What is life? How biologists tell living from non-living

Life has no single agreed definition, so biologists decide what is alive by a set of traits all living things share: they are made of cells, run chemical reactions to use energy, keep their insides steady, grow, reproduce, respond to their surroundings and evolve over generations. NASA's working definition compresses this to one line: life is "a self-sustaining chemical system capable of Darwinian evolution."

In short

• Life is recognized by a checklist of traits, not by one test, and every known living thing on Earth passes all of them.
• Edge cases such as viruses, prions, fire and crystals each pass some tests and fail others, which is why definitions are still argued.
• The trait that does the most work is evolution: copying with errors that are themselves inherited.

What do all living things have in common?

Every organism known, from a bacterium to a blue whale, shares seven properties. Textbooks word them differently, but the list is stable.

1. Organization in cells. Every living thing is made of one or more cells, each wrapped in a membrane that separates inside from outside.
2. Metabolism. Living things take in matter and energy and run chemical reactions with them, building some molecules and breaking down others. All of those reactions together are its metabolism.
3. Homeostasis. They hold their internal conditions within narrow limits, such as temperature, water and acidity, against a changing outside: this is homeostasis.
4. Growth. They get bigger or more complex by building new material, not just by piling it on from outside.
5. Reproduction. They make more of their kind, by splitting, budding or sex.
6. Response to stimuli. They react to their surroundings: a plant bends toward light, a bacterium swims toward sugar.
7. Evolution. Over generations, populations change, because offspring inherit their parents' traits with small variations, and some variations survive better than others.

The traits depend on each other. A cell is the container that lets metabolism run without everything diffusing away. Metabolism pays for growth, repair and copying. Reproduction with inherited variation is what lets populations adapt, as evolution by natural selection explains.

Why isn't there one definition everyone accepts?

Because every short definition either lets in things that aren't alive or shuts out things that are. One compilation, cited by Wikipedia's article on life, counted at least 123 published definitions. Part of the trouble is that life is a process, something matter does, rather than a substance such as gold or water that can be pointed at.

The chemist Steven Benner tells the story in a 2010 paper, Defining Life. The biochemist Daniel Koshland recalled a committee that agreed "the ability to reproduce" was life's essential feature, until one voice objected: "Then one rabbit is dead. Two rabbits—a male and female—are alive but either one alone is dead." Lists of traits have the same weakness: a mule is sterile, yet nobody doubts it is alive.

The definition NASA uses came from a committee it assembled in 1994. Following a suggestion by Carl Sagan, the committee, summarized by the chemist Gerald Joyce, proposed that life is "a self-sustaining chemical system capable of Darwinian evolution," a wording the NASA Astrobiology Program still quotes. Each phrase does a job. "System" means a population over time, so a single sterile mule belongs to something that is alive. "Self-sustaining" rules out anything that needs constant outside help to keep going. And "Darwinian evolution" means copying with errors, where the errors themselves are passed on. A definition is not a test, though. In 1976 the two Viking landers ran three life-detection experiments on Martian soil, and, as Benner recounts, all three gave positive results. Scientists still concluded there was no life there, because another instrument found no organic molecules in the soil to speak of, and life as we know it is built from them. Gilbert Levin, who designed one of the experiments, still argued when Benner wrote that it had detected life.

Is it alive? Testing the edge cases

The checklist is most useful on things that fail part of it. Here is how five well-known edge cases score, with a bacterium for comparison.

Case	Cells	Own metabolism	Reproduces	Evolves	Usual verdict
Bacterium	Yes	Yes	Yes, by splitting	Yes	Alive
Virus	No	No	Only inside a host cell	Yes	Argued
Prion	No	No	Converts normal proteins into copies of itself	Strains can change, but carry no genes	Not alive
Fire	No	Consumes fuel and oxygen	Spreads	No inherited variation	Not alive
Salt crystal	No	No	A broken crystal seeds new ones	Defects aren't inherited	Not alive
Dried seed or tardigrade	Yes	Almost none while dormant	Yes, once revived	Yes	Alive, but dormant

Three of these repay a closer look.

Viruses carry genes, mutate and evolve by natural selection, but they have no metabolism and can't make anything outside a host cell. Whether that makes them alive is argued, as viruses versus bacteria sets out.

Prions are proteins, not organisms. The U.S. National Institute of Allergy and Infectious Diseases describes how a host's normal prion protein can malfunction and assemble into aggregates that cause infectious, fatal brain diseases such as Creutzfeldt-Jakob disease in people and mad cow disease in cattle. A prion spreads by refolding the healthy protein it touches. It copies a shape, not a genetic message.

Crystals are Benner's example of why heredity matters. A crystal of sodium chlorate can be crushed and used to seed new crystals, and even its handedness carries over. But the countless defects in the parent crystal are not copied into the offspring, so there is nothing for natural selection to work on.

Dormancy shows the opposite problem. A seed in a packet, or a dried-out tardigrade, has its metabolism almost stopped and can stay that way for years, then revive with water. Biologists count these as alive because the machinery is intact and only paused.

Where does the energy for life come from?

Every living thing is a pocket of order that has to be paid for. Left alone, organized systems run down, which physics describes as rising entropy. A living cell stays organized only by taking in energy, from sunlight or from food, and releasing heat and waste to its surroundings. The physicist Erwin Schrödinger put this question at the center of his 1944 book What Is Life?, based on lectures he gave in Dublin in 1943.

That is why metabolism appears on the checklist at all. Fire also burns fuel and releases heat, but it builds nothing that lasts and passes nothing on. A cell uses part of the energy it takes in to rebuild itself and copy its instructions, which fire never does.

What is the most common misconception about life?

The most common one is that life is whatever moves, breathes or eats. Plants don't move from place to place, many bacteria live without oxygen, and a dormant seed does none of these. Movement and breathing are things many animals do, not what makes something alive.

A second is that "alive" has a sharp edge. The edge cases above show that it doesn't: viruses sit on a border that depends on which traits a definition counts most. Biologists treat the checklist as a description of the life we know, which all descends from a common ancestor and has existed on Earth for at least 3.5 billion years, according to Wikipedia's summary of the evidence. Whether life elsewhere would share it is exactly what astrobiologists want to find out.

Still argued

Whether viruses are alive has no settled answer. One side counts genes and evolution as the heart of life and includes them; the other counts cells and metabolism and leaves them out. The facts about viruses are not in dispute, only which traits a definition should require.

Check yourself

• Without looking back, name the seven traits of living things and give one example of each.
• Why does a crystal that seeds new crystals still fail NASA's definition of life?
• Explain why a sterile mule is alive even though it can't reproduce.

Sources

• About Life Detection — NASA Astrobiology Program
• Defining Life — Steven A. Benner, Astrobiology, 2010
• Prion Diseases — National Institute of Allergy and Infectious Diseases
• Life — Wikipedia
• What Is Life? — Wikipedia