Definition of life
Defining life is hard. Scientists in biology, chemistry, and space study it in different ways. