General · EdgepediaSeptember 17, 20266 min read

What Is Life?

What is life? is the question of what distinguishes living systems from non-living matter, and it remains one of the few fundamental scientific questions without an agreed answer. The most widely used working definition, adopted by NASA, is that "life is a self-sustaining chemical system capable of Darwinian evolution"¹. Despite significant progress in biology, no single definition has achieved universal acceptance², and the question has become more urgent as synthetic biology, artificial intelligence, and astrobiology each challenge traditional conceptions of what it means to be alive².

Key fact	Detail
NASA working definition	"A self-sustaining chemical system capable of Darwinian evolution," derived in the 1990s by a NASA exobiology working group; it is not used to guide mission design³
Consensus status	No single definition of life has achieved universal acceptance²
First recorded definition	Aristotle defined life as that which "grows, is self-sustaining and reproduces"⁴
Characteristics of known life	Terran life is chemical in essence, uses inherited enzyme catalysts and molecule-borne heritable information, exploits thermodynamic disequilibrium, and evolves by random variation plus natural selection¹
Requirements for life as we know it	Liquid water (all life on Earth seems to require it) and an energy source³
Boundary case	Viruses have their own genetic material and evolve but cannot live without a host, blurring the life/non-life distinction⁵
Practical use	Because a settled definition is lacking, astrobiology relies on frameworks such as the Ladder of Life Detection to assess biosignature evidence⁶

The NASA working definition

The definition NASA uses is described as less-than-formal, non-binding but helpful. It was derived in the 1990s by a NASA exobiology working group, and its purpose is to set expectations and focus debate on when non-life becomes life, rather than to guide mission design³. Gerald Joyce, a research professor at the Salk Institute in La Jolla, California, and a member of the working group that helped create the definition, summarized the underlying idea as "biology is chemistry with history": the chemistry records its own history, in our case inscribed in DNA³.

The definition has two components. "Self-sustaining chemical system" points to metabolism, a network of chemical reactions that sustains the organism, and "capable of Darwinian evolution" points to heredity, variation, and selection. Joyce suggests that a genuine record of information in molecules from another world, whether DNA, RNA, or something else, robust, complex, self-replicating, and open-ended, would be a "smoking gun" for life³.

Characteristics of the one known example

Because only one example of life is known, NASA's life-detection program lists the characteristics of Terran life rather than claiming a universal definition¹. Terran living systems are chemical in essence: they contain molecular species that undergo chemical transformations (metabolism) under the direction of enzyme catalysts whose structures are inherited, and the heritable information itself is carried by molecules. They exploit a thermodynamic disequilibrium to drive those transformations. Their biomolecules take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, and they interact with water in ways that confer fitness on the host organism. Living systems on Earth emerged through random variation in inherited biomolecules on which natural selection was superimposed, the central elements of the Darwinian paradigm¹.

At the level of everyday traits, living things are made of cells, maintain homeostasis, grow and develop, reproduce, metabolize, respond to the environment, and evolve over time; many nonliving things show a few of these characteristics but not all of them⁵.

Why no definition has stuck

The definitional effort has a long history. Aristotle was the first-recorded person to define life, as that which grows, is self-sustaining, and reproduces; as cell theory became universally accepted, a strictly cellular definition of life was also accepted⁴. Vitalism, the idea that life depends on a special non-physical force beyond chemistry and physics, arose in the 17th to 19th centuries in the work of thinkers such as Georg Stahl and Xavier Bichat⁷.

Modern analyses distinguish two kinds of definitions. Fixed definitions carry an ontological claim, aiming to carve out natural kinds by generalizing the characteristics of all living forms; flexible operational definitions carry an instrumental claim, serving as theoretical tools that guide debates and experiments⁸. A 2025 meta-analysis that used large language models to computationally analyze 68 expert-provided answers to "what is life?" found no universally accepted definition and offered the study as a methodological bridge between reductionist and holistic approaches². Some researchers argue the pursuit is premature while new species are still being discovered and hypotheses for life's origin still compete⁴.

Philosophers Carol Cleland and Christopher Chyba compared the problem to water: water was precisely defined as H₂O only after the advent of molecular theory, suggesting that a theory of life may be needed before life can be precisely defined⁹.

The virus problem

Viruses are the standard boundary case. They carry their own genetic material, act as biological machines, and evolve, yet they cannot live without a living host⁵. One position holds that viruses are strict molecular parasites unable to transform energy and matter, so that none of the discoveries about viral diversity challenge this basic essence¹⁰. Another view treats viruses as complex living entities that transform the infected cell into a novel organism, the virus, which produces virions¹¹.

The discovery of the mimivirus in 2003 and the later giant viruses rekindled the discussion, because these viruses build complex viral factories, reorganize cellular metabolism, and manipulate the expression of hundreds or thousands of genes with only a handful of expressed viral proteins⁴^•¹². Some writers on the subject conclude that whether viruses are living is a philosophical choice rather than a scientific question¹¹.

Life detection in practice

In astrobiology, the definition matters mainly because missions need criteria for what to look for. NASA has not tried to detect living organisms directly since its Viking missions of the late 1970s; to help scientists and engineers evaluate and deliberate about candidate biosignatures and measurements for detecting life, the Ladder of Life Detection offers a framework.⁶

For life as we know it, the leading requirements are liquid water, which all life on Earth seems to require and which provides a medium for chemical components to persist and react, and an energy source, needed both for building structures and for creating order against entropy³. A tell-tale sign is chemical disequilibrium: in Earth's atmosphere, oxygen and methane are highly reactive with each other and should not coexist, so something must be constantly replenishing them, and on Earth that something is life³.

Because any single sign can have non-biological causes, astrobiologists have proposed a confidence framework: a 2021 sample framework uses a scale from 1 to 7, from hints of other life at level 1 to certainty of life elsewhere at level 7, and it is still being discussed and revised³. Identifying definitive signs is difficult enough for life as we know it; evidence for life with unfamiliar molecular combinations or a solvent other than water would be still more uncertain³.

References

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026