads to questions: How do we distinguish life or non-life? What are the attributes that all living organisms on Earth share, and which of these attributes are exclusive to living organisms? What about the possibility of life on other planets with environments utterly unlike our planet? Mars may have once had life, and scientists hypothesize that Jupiter’s moon Europa has briny seas and hydrothermal vents beneath a layer of ice (Jaggard 2015). Suppose that we launched an exploratory mission to Mars or Europa, sending a laboratory module equipped with any type of analytical instrumentation you can think of. How would you search for evidence of life on Mars or Europa?

If we look at the fundamental properties of life, we can define some emergent properties:

• Need for energy
• Organization in membrane-bound cells
• Genetic information
• Ability to replicate
• Change (in a population) over time

NASA uses these properties to search for extraterrestrial life. NASA defines life as a self-sustaining chemical system capable of biological evolution.

Evolution as an emergent property of life

A key part of any definition of life is that living organisms reproduce. Let’s now add a couple of observations:

• Reproduction is a mostly accurate but imperfect process. When cells divide, they have to replicate their genetic material, called DNA, that encodes all the heritable traits of the organism. The unit of DNA is called a base, or nucleotide more formally. Although DNA replication is highly accurate, it still makes about 1 mistake in 10 million bases. For scale, human cells have ~3 billion bases. Over generations, a population will contain lots of heritable DNA variation.
• Reproduction causes a population of a given type of organism to grow exponentially, but the population size will reach a limit, where the individuals have to compete with each other for the limiting resource (food, space, mates, sunlight, etc.)

Suppose some heritable trait variants (speed, strength, sharper claws, bigger teeth) make some individuals more competitive for the limiting resource – what will happen?

The individuals with superior variants will acquire more resources, and have more progeny. If the superior variants are heritable, then their progeny will have the same superior traits. Over generations, a larger and larger proportion of the population will consist of individuals with the superior heritable variants. This process, called biological evolution, is change in the heritable characteristics of a population over succeeding generations.

Suppose there is heritable variation in a population, and the heritable variation makes a difference in the survival and reproduction of individual organisms. If these conditions exist, and they do for all natural populations of living organisms, evolution must occur. Therefore, by definition, life evolves!

Charles Darwin (1859) called this process natural selection. He and Alfred Wallace were the first to propose that evolution by natural selection could explain the origin of all the multitudes of species on Earth and how they appear so well-adapted in form and function to their particular environments. Moreover, Darwin proposed that all of life on Earth descended from a common ancestor via slow, incremental accumulation of heritable (genetic) changes. This big idea, that all life is related, turns out to be true: we all share the same genetic information, and the small changes accrued in DNA can be compared between species to reveal that every living organism sorts into one of three “domains” of life.

Three domains of life

The genetic information in all  living organisms on earth is DNA. DNA sequence comparisons, as well as comparisons of structure and biochemistry, consistently categorize all living organisms into 3 primary domains : Bacteria, Archaea, and Eukarya. Both Bacteria and Archaea are prokaryotes, single-celled microorganisms with no nuclei, and Eukarya includes us and all other animals, plants, fungi, and single-celled protists – all organisms whose cells have nuclei to enclose their DNA apart from the rest of the cell. The fossil record indicates that the first living organisms were prokaryotes (Bacteria and Archaea), and eukaryotes arose a billion years later.

Humans are eukaryotes, but we are made up of cells from all three domains of life. Our bodies have about 37 trillion (3.7 x 10^13) human cells (hosts of the DNA inherited from our parents), and about 100 trillion (1 x 10^14) bacteria, mostly in the gut (American Society for Microbiology, Human Microbiome FAQ ). We also have Archaea, primarily methanogens (responsible for flatulence!), though they appear to comprise less than 1% of our intestinal microflora ( Lurie-Weinberger MN, Gophna U, 2015 ).

Previously, you may have learned that scientists classify organisms by the Linnaean System (Kingdom/Phylum/Class/Order/Family/Genus/Species), which is still true for Eukarya. The Linnaean system doesn’t work very well for bacteria and archaea, though. The “domain” level captures all three groups, and sits one level higher than Kingdom. Since the advent of DNA sequencing, we now understand that the higher classifications of that system do not always accurately reflect the genetic relatedness between those groups. Carl Linnaeus’ classification system stems from his work in the 1700s on how organisms look, long before we knew about DNA. The amazing thing to scientists now is how accurate the system is, given that it depends solely on morphological characteristics (physical form and structure).

What about Viruses?

Given their impact on living things, you are probably wondering where viruses fit into this organizational system. Are they alive? Viruses are not composed of cells and cannot reproduce on their own, but rather have to ‘take over’ a cell to replicate. Most biologists do not lose much sleep over the debate of whether viruses are classified as living. We think instead about how viruses operate in the world. Viruses act as obligate cellular parasites. That means that they can survive, reproduce, and create new variants when they live inside of and harm another organism.

How long can viruses survive outside a host? In a review of infections originating in hospitals, Kramer et al (2006) found that respiratory viruses, including influenza, corona, and rhino viruses, persisted on surfaces for up to several days. Gastrointestinal viruses often persist in humans for a couple of months, while blood-borne viruses like HIV can persist for more than one week outside a host.

We’ll look into the characteristics of viruses at several points this semester. For now, we can say that viruses use either DNA or RNA for their hereditary material. They also require a host cell to carry out their metabolic activities.

How do we organize all these organisms ?

Sweden’s Carl Linnaeus published Systema Naturae (1735), which established the organizational system that we still use to name and classify life on earth. Linnaean classification gives us the ability to give a unique, two-word scientific or Latin name to each species. Ours is Homo sapiens , with the full classification of:

• Domain: Eukaryota
• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Family: Hominidae
• Genus: Homo
• Species: Homo sapiens

Humans happen to be all alone in their genus, but other members of the family Hominidae include orangutan, gorilla, chimpanzee, and bonobo. Linnaeus didn’t have access to the evolutionary thinking that Darwin and Wallace published over 120 years later, but his organizational categories of species, genus, family, order, class, and phylum often align with the evolutionary relationships we find in the tree of life. (His top level of Kingdom isn’t considered evolutionary robust, so most biologists use domain now instead.)

Wrap up by viewing the Crash Course take on taxonomy, the classification system for all life on Earth:

Traits of the three Domains of Biological Diversity [adapted from OpenStax Biology ]

As Linnaeus did, we still classify organisms according to particular traits they have in common. At the domain level, we can consider representatives of each of the domains to help us define the characteristics of eukarya, archaea, and bacteria.

Figure:  Eukarya (represented by the Australian green tree frog , left),  Bacteria (represented by Staphylococcus aureus , middle) and  Archaea (represented by Sulfolobus , right). [Photo credits: LiquidGhoul, Janice Haney Carr (CDC), Xiaoyu Xiang, in the Public Domain on Wikimedia Commons.]

