There is no single, universally accepted scientific definition of life. Biologists, chemists, physicists, and philosophers have proposed more than a hundred definitions over the past century, and none has won consensus. What scientists do agree on is a set of overlapping frameworks, each capturing something important about what separates a bacterium from a rock. The most widely used working definition comes from NASA and focuses on chemistry and evolution, but it has well-known blind spots, and the boundary cases (viruses, dormant organisms, synthetic cells) keep forcing the conversation open.
The Checklist Approach
The version most people encounter in a biology classroom treats life as a set of observable properties. A living thing takes in energy and converts it to useful work. It grows. It reproduces. It responds to its environment. It maintains internal stability. It is made of cells. If an entity checks every box, it counts as alive; if it fails one or more, the answer gets murky. This approach is intuitive and useful for everyday purposes, but it runs into trouble quickly. Fire grows, consumes fuel, and responds to its environment, yet nobody calls a campfire alive. A mule cannot reproduce, but nobody calls it dead.
Some researchers have tried to refine the checklist by expanding it. A 2023 paper proposed eighteen distinct characteristics of living beings, including agency, purposiveness, information processing, and hierarchy, arguing that agency with purpose, knowledge, and power is the key feature without which the behavior of living things cannot be explained.1Europe PMC / Heliyon. Eighteen distinctive characteristics of life That kind of expansion reveals a core problem with checklist definitions: different researchers weight different properties, and adding more criteria does not necessarily sharpen the line between living and nonliving. It just makes the list longer.
The NASA Definition
The most frequently cited working definition in modern science comes from NASA’s astrobiology community: life is a self-sustained chemical system capable of Darwinian evolution.2PubMed. The Origin, Extension, and Future of the “NASA Definition” of Life This definition packs a lot into a single sentence. “Self-sustained” means the system maintains itself through its own chemistry, taking in energy and raw materials. “Chemical system” means it is made of matter, not just information or patterns. “Capable of Darwinian evolution” means the system can produce imperfect copies of itself, and those variations can be selected by the environment over time.
The appeal of this definition is that it avoids Earth-centric assumptions. It does not require DNA, carbon, or water. It does not specify cells. In principle, a chemical system built on completely different molecules on another world would still qualify as alive if it met these criteria. That flexibility is why astrobiologists lean on it so heavily. But the definition has critics. A single rabbit is alive but cannot evolve on its own; evolution is a property of populations, not individuals. And a sterile organism, one that will never reproduce, seems alive by any common-sense measure yet does not fit neatly into a definition built around Darwinian evolution.3PubMed Central. Defining life
Life as a Fight Against Disorder
Physics offers a different lens. Living systems are striking because they maintain themselves in a highly ordered, far-from-equilibrium state. In plain terms, they stay organized in a universe that constantly trends toward disorder. Your body is full of chemical reactions that, left to themselves, would settle into a dull equilibrium where nothing interesting happens. Instead, those reactions are kept off-balance, compartmentalized inside membranes, and continuously driven by the energy you extract from food.
The numbers are small but telling. The compartmentalization of molecules and the maintenance of non-equilibrium chemical reactions inside living tissue produce measurable reductions in entropy on the order of about 40 to 50 joules per kelvin per liter, compared with what you’d see in the same chemicals at equilibrium. The entropy produced by the body’s ongoing processes, roughly 480 joules per kelvin per liter per day in a human, must be rapidly dumped as heat to preserve that internal order.4PubMed Central. Entropy Perspectives of Molecular and Evolutionary Biology You are, in a real physical sense, a system that continuously pushes disorder outward so it can remain ordered inside.
This thermodynamic framing captures something that checklist definitions miss: the ongoing, active nature of being alive. Life is not a thing; it is a process. The moment that process stops maintaining its internal order, the system slides toward equilibrium, and what remains is no longer alive.
Autopoiesis and Self-Making Systems
In the 1970s, Chilean biologists Humberto Maturana and Francisco Varela proposed a concept called autopoiesis, meaning “self-making.” An autopoietic system is one that continuously produces and replaces its own components, maintaining a boundary that separates it from its surroundings. A cell fits this definition neatly: it manufactures its own membrane, its own enzymes, its own internal structures, all while exchanging matter and energy with the outside world.5PubMed. Autopoiesis 40 years later. A review and a reformulation
Autopoiesis was proposed explicitly as a definition of a living being, and it remains influential in systems biology. It offers a clear definition of minimal life tied to concrete cellular machinery rather than abstract properties like “evolution.” A cell is alive because it makes itself, sustains itself, and rebuilds itself. This view also links cleanly to related ideas in complexity science, including self-organization, emergence, and biological autonomy.6PubMed. Autopoiesis: a review and a reappraisal The limitation is that autopoiesis is tightly bound to cellular life. It has little to say about whether a computer program or an alien chemistry without cell-like compartments could be alive.