Bacteria and archaea, together called prokaryotes, are abundant and ubiquitous; that is, they are present everywhere. Prokaryotes are single-celled, and were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. In addition to living in moderate environments like the digestive systems of mammals, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively contaminated regions, such as Chernobyl.

Bacteria and Archaea [adapted from OpenStax Biology ]

Bacteria and Archaea are structurally similar. They have a plasma membrane that houses the cell’s internal content in a single compartment filled with cytoplasm, ribosomes, and DNA. Their DNA is not separated from the rest of the cell by a membrane. Most prokaryotes have cell walls made of peptidoglycan, and many have polysaccharide capsules. Their cells are small, ranging in diameter from 0.1 to 5.0 μm (micrometer, or 1/1,000,000th of a meter, or very small).

Here’s why cell size matters: As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the surface area becomes too low relative to the increased volume and essential elements cannot diffuse through the cell’s membrane. So, single-celled organisms have a size limit that is set by the diffusion rate across the cell’s membrane surface area.

While they have many similarities, here are three key differences between Bacteria and Archaea that you should know: 1) Bacteria cell walls contains peptidoglycan while Archaea cell walls do not. Archaea cell membranes have a different biochemical structure than either bacteria or eukarya. 2) Bacteria have a simple RNA polymerase enzyme, while Archaea have a complex RNA polymerase enzyme, similar to Eukarya. RNA polymerase builds strands of RNA, an important biological molecule that is a middle step in the path between the genetic information of DNA and the action molecules of proteins in a cell. We’ll learn more details about this later in the course. 3) When RNA is translated into proteins in archaea and eukarya, the first amino acid is called methionine (Met), but bacteria use a slightly different version of this amino acid, called formylmethionine (f-Met).

The following two videos explain in lecture format the characteristics that define bacteria  (just watch from time index 0:00 to 3:46).

and the characteristics that define archaea (just watch from time index 0:00 to 3:19).

Eukaryotes and Eukaryotic Origins [adapted from OpenStax Biology ]

The earliest fossils found appear to be Bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for prokaryotes, relatively large cells. Most other prokaryotes have small cells, 1 or 2 µm in size, and would be difficult to pick out as fossils. Most living eukaryotes have cells measuring 10 µm or greater. Structures this size, which might be fossils, appear in the geological record about 2.1 billion years ago.

Characteristics of Eukaryotes

Data from these fossils have led comparative biologists to the conclusion that all living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the most recent common ancestor of the eukaryotes because these characteristics are present in at least some of the members of each major lineage.

• Cells with nuclei surrounded by a nuclear envelope with nuclear pores. This is the single characteristic that is both necessary and sufficient to define an organism as a eukaryote. All extant eukaryotes have cells with nuclei.
• Mitochondria. All eukaryotic lineages have mitochondria, though in a few species they are reduced to remnants of mitochondria.
• A cytoskeleton. All extant eukaryotes have the cytoskeletal elements called actin microfilaments and microtubules.
• Flagella and cilia, organelles associated with cell motility. Some extant eukaryotes lack flagella and/or cilia, but they are descended from ancestors that possessed them.
• Chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them.
• Mitosis, a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton. Mitosis is universally present in eukaryotes.
• Sex, a process of genetic recombination unique to eukaryotes in which diploid cells undergo meiosis to yield haploid cells called gametes. Gametes fuse together to create a diploid zygote nucleus.

Endosymbiosis and the Evolution of Eukaryotes [Adopted from Openstax Biology ]

All living eukaryotes now are descendants of an organism that was a composite of a host cell and an oxygen-using bacterial cell that “took up residence” inside the host cell. This process of one cell engulfing another, where the engulfed cell survives and both cells benefit is called endosymbiosis. Endosymbiosis explains the fundamental characteristics of eukaryotes. Evidence for this includes cell membrane structure similar to bacteria and separate genomes inside mitochondria and chloroplasts that show closer relatedness to bacteria than archaea or eukarya. Endosymbiotic events likely contributed to the origin of the most recent common ancestor of today’s eukaryotes and to later diversification in certain lineages of eukaryotes.

The diagram below explains, in three steps, the theory of how eukaryotes evolved. First, the ancestral eukaryote evolved some internal organization in the form of organelles, including a nucleus to sequester the DNA.  Second, this ancestral eukaryote engulfed an energy-producing bacterium that evolved into the modern mitochondria that all eukaryotes possess. Third, one lineage of eukaryotes acquired a second endosymbiont that evolved into the chloroplasts of modern photosynthesizers like plants and green algae. The chloroplast is the organelle that allows plants and others to harvest their energy from sunlight.

Image Credit: Open Stax Biology, modified by Chrissy Spencer.

Review these ideas by adding your own characteristics to the table below to summarize the similarities and differences in bacteria, archaea, and eukarya:

Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. London: John Murray, Albemarle Street.

Jaggard, V. 2015. These Instruments Will Help NASA Figure Out If Life Can Thrive on Europa. Smithsonian Magazine. 

Kramer, A., Schwebke, I. & Kampf, G. 2006. How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infect Dis 6, 130. https://doi.org/10.1186/1471-2334-6-130.

Lurie-Weinberger MN, Gophna U (2015) Archaea in and on the Human Body: Health Implications and Future Directions. PLoS Pathog 11(6): e1004833. https://doi.org/10.1371/journal.ppat.1004833

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30 Great Essays About Biology

If you read one a day, you may just fall in love..

The world needs more essays about biology. So last month, I tweeted a link to one of my favorite essays (#1 below) and promised that I would continue to share an additional essay every day for the next 29 days. I titled the series, “30 Essays to Make You Love Biology.”

I’ve now assembled all 30 essays in this article. I hope you’ll read them and emerge with a deeper appreciation for the cell, atoms and their confluence with physics and math.

I scoured the internet for non-paywalled versions of each article, so all links go to open-source versions. This effort was inspired by the website “ Read Something Wonderful .” Enjoy!

"I should have loved biology" by James Somers. An easy-to-read essay about how biology is poorly taught in schools, and how this poor teaching masks its most intriguing bits. Students are typically told to read textbooks and memorize facts about the cell ( Mitochondria are the powerhouse of the cell! ) without ever appreciating its miraculous complexity. Tests are often given as multiple choice, with little to no problem-solving involved. As Somers writes: "It was only in college, when I read Douglas Hofstadter’s Gödel, Escher, Bach, that I came to understand cells as recursively self-modifying programs." Link

"Cells are very fast and crowded places" by Ken Shirriff. A short essay about some awe-inspiring numbers in cell biology. My two favorite lines are: "A small molecule such as glucose is cruising around a cell at about 250 miles per hour" and "a typical enzyme can collide with something to react with 500,000 times every second." Link

"Seven Wonders," by Lewis Thomas. When Thomas was asked by a magazine editor “to join six other people at dinner to make a list of the Seven Wonders of the Modern World,” he declined and instead drafted this article about the seven wonders of biology . Number 2 on the list: Bacteria that survive in 250°C waters. Link

"Life at low Reynolds number," by E.M. Purcell. An all-time classic. One of the best biology lectures of all time. This essay opened my eyes to the weirdness of life at the microscale, where "inertia plays no role whatsoever." Or, as Purcell says, "We know that F = ma, but [microbes] could scarcely care less." Link

"The Baffling Intelligence of a Single Cell," by James Somers & Edwin Morris. This interactive article, about chemotaxis and flagella, gives "an intuition for how a bag of unthinking chemicals could possibly give rise to a being." It’s stunning and slightly emblematic of the great Bartosz Ciechanowski’s blog. Link

"Thoughts About Biology," by James Bonner. A little-read essay, I think, that deserves more attention. Published in 1960, Bonner argues that biology is ever-changing and progress, often, comes from those outside the field. Part of biology’s beauty is that you can push it forward regardless of background. Link

"Biology is more theoretical than physics," by Jeremy Gunawardena. It is often said "that biology is not theoretical," writes Gunawardena, but that's not true. This essay gives examples where theory preceded and informed major discoveries in biology. It’s a must-read, especially for those who want to work on biology but don't feel compelled to work at the bench with a pipette in hand. Link

"Can a biologist fix a radio?" by Yuri Lazebnik. One of my favorites. Biologists tend to catalog things by breaking them apart. But without quantitative insights, it is difficult to piece them back together into a holistic understanding. Even if you think a line of inquiry in biology has been exhausted, there is always room to go deeper. Link

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"Schrodinger’s What Is Life? at 75" by Rob Phillips. In 1944, physicist Erwin Schrödinger wrote a book, called “What is Life?” that pondered a single question: “How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?" This essay is an ode, synopsis, and expansion of that classic book. "Names such as physics and biology are a strictly human conceit,” writes Phillips, “and the understanding of the phenomenon of life might require us to blur the boundaries between these fields." Link

"Molecular 'Vitalism'" by Marc Kirschner, John Gerhart & Tim Mitchison. Students are often taught that genes are the bedrock, or blueprint, for biology. But this picture is quickly changing, unraveling, fading. "Although...proteins, cells, and embryos are...the products of genes, the mechanisms that promote their function are often far removed from sequence information." Link

"Escherichia coli," by David Goodsell. Goodsell is a computational biologist who also makes brilliant watercolor paintings of living cells. His paintings are based on atomic truth —that is, the ribosomes, mRNAs, and DNA molecules are all painted to scale. This short essay explains how he does it. Link

"How Life Really Works," by Philip Ball. This essay challenges much that students are taught about how cells actually work. DNA is not some all-powerful blueprint of the cell, as textbooks often suggest. To truly understand life, argues Ball, one must first realize that cells are far more complex than that. They are, in fact, intelligent agents that change their surroundings to their own benefit. Link

"A Long Line of Cells," by Lewis Thomas. Another masterful essay that traces one man's life, and mankind's progress, through the lens of evolutionary biology. It helped me appreciate how my own life is deeply intertwined with the lives of organisms all around me. Link

"AlphaFold2 @ CASP14," by Mohammed AlQuraishi. Biological progress is swift, and that is one reason it is so exciting. In this first-person essay, a computational biologist marvels at a scientific breakthrough in predicting protein structures from their amino acid sequences. Link

"Theory in Biology: Figure 1 or Figure 7?," by Rob Phillips. Another great essay about theory —and not just wet-lab experiments—as a key driver of scientific progress. "Most of the time, if cell biologists use theory at all, it appears at the end of their paper, a parting shot from figure 7. A model is proposed after the experiments are done, and victory is declared if the model ‘fits’ the data." But such an approach is misguided, writes Phillips. As Henri Poincaré once said: "A science is built up of facts as a house is built up of bricks. But a mere accumulation of facts is no more a science than a pile of bricks is a house." Link

" On Being the Right Size," by J.B.S. Haldane. Published in 1926, this essay made me appreciate the myriad forms and functions of lifeforms all around me. I learned why an insect is not afraid of gravity; why a flea as large as a human couldn't jump as high as that human; why a tree spreads its branches, and much more. Simple, beautiful. Link

"I Have Landed," by Stephen Jay Gould. The final essay in a 300-essay series, Gould  writes about how he often lies awake at night, pondering his purpose in the Universe and his fear of death. And how, upon deep reflection, he is most stunned by the fact that life—after more than 3.5 billion years of evolution—continues to exist at all “without a single microsecond of disruption." Link

"A Life of Its Own," by Michael Specter. Published in The New Yorker in 2009, this piece explores the then-nascent field of synthetic biology. It opens by telling the story of Jay Keasling, a professor at UC Berkeley, who engineered yeast to make an antimalarial drug called artemisinin, which has been used to save at least 7.6 million lives. Artemisinin was historically extracted from the sweet wormwood plant in a painstaking and low-efficiency process. Link

"Slaying the Speckled Monster," by Jason Crawford. Smallpox killed an estimated 300 million people in the 20th century alone. This essay explains how a long line of brilliant scientists—from John Fewster and Edward Jenner to D.A. Henderson—invented the first vaccines against the disease and then, in the 1960s, launched campaigns to eradicate smallpox entirely. An inspiring story about how biological discoveries can save lives. I also learned this: "The origin story [about smallpox vaccines] that is usually told, where Jenner learns of cowpox’s protective properties from local dairy worker lore or his own observations of the beauty of the milkmaids, turns out to be false—a fabrication by Jenner’s first biographer, possibly an attempt to bolster his reputation by erasing any prior art." Link

"Why we didn’t get a malaria vaccine sooner," by Saloni Dattani, Rachel Glennerster & Siddhartha Haria. Malaria has killed billions of humans in the last few centuries and continues to kill 600,000+ each year. This is, simply put, the best essay ever written on the history of malaria and the invention of vaccines to prevent it. We are living through a revolutionary time, considering these vaccines were only approved for the first time in 2021. Link

"Biology is a Burrito" and "Fast Biology," by Niko McCarty. Cells are often envisioned as wide-open spaces, where molecules diffuse freely. But this isn't true. In reality, cells are so crowded, it’s a wonder they work at all. Every protein in the cell collides with about 10 billion water molecules per second. Protein ‘motors’ make energy-storing molecules by spinning around thousands of times a minute. Sugar molecules fly by at 250 miles per hour, nearly double the speed of a Cessna 172 airplane at cruising speed. When I first heard these numbers, I thought they were made up. After all, how is it even possible to measure such things? The world’s most powerful microscope cannot necessarily “see” a protein motor spinning, or watch a sugar molecule move through a cell. As a PhD student, I jumped head-first into the world of biological speed. My goal was to collect some "remarkable" numbers in biology and understand the experiments that brought them to light. My search made me appreciate how remarkable it is that life functions at all, considering the chaotic conditions in which cells exist. It also gave me a new appreciation for biology, and the incredible exactitude that one must have to engineer it — let alone engineer it successfully. Link | Link