The Virus Problem
Viruses are the most famous challenge to every definition of life. A virus particle sitting on a doorknob does nothing. It has no metabolism, no energy production, no ability to reproduce on its own. By most definitions, it is not alive. But inside a host cell, a virus hijacks the cell’s machinery, replicates its genome, builds new copies of itself, and evolves under natural selection at blistering speed. If you only look at the particle in isolation, it seems inert. If you look at the full cycle, it looks very much alive.
Some researchers have argued that the entire debate is misframed. One influential idea is the “virocell” concept: instead of thinking of a virus as the particle alone, think of it as the infected cell during the phase when viral genes are being expressed and reproduced. Under this view, the virus is alive during the intracellular part of its cycle, and the free-floating particle is just a dispersal stage, analogous to a seed or spore.7PubMed. To be or not to be alive: How recent discoveries challenge the traditional definitions of viruses and life This reframing does not settle the question but it shifts it in a productive direction, asking not “is this entity alive?” but “is this entity participating in the process of life?”
The discovery of giant viruses over the past two decades has complicated things further. Some giant viruses have genomes larger than those of the smallest free-living bacteria, carry genes for metabolic functions, and even get infected by their own parasitic viruses. Meanwhile, all biological replicators, from fully cooperative cells to completely parasitic genetic elements, fall on a continuum rather than neatly into “alive” and “not alive” bins.8PubMed Central. Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question The sharp line we want to draw may not exist in nature.
Prions and Life Without DNA
If viruses blur the boundary, prions practically erase it on one axis. Prions are misfolded proteins that can convert normal copies of the same protein into the prion shape, effectively “replicating” without any nucleic acid at all. In yeast, prion-based phenotypes are heritable: daughter cells inherit the misfolded protein and pass it on in turn.9PLoS Biology. Hsp104-Dependent Remodeling of Prion Complexes Mediates Protein-Only Inheritance The propagation happens through structural transitions in existing protein rather than through the synthesis of new genetic polymers.10PubMed Central. Prion propagation: the role of protein dynamics
Nobody seriously argues that a prion is a living organism. But prions demonstrate that hereditary information can exist and be transmitted without DNA or RNA, which undercuts definitions that treat nucleic-acid-based information as essential to life. If we ever encounter an alien system that stores and copies information in some completely different molecular form, prions are a reminder that our own biology already has a precedent for non-genetic inheritance.
Cryptobiosis and the Third State
Tardigrades, the microscopic creatures sometimes called water bears, offer another definitional headache. Under extreme stress, like desiccation, freezing, or radiation, a tardigrade can enter a state called cryptobiosis in which its metabolism drops to undetectable levels. It does not eat, does not respire, does not grow. By any metabolic measure, it has stopped doing everything a living thing is supposed to do. Yet it is not dead: when conditions improve, it resumes normal activity.11PubMed. Cryptobiosis: a new theoretical perspective
Cryptobiosis has been called a third state between life and death, and it is not unique to tardigrades. It occurs across multiple branches of the tree of life, including bacteria, fungi, plants, and various invertebrates.12PubMed. New insights into survival strategies of tardigrades These organisms force an uncomfortable question: if metabolism is a defining feature of life, is a desiccated tardigrade alive? Most biologists would say yes, because the potential for metabolism is preserved in its molecular structures. But “potential” is a slippery criterion. A dried tardigrade and a freshly dead tardigrade look nearly identical under a microscope. The difference is whether the molecular machinery is intact enough to restart.
How Small Can Life Get?
One way to approach the question of what life is, is to strip away everything non-essential and see what remains. Researchers have done this both by studying nature’s most reduced organisms and by building minimal cells from scratch.
In nature, bacterial endosymbionts that live permanently inside insect cells have undergone radical genome reduction over millions of years. They have lost so many genes that their genomes are far smaller than those of their free-living relatives. Eventually they hit a tipping point where they can no longer build a proper cell envelope and their translation machinery begins to erode.13PubMed Central. How do bacterial endosymbionts work with so few genes? At that stage they start to resemble organelles more than independent organisms, and the line between “living symbiont” and “cellular component” blurs.
In the lab, the synthetic biology team behind JCVI-syn3.0 built a minimal bacterial cell with just 473 genes, giving it a genome smaller than any autonomously replicating cell found in nature.14PubMed. Design and synthesis of a minimal bacterial genome Even in that stripped-down cell, roughly a third of the genes had no known function at the time. The minimal translation machinery alone, the set of genes needed just to read genetic instructions and build proteins, has been estimated at about 142 genes.15PubMed Central. The Minimal Translation Machinery: What We Can Learn From Naturally and Experimentally Reduced Genomes Life, it turns out, has a significant minimum overhead even at its simplest.