"Jonas Salk, the People’s Scientist," by Algis Valiunas. Salk made one of the first successful polio vaccines. A double-blind clinical trial, launched in 1954, showed that patients who received his vaccine "developed paralytic polio at about one-third the rate of the control groups. On average across the different types...the vaccine was eighty to ninety percent effective." Shortly after the trial's results were made public, journalist Edward R. Murrow interviewed Salk. When Murrow asked Salk who held the patent on the vaccine, Salk replied: “Well, the people, I would say. There is no patent. Could you patent the sun?” Reading this essay helped me to appreciate the struggle and strife of biological research, the fickleness of fame, and the positive impact that a small group of scientists can have on the world. Link

"On Protein Synthesis," by Francis Crick. Arguably the most important essay in biology’s history, this was adapted from a lecture that Crick gave in 1957 during which the famed geneticist made several accurate predictions about how cells work well before experimental evidence existed to support them. “I shall…argue that the main function of the genetic material is to control (not necessarily directly) the synthesis of proteins," wrote Crick. "There is a little direct evidence to support this, but to my mind the psychological drive behind this hypothesis is at the moment independent of such evidence.” At the time, scientists weren't sure DNA had anything to do with proteins. In this essay, Crick also predicted the existence of a small ‘adaptor’ molecule that brings amino acids to the ribosome for protein synthesis (now known as tRNAs) and that future scientists would chart evolutionary lineages by comparing DNA sequences between organisms. Crick was years ahead of his time. This essay is a masterclass in scientific thinking. Link

"The People Who Saw Evolution," by Joel Achenbach. My favorite article on this list. Every year, for 40 years, Peter and Rosemary Grant traveled to Daphne Major, a volcanic island in the Galápagos, to study Charles Darwin's finches. During that time, they watched "evolution happen right before their eyes." In 1977, for example, just 24 millimeters of rain fell on Daphne Major, causing major food sources—including small, soft seeds—to become scarce. When the Grants returned to the island in 1978, they found that smaller finch species had died off, whereas “finches with larger beaks were able to eat the seeds and reproduce. The population in the years following the drought in 1977 had ‘measurably larger’ beaks than had the previous birds." I also strongly recommend the book, “ 40 Years of Evolution ,” from Princeton University Press.  Link

"Is the cell really a machine?" by Daniel J. Nicholson. Living cells are far more complex—and beautiful—than any machines made by human hands. In this essay, a philosopher points to four areas of current research where the metaphor of "cells as machines" breaks down. For example: Even though proteins are depicted as static or unmoving molecules, they actually “behave more like liquids than like solids." Link

"Biological Technology in 2050" by Rob Carlson. "In fifty years,” writes Carlson, “you may be reading The Economist on a leaf. The page will not look like a leaf, but it will be grown like a leaf. It will be designed for its function, and it will be alive. The leaf will be the product of intentional biological design and manufacturing." This is a futuristic essay about the potential of manipulating atoms via living cells. Link

"Research Papers Used to Have Style. What Happened?" by Roger's Bacon . This is an ode to beautiful scientific writing. The essay draws from classic biology research papers to make its case. Link

"Night Science," by Itai Yanai & Martin Lercher. A personal essay about scientific discoveries that do not emerge from the scientific method as it’s taught in school, as told by two biologists. Perhaps it will inspire you to take up night science experiments of your own. Link

"Atoms Are Local," by Elliot Hershberg. Biology is the ultimate distributed manufacturing platform. Cells harvest atoms from their environments—air and soil—and rearrange them to build materials, medicines, and everything we need to live. Link

"The Mechanistic Conception of Life," by Jacques Loeb. This is the article that got me hooked on biology a decade ago. Written by one of history’s greatest biologists, it poses a number of questions that I suspect will keep scientists busy for many decades to come. "We must either succeed in producing living matter artificially,” writes Loeb, “or we must find the reasons why this is impossible." Link

What essays did I miss? Let me know in the comments and I’ll expand the list :)

Discussion about this post

Ready for more?

Module 1: Introduction to Biology

The characteristics of life, list the defining characteristics of biological life.

Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life. Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term.

From its earliest beginnings, biology has wrestled with these questions: What are the shared properties that make something “alive”? And once we know something is alive, how do we find meaningful levels of organization in its structure?

Learning Objectives

• List the properties of life
• Order the levels of organization of living things

Properties of Life

All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.

Figure 1. A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems.

Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions.

In multicellular organisms (Figure 1), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.

Sensitivity or Response to Stimuli

Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2).

Figure 2.The leaves of this sensitive plant ( Mimosa pudica ) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. (credit: Alex Lomas)

Even tiny bacteria can move toward or away from chemicals (a process called  chemotaxis ) or light ( phototaxis ). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.

Watch  this video to see how plants respond to a stimulus—from opening to light, to wrapping a tendril around a branch, to capturing prey.

Reproduction

Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism’s offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.

Growth and Development

Figure 3. Although no two look alike, these puppies have inherited genes from both parents and share many of the same characteristics.

Organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species’ young (Figure 3) will grow up to exhibit many of the same characteristics as its parents.

Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.

Homeostasis

Figure 4. Polar bears ( Ursus maritimus ) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin. (credit: “longhorndave”/Flickr)

In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through  homeostasis (literally, “steady state”)—the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.

Energy Processing

All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food (photosynthesis); others use chemical energy in molecules they take in as food (cellular respiration).

Figure 5. The California condor ( Gymnogyps californianus ) uses chemical energy derived from food to power flight. California condors are an endangered species; this bird has a wing tag that helps biologists identify the individual.

Levels of Organization of Living Things

Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules , large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 6), which contains the instructions for the structure and functioning of all living organisms.

Figure 6. All molecules, including this DNA molecule, are composed of atoms. (credit: “brian0918″/Wikimedia Commons)

Some cells contain aggregates of macromolecules surrounded by membranes; these are called  organelles . Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.

In larger organisms, cells combine to make  tissues , which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

All the individuals of a species living within a specific area are collectively called a  population . For example, a forest may include many pine trees. All of these pine trees represent the population of pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure 7), the biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.

Practice Question

From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy.

Figure 7. The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit “organelles”: modification of work by Umberto Salvagnin; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer/ National Cancer Institute; credit “tissues”: modification of work by Kilbad; Fama Clamosa; Mikael Häggström; credit “organs”: modification of work by Mariana Ruiz Villareal; credit “organisms”: modification of work by “Crystal”/Flickr; credit “ecosystems”: modification of work by US Fish and Wildlife Service Headquarters; credit “biosphere”: modification of work by NASA)

Which of the following statements is false?

• Tissues exist within organs, which exist within organ systems.
• Communities exist within populations, which exist within ecosystems.
• Organelles exist within cells, which exist within tissues.
• Communities exist within ecosystems, which exist in the biosphere.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does  not  count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

• Introduction to Characteristics of Life. Authored by : Shelli Carter and Lumen Learning. Provided by : Lumen Learning. License : CC BY: Attribution
• Biology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]
• fresh litter. Authored by : Magalie LAbbe. Located at : https://flic.kr/p/9yeYXd . License : CC BY-NC: Attribution-NonCommercial
• Bufo viridis/European Green Toad. Authored by : Ivengo(RUS). Located at : https://commons.wikimedia.org/wiki/File:Bufo_viridis.jpg . License : Public Domain: No Known Copyright
• Young California condor (Gymnogyps californianus) ready for flight. Provided by : US Fish and Wildlife Service. Located at : https://en.wikipedia.org/wiki/File:California-condor.jpg . License : Public Domain: No Known Copyright

What is life?

• Open access
• Published: 27 July 2021
• Volume 48 , pages 6223–6230, ( 2021 )

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• Jaime Gómez-Márquez   ORCID: orcid.org/0000-0001-6962-1348 1  

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Many traditional biological concepts continue to be debated by biologists, scientists and philosophers of science. The specific objective of this brief reflection is to offer an alternative vision to the definition of life taking as a starting point the traits common to all living beings.

Results and Conclusions

Thus, I define life as a process that takes place in highly organized organic structures and is characterized by being preprogrammed, interactive, adaptative and evolutionary. If life is the process, living beings are the system in which this process takes place. I also wonder whether viruses can be considered living things or not. Taking as a starting point my definition of life and, of course, on what others have thought about it, I am in favor of considering viruses as living beings. I base this conclusion on the fact that viruses satisfy all the vital characteristics common to all living things and on the role they have played in the evolution of species. Finally, I argue that if there were life elsewhere in the universe, it would be very similar to what we know on this planet because the laws of physics and the composition of matter are universal and because of the principle of the inexorability of life.

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General Introduction

Reflections upon a new definition of life

Rethinking Life

Avoid common mistakes on your manuscript.

Introduction

Life is a wonderful natural process that occurs in highly organized dynamic structures that we call living beings. Today, thanks to the enormous advance of Biology, we know and understand much better the vital phenomenon, the molecular biology of the cells, the enormous biodiversity on our planet, the evolutionary process, and the complexity of ecosystems. However, despite these enormous advances, biology still lacks a solid theoretical framework necessary to understand the vital phenomenon and to answer questions such as what is life? or are viruses living entities? To answer these and other fundamental questions related to life, in addition to the universal laws of physics, biology needs its own principles to help us find answers to major theoretical challenges such as the origin of life, the construction and maintenance of genomes, or the concept of life itself. Regarding the principles governing life, there have been several contributions from different perspectives (e.g. [ 1 , 2 , 3 , 4 , 5 ]) and I myself have proposed a series of principles (named as the commandments of life) to explain and understand the vital phenomenon from an evolutionary perspective, far from any vitalist, pseudo-scientific or supernatural considerations [ 6 ].

In the words of B. Clark, a definition of life is needed more than ever before to provide defendable objective criteria for searches for life on other planets, to recognize critical distinctions between machine life and robots, to provide insight into laboratory approaches to creating test-tube life, to understand the profound changes that occurred during the origin of life, and to clarify the central process of the discipline of biology [ 7 ]. It is worth noting what E. Koonin wrote about the complexity of defining life: “In my view, although life definitions are metaphysical rather than strictly scientific propositions, they are far from being pointless and have potential to yield genuine biological insights” [ 8 ]. However, despite its importance there is no widely accepted definition of what life is and some of the most commonly employed definitions (see below) face problems, often in the form of robust counter-examples [ 5 , 9 ]. Even some scientists and philosophers of science suggest that it is not possible to define life [ 5 , 8 ].

We can define life in very different ways depending on the context and the focus we want to give to the definition. For example, we can define life as the period from birth to death or as the condition that occurs only in living organisms. We can also say that life is a wonderful and ever-changing process that occurs in highly organized receptacles that we identify as living entities. Likewise, the popular encyclopedia Wikipedia define life as “a characteristic that distinguishes physical entities that have biological processes ….. from those that do not …” [ 10 ]. However, with these expressions we are not defining precisely what life is and therefore we need to create a definition that concisely but informatively reflects our scientific knowledge of the vital phenomenon. We have to distinguish between life and living matter, which is the place where life lives, and between living beings and non-living matter. In reality, when we ask ourselves “what is life?” we are asking “what are the characteristics that distinguish a living organism from a non-living entity?

There are numerous definitions of life formulated from different characteristics of living beings (replication, metabolism, evolution, energy, autopoiesis, etc.) and from different approaches (thermodynamic, chemical, philosophical, evolutionary, etc.). Often, definitions of life are biased by the research focus of the person making the definition; as a result, people studying different aspects of biology, physics, chemistry, or philosophy will draw the line between life and non-life at different positions [ 11 ]. These strategies create a panoply of alternative definitions that makes it very difficult to reach a consensus on the best definition of life because they all have pros and cons. [ 12 , 13 ]. Let me briefly discuss some of the most representative definitions of life. There is a short definition “Life is self-reproduction with variations” [ 14 ] that is interesting for its brevity and because it includes two fundamental characteristics of living organisms: reproduction and evolution. However, this minimalist definition is clearly insufficient [ 8 ] and it does not include some of the most important traits we see in living things. Along these lines, there is also the definition coined by NASA: “Life is a self-sustaining chemical system capable of Darwinian evolution” [ 15 , 16 ]. This is a more complete and, I believe, better definition than the previous one, as it incorporates, in addition to reproduction and evolution, metabolism. However, both definitions are unsatisfactory because neither cell nor multicellular organisms are self-sufficient as there is always a dependence on other organisms and external factors to live and reproduce. Furthermore, these definitions say nothing about the chemical nature of living matter, the interactions with the environment or the low entropy of living things. Apart from that, reproduction being essential for the perpetuation of species and evolution, not all living beings are able to reproduce (e.g. the mule, most bees, etc.) and do not thereby lose their living status. A more recent definition of life states: “Life is a self-sufficient chemical system far from equilibrium, capable of processing, transforming and accumulating information acquired from the environment” [ 17 ]. Although this definition is more comprehensive than the previous ones and includes a reference to thermodynamics, in my opinion it has four drawbacks: (i) the term “self-sufficient” is not adequate because the quality of life does not provide self-sufficiency; (ii) the thermodynamic component does not highlight how fundamental low entropy or high order is for any living being; (iii) information can be acquired from “within” and not only from the environment; (iv) life is not a system is a process and living beings are the system where that process occurs (I discuss this point below). From a very different perspective it was defined life as “matter with the configuration of an operator, and that possesses a complexity equal to, or even higher than the cellular operator” [ 18 ]. This proposal introduces a new term, the operator, which is somewhat confusing, excludes viruses and makes a strange classification of living systems. On the other hand, some scientists have also attempted to define life from a handful of key features. Thus, seven “pillars” (the essential principles by which a living system functions) have been proposed on which life as we know it can be defined [ 19 ], but no definition was provided. Life has also been considered as any system that from its own inherent set of biological instructions and the algorithmic processing of that "prescriptive information" can perform the nine biofunctions [ 20 ] which are basically the same as the "pillars" mentioned above. However, no definition of life was proposed, and again it was considered as a system rather than a process. Both definitions exclude viruses as living beings, mainly because the existence of a membrane, a metabolic network and self-replication are set as conditions for life. In short, there are many more definitions of life but as R. Popa says “We may never agree on a definition of life, which will remain forever subject to a personal perspective” [ 21 ].