The ability to create life synthetically also raises ethical and philosophical questions that go beyond biology. Synthetic biology has significantly changed established ideas about what life is and how it can be made, and the ethical implications of creating life from scratch are now a subject of active scholarly discussion.16PubMed Central. The view of synthetic biology in the field of ethics: a thematic systematic review
From Chemistry to Biology
If life is hard to define as a category, it may be because it emerged gradually rather than all at once. Origin-of-life research increasingly treats the transition from non-life to life as a process of protocell development rather than a single event. Protocells, simple compartments made of fatty-acid membranes enclosing simple chemical reactions, allow researchers to study how cooperation among chemical compounds can give rise to new, emergent properties that no individual compound possesses on its own.17PubMed Central. Investigating Prebiotic Protocells for A Comprehensive Understanding of the Origins of Life: A Prebiotic Systems Chemistry Perspective
Framing the origin of life as a series of stages in protocell evolution, rather than a single spark, shifts the question from “when did life begin?” to “at what point in the process did the system become alive?” And the honest answer may be that there was no clean threshold. A protocell population that could grow, divide, and respond to environmental pressure would have been on its way to being alive without clearly being alive yet. This gradualist view aligns well with the idea that the transition from chemistry to biology was a continuum, not a switch.18PubMed Central. Framing major prebiotic transitions as stages of protocell development: three challenges for origins-of-life research
Searching for Life We Would Not Recognize
Every definition of life discussed so far is shaped, to some degree, by the one example of life we know: Earth’s. When scientists search for life on Mars, Europa, or Enceladus, they typically look for familiar signatures like amino acids, lipid membranes, or metabolic byproducts. But a growing community of astrobiologists argues this approach is dangerously narrow. If alien life uses fundamentally different chemistry, familiar biosignatures would miss it entirely.
The alternative is to develop agnostic biosignatures: detection strategies based on the fundamental features of living systems, like chemical complexity or thermodynamic disequilibrium, rather than on Earth-specific molecules. These frameworks challenge both how we seek life and how we define it, because designing a detector forces you to articulate exactly what you think life must do regardless of its chemical basis.19Annual Review of Earth and Planetary Sciences. Agnostic Biosignatures: Expanding the Search for Life in the Solar System Mass spectrometry instruments designed for planetary missions are now being evaluated for their ability to detect non-Terran chemistry, casting a wider net than any previous life-detection mission.20Frontiers in Astronomy and Space Sciences. Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System
Meanwhile, theoretical work on alternative biochemistries is expanding what “self-sustained chemical system” could mean in practice. Researchers have analyzed candidate solvents beyond water, evaluating each for its ability to dissolve complex molecules, stabilize the building blocks of large polymers, and support chemical reactions.21PubMed. Alternative Solvents for Life: Framework for Evaluation, Current Status, and Future Research One particularly speculative proposal describes an entirely oxygen-free biochemistry built on carbon, hydrogen, and nitrogen, which could produce functional large molecules analogous to sugars, amino acids, and nucleic-acid bases. Such a system could, in principle, operate in the liquid hydrocarbon lakes of Saturn’s moon Titan.22PubMed. Oxygen-Free Biochemistry: The Putative CHN Foundation for Exotic Life in a Hydrocarbon World? Computational work has even shown that several alternative solvents, not just water, are thermodynamically compatible with the folding of complex polymers, a basic requirement for biochemistry to work at all.23PubMed Central. Solvent constraints for biopolymer folding and evolution in extraterrestrial environments
None of this means alien life exists, let alone that it uses exotic chemistry. But the research reveals something important about the definition question: the more seriously we think about what life could be, the harder it gets to write a definition that includes everything we’d want to call alive while excluding everything we would not.
Can Software Be Alive?
Digital organisms, self-replicating computer programs that mutate and evolve inside simulated environments, have been around since the early 1990s. They reproduce, they undergo selection, and they adapt. By the NASA definition, a population of digital organisms capable of Darwinian evolution looks like it should qualify as alive. Yet most biologists hesitate. The programs have no chemistry, no thermodynamic relationship with their environment, no physical boundary separating inside from outside.
The philosophical debate continues. Some researchers argue that certain digital organisms can be considered alive, while acknowledging that others clearly cannot, and that digital life may not even represent the most challenging boundary case.24arXiv.org. On the Liveliness of Artificial Life The question is whether life must be a physical, chemical process or whether the pattern of organization is what matters, regardless of what it is made from. This is not an idle thought experiment. As artificial intelligence systems grow more complex and autonomous, and as researchers build increasingly sophisticated digital simulations of biological processes, the question of where information processing ends and living begins is likely to get more urgent rather than less.