My definition of life

Traits are measurable attributes or characteristics of organisms and trait-based approaches have been widely used in systematics and evolutionary studies [ 22 ]. Since any definition of life must connect with what we observe in nature, my strategy for finding a definition of life was to establish what are the key attributes or traits common to all living things. What do bacteria, yeasts, lichens, trees, beetles, birds, whales, etc. have in common that clearly differentiates them from non-living systems? In my opinion, living organisms share seven traits: organic nature, high degree of organization, pre-programming, interaction (or collaboration), adaptation, reproduction and evolution, the last two being facultative as they are not present in all living beings.

Organic nature and highly organized structures. Living matter is organic because it is based on carbon chemistry and molecular interactions take place following the laws of chemistry. As R. Hazen wrote “Carbon chemistry pervades our lives. Almost every object we see, every material good we buy, every bite of food we consume, is based on element six. Every activity is influenced by carbon—work and sports, sleeping and waking, birthing and dying.” [ 23 ]. Living organisms are highly organized structures that maintain low entropy (the vital order) by generating greater disorder in the environment, thus fulfilling the postulates of thermodynamics [ 24 , 25 ]; when this vital order is lost, life disappears and the only way to restore life is to generate a new vital organized structure through reproduction [ 6 ]. Living organisms resist entropy thanks to biochemical processes that transform the energy they obtain from nutrients, sun or redox reactions. It could be said that vital order and energy are two sides of the same coin.

Pre-programming. Every living entity has a software (a pre-programme) in its genetic material that contains the instruction manual necessary for both its construction (morphology) and its functioning (physiology). This programme has been modified in the course of evolution, as a consequence of contingency and causality, so it is not a static or immutable program but a dynamic one. Furthermore, there is also another preestablished program that conditions the vital phenomenon and that I have called the principle of inexorability [ 6 ]. Let me give few examples of the principle of inexorability at different levels of complexity. The shape of ribosome is determined (pre-programmed) by the chemical bonds that are established between ribosomal proteins and rRNA. A similar example is the λ phage morphogenesis that depends only on interactions protein–protein and protein-DNA. Evolutionary convergence or the need for wings to fly are other examples of this inexorability guided by the laws of nature.

Interaction and adaptation. If we look at nature in its purest state or at the complex human society, we can see countless interactions between living beings and with their environment necessary for survival and reproduction. We can see interactions at the molecular level (e.g., allosteric interactions, metabolic pathways, cellular signaling, quorum sensing), in the relationships between organisms of the same or different species (e.g., sexual reproduction, symbiosis, infection, parasitism, predator–prey, or sound language), or between living forms and the environment (e.g., photosynthesis or physiological/anatomical interactions for swimming or flying). Interaction is collaboration, it is cooperation at all levels [ 6 ], the ecosystem being the best example of multiple collaborative interactions between very different organisms. In terms of adaptation, living organisms show a great capacity to adapt both to their surroundings and to environmental circumstances; furthermore, adaptations involving new biological characteristics can be seen as an opportunity to find a different way to evolve. In this sense, the evolutionary process reflects this continuous adaptation and anatomy, physiology and genome bear witness to this. Life is adaptative because species adapt to environmental changes modifying their physiology or metabolism, for instance reducing heartbeat during hibernation (e.g. the grizzly bear Ursus arctos horribilis ) or synthesizing fat from excess sugar to increase the energy reserves of the body (e.g. Homo sapiens ). In addition to these temporal adaptations in response to environmental changes [ 26 ], there are also changes in genotype or phenotype since the adaptation process is the result of natural selection acting upon heritable variations [ 27 ]; a well-known example of this is the peppered moth Biston betularia whose allele frequencies of the locus that controls the distribution of melanin in the wings changed with the industrial revolution in England [ 28 ]. Epigenetic variations also contribute to rapid adaptative responses [ 29 , 30 ].

Reproduction and Evolution. Another property of living beings is their ability to perpetuate themselves and thus make it possible for the species not to disappear and to evolve. Reproduction can be observed at the molecular (DNA replication), cellular (mitosis, meiosis, binary division), and organismal (sexual and asexual) levels. From a different perspective, reproduction is also the way to overcome the second law of thermodynamics and the tyranny of time because when we reproduce, we are creating a new order and resetting the vital clock to zero [ 6 ]. What about individuals such as the mule or the male and female of a species, or the hermaphrodite that cannot self-fertilize, who cannot reproduce because they are sterile or because they need another member of their species to reproduce? Are not these organisms living beings? Of course, they are! In this context, reproduction must be considered as a facultative trait because not all living organisms are fertile or can produce offspring on their own but maintain all other traits necessary for the life process. If an individual is sterile, the species will continue to exist because the evolutionary process must be analyzed at the population level, not at the level of individual organisms; obviously, if the entire population were sterile, then the species would disappear and there would be no life. All species have the capacity to evolve, and this property is unique to life. Evolution allows living beings to adapt to new circumstances and the best genomes are selected and transmitted to the next generations. The concept of evolution (reproduction with variations and permanence in time) allows us to interpret the reality of the life we observe now and to guess what it has been like in the past. We cannot predict the future because evolution is not a finalistic process, it is, to use the words of J. Monod, the fruit of chance and necessity.

There is nothing on this planet, apart from a living being, that complies with all these characteristic features of living beings. It should therefore be possible to define life by logically combining them. Consequently, I define life as a process that takes place in highly organized organic structures and is characterized by being preprogrammed, interactive, adaptative and evolutionary. If life is the process, a living organism is the system in which that process takes place and which is characterized as organic, highly organized, pre-programmed, interactive, adaptative, and evolutionary. Why do I say that life is a process and not a system? According to the Merriam-Webster dictionary, a process is a natural phenomenon characterized by gradual changes that lead towards a certain result. A second meaning defines it as a continuous natural or biological activity or function; and a third one as a series of actions or operations conducing to an end. These three meanings of what a process is fit very well with what we observe happening in living beings, which is none other than the vital process or life. The dictionary itself defines a system as a regularly interacting or interdependent group of items forming a unified whole, and as an assemblage of substances that is in or tends to equilibrium or a group of body organs that together perform one or more vital functions. Once again, these definitions fit very well with what a living being represents.

What is the difference between life, living being and a robot? [ 31 ] Life is the vital process and the living being is the system, the “container” in a metaphorical way, where the vital process takes place. Following this reasoning, a robot would be an artificially organized, pre-programmed and interactive system, but unlike a living being it is not alive because it is neither organic, nor does it reproduce, adapt, or evolve. A robot or a population of robots cannot “reproduce and evolve” on its own, without the intervention of its "creator" (the human being), it will always need to be built or programmed by an engineer to do so. I do not dispute that the robot can adapt, especially thanks to advances in artificial intelligence, although I am not sure that it can do so in the biological sense of the term. Biological adaptation is a process by which a species eventually adapts to its environment as a result of the action of natural selection on phenotypic characteristics [ 32 ]. A robot may be able to adapt to its environment, but what it cannot do is adapt itself through a selective process (without intervention from its creator) and change into a new type of robot (evolve). On the other hand, regarding the synthetic lifeforms named as xenobots [ 33 ], I think they cannot be considered as pure robots, but as an interface between living beings and artificial robots, as they are made from cells. In the future we will probably build robots so perfect that we can consider them as almost living beings and as the result of the intervention of a creator (their engineer), something that we cannot say about living beings unless we are creationists.

Are viruses alive?

A. Turing, one of the pioneers in the development of computer sciences, wrote: “Can machines think? This should begin with definitions of the meaning of the terms “machine” and “think” [ 34 ]. To paraphrase Turing, we could ask ourselves: can viruses be considered living entities? And the answer to this question, so important for biology and still controversial [ 35 ], is to define what a virus is and what life is. At least from a theoretical point of view, biology should seek a clear and definitive answer to this question instead of adopting a skeptical attitude and assuming what K. Smith wrote in his classic book on viruses, “As to the question asked most frequently of all, are viruses living organisms? that must be left to the questioner himself to answer” [ 36 ].

Viruses are entities that straddle the boundary between living and non-living and therefore their biological status is controversial. A virus can be defined as an acellular infectious agent whose structure consists of a macromolecular complex of proteins and nucleic acids. Viruses are not cells, they do not metabolize substances, nor can they reproduce by themselves, grow, or breathe. Yet, regardless of whether we consider viruses to be living beings or not, they are an inescapable part of life and there is an undeniable biological connection between the virus and the organism it infects. Given the close interconnection between viruses and their hosts, it seems plausible that viruses play essential roles in their hosts [ 37 ]. For example, endogenous retroviral elements have shaped vertebrate genome evolution, not only by acting as genetic parasites, but also by introducing useful genetic novelty [ 38 ]. More recently, it was found in the human genome a gene regulatory network based on endogenous retrovirus that is important for brain development [ 39 ] and a new tamed retroviral envelope that is produced by the fetus and then shed in the blood of the mother during pregnancy [ 40 ].

Viruses are capsid-encoding particles that infect all kind of cells and share hallmark genes with capsidless selfish genetic elements, such as plasmids and transposons [ 41 ]. Traditionally, they have been regarded as lifeless agents because they have no metabolism of their own and need a cell to replicate and generate new viruses [ 42 ]. However, while this is true, I believe that this is not a definitive criterion for excluding them from the tree of life (more on this below). There are scientists in the opposite side that consider viruses as living beings that can evolve [ 43 ] and classify them as capsid-encoding organisms as opposed to the ribosome-encoding organisms that include all cellular life forms [ 37 , 44 ]. Viruses have played a key role in the evolution of species [ 35 ] because they are the most abundant source of genetic material on Earth, are ubiquitous in all environments, and have actively participated in the exchange of genes or DNA fragments with their hosts [ 41 , 45 , 46 ].

We cannot say whether a virus is a living thing or not without defining what is life and what is a living thing. Obviously, if we take the cell as the minimum vital unit, we cannot consider viruses as living entities, and any discussion of this is superfluous. As far as I am concerned, considering viruses as non-living creatures because they need a cell to reproduce is not a very strong argument for two reasons. First, viruses are obligate intracellular parasites, and, like all parasites, they use the host for their own benefit, and this is their survival strategy. Viruses need nothing else to pursue the same goal as all species on this planet, which is to generate more viruses better adapted to infect new organisms. They apply the “law of least effort” to achieve this goal and may even decide to remain inside the host cell in a lysogenic manner, as in the case of bacteriophage lambda [ 47 ], or by establishing latency as herpesviruses do [ 48 ]. Second, as I said before no cell or organism is self-sufficient, as it needs at least a supply of food/energy to survive and reproduce. We know that life is absolutely interdependent. For example, we depend directly on our intestinal bacterial flora for our survival, and indirectly on nitrogen-fixing bacteria or photosynthesis. We could take to absurdity the argument that because viruses need a cell to reproduce, they are not alive and say that a man or a woman is not a living being because they cannot reproduce by themselves. The argument that a virus is not a living thing because it is an inert entity outside the cell is also not valid because such a virus could still have the ability to infect cells. Similarly, a spore or a seed cannot be considered lifeless because it is inert, as it is only waiting for the right environmental conditions to germinate, and that wait can last for thousands of years.

To answer the question of whether viruses are alive or not, I base my argument in support of considering viruses as living entities obviously on my own definition of life (this paper), as well as on what we know about the biology of viruses. First, viruses, like all cellular entities in nature, are composed of organic molecules; a virus consists of a nucleic acid (DNA or RNA), which is its genetic material as in all living things, and a protein capsid encoded by the viral genome that protects the viral genetic material and participates in the propagation of the virus in the host; viral capsids show fascinating dynamics during the viral life cycle [ 49 ]. Secondly, viruses are highly organized structures. There is an astonishing diversity of organization and geometric design of viruses, requiring only a few different structural subunits of the capsid to construct an infectious particle. Many viruses have developed very successful self-assembly systems; so much so that the viral capsid can self-assemble even outside the host cell [ 50 ]. The third feature common to all living things is that they are pre-programmed, and viruses also fulfill this characteristic because in their genetic material are written the instructions to make new viruses capable of infecting new cells or organisms. Viruses in their genome have the necessary (though not sufficient because they need elements provided by the host cell) instructions to make new viruses, and in this they are the same as any other living thing. In addition, the process of self-assembly to generate new viruses occurs spontaneously because the instructions to do it autonomously are both in the capsid-forming molecules themselves and in the nucleic acid, either DNA or RNA [ 49 ].

Two other characteristics of living organisms are the ability to interact with other living organisms (interaction) and to adapt genetically to new circumstances (adaptation). Viruses interact with their host in multiple ways: during infection, when their genes are expressed and their genome replicated, when virions are formed, when they integrate into the genome of the host cell, or when they engage in horizontal gene transfer processes. Viruses not only interact with their host, but also adapt by generating new variants that increase their ability to infect other cells, or by taking control of cell metabolism for their own benefit, or even to escape the immune response [ 51 ]. In terms of reproduction and evolution, which are two closely related processes, viruses reproduce in the host cell and evolve through changes in their genome. Viral evolution, like that of all living things, refers to the heritable genetic changes that a virus accumulates during its life cycle, which may arise from adaptations in response to environmental changes or host immune response. Because of their short generation times and large population sizes, viruses can evolve rapidly [ 52 ].

Microbiologist and Nobel laureate J. Lederberg said that “The very essence of the virus is its fundamental entanglement with the genetic and metabolic machinery of the host”. As far as I am concerned, this statement is essentially true and its profound meaning is, at least for me, further proof that viruses are living things. Viruses form part of many integrated biological systems, and they played an important role in the evolution of species [ 53 ]. They can exchange genetic material and participate in horizontal gene transfer [ 43 ] even between individuals from different species [ 54 ]. Due to their high frequency of mutation [ 55 ], viruses are so abundant in nature and present such a high degree of diversity that they constitute by themselves the virosphere [ 46 ]. This great viral biodiversity is proof that these living entities perform fundamental evolutionary and ecological functions [ 56 , 57 ]. In conclusion, I believe that viruses should be considered as living entities that can participate in events as diverse as causing pandemics, destroying bacteria, causing cancer, or participating in horizontal gene transfer.

Following the metaphor of the “container” as the vessel or system (the living being) in which the life process takes place, the fact that viruses are obligated intracellular parasites and do not have a cellular structure and metabolism of their own does not seem to fit this metaphor. It is obvious that the virus cannot be the “container” where the life process takes place, since the virus, when outside the cell, is in a “dormant” state waiting to find a suitable host to infect and complete its life cycle; we could say that it is inert but not yet dead. Therefore, in the special case of viruses, the “container” is the cell. Once the virus finds its specific “container”, it can then reproduce, or integrate into the genome of the host cell, or remain as an episome, or intervene in the evolutionary process through exchange of genetic material. From genomic and metagenomic data, we know that co-evolution between viral and host genomes involves frequent horizontal gene transfer and the occasional co-option of novel functions over evolutionary time. We can say that viruses and their cellular hosts are ecologically and evolutionary intertwined [ 58 ].

I would like to refer to an interesting reflection on the defining characteristics of life and how viruses fit into this conceptual framework [ 59 ]. Thus, Dupré and O'Malley consider collaboration as a common criterion of life and I can only agree with this assessment; in this sense, in a previous paper on the principles that govern life [ 6 ], I use the expression “cooperative thrust” to refer to the importance of collaboration in the origin and evolution of living beings. Without considering collaboration or cooperation as a key interaction, we could not explain endosymbiosis, eukaryogenesis, metabolism, multicellularity, etc. In the present paper, collaboration is implicit in what I call interaction as a common and fundamental feature of all living things. Interestingly, these authors point out that “leaving viruses out of evolutionary, ecological, physiological or conceptual studies of living entities, would allow only an incomplete understanding of life at any level” [ 59 ]. Considering this emphasis on collaboration as a sine qua non condition for life, how does the world of viruses fit in? Dupré and O'Malley propose, and I agree, that viruses can be understood as alive when they actively collaborate (I mean when they are infecting the target cell) and when they do not collaborate (I would say they are inactive), they have at most a potential for life.

Finally, I would like to add that I am aware that there are many scientists who consider that viruses are not living beings basically because they do not have a cellular structure with all that this means. Therefore, this biological dilemma will probably be with us for a long time to come. I think it will only be resolved when we reach a consensus on what life is because only then will we be able to say categorically whether something is alive or not. This is what I have modestly tried to do in this paper.

What would life be like elsewhere in the universe?

The massive number of exoplanets strongly suggests that there is a high probability that life evolved elsewhere in the universe. Astrobiologists are committed to the search for life in the cosmos and for that purpose it is very convenient to have a criterion about what life is [ 16 ]. How can we be sure that there is life on a distant planet? To do so, we need to define some biosignatures that can establish the possible existence of living things elsewhere in the universe [ 60 ], otherwise what are we looking for? In addition to this, it would also help a lot in this search for life on other planets, finding out how life began on Earth.

Some scientists and philosophers of science think that this preconception of what life is may be a problem rather than a solution in the search for life in other planets. C. Cleland in her book about the nature of life states, “Life is not the sort of thing that can be successfully defined. In truth, a definition of life is more likely to hinder than facilitate the discovery of novel forms of life” [ 5 ]. I do not entirely agree with this double statement because although we must be open-minded in the search for life outside our galactic home, at the same time I think it is a good idea to have a hypothesis based on the only certainty we have about vital phenomena, which is life on Earth, that will help in the design of the search for extraterrestrial life.

Is there life elsewhere in the universe? We don't know yet and it is probably only a matter of time before we find life on other planets or aliens find us. In my opinion if there is life elsewhere in the universe, it will most likely be similar to what existed, exists or will exist on our planet. Let us see why. First of all, the laws of physics and chemistry are universal and these laws, directly or indirectly, govern everything that happens with the matter of the universe. According to the cosmological principle, the same physical laws and models that applies here on Earth also works in all parts of the universe [ 61 ]; it is also assumed that physical constants (gravitational constant, speed of light, etc.) remain the same everywhere in the universe. Second, the elements that make up the matter of the stars are the same everywhere in the universe although in different proportions; the “periodic table” is the same for the whole universe. Whether life exists elsewhere in the universe based on a chemistry other than carbon we do not know and can only speculate, but what we do know for sure is that life on Earth is based on carbon chemistry, perhaps because it cannot be otherwise. Third, there is the aforementioned principle of inexorability [ 6 ]. In this context, what does this principle mean? It means that if the environmental conditions are suitable, glucose will be converted into pyruvate in an aqueous medium, chemiosmotic processes will be an important mechanism for generating chemical energy, flying organisms will have wings, or genetic information will be encoded in a language analogous or identical to what we know on Earth. According to this, the differences between the Earth living forms and the “space creatures” could be attributed to a different evolutionary stage or to specific environmental conditions. This hypothetical premise could be very important when developing projects that seek life elsewhere in the universe.

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Acknowledgements

I would like to thank my colleagues M. L. González Caamaño and R. Anadón for their useful discussions. This work is dedicated to my parents.

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Gómez-Márquez, J. What is life?. Mol Biol Rep 48 , 6223–6230 (2021). https://doi.org/10.1007/s11033-021-06594-5

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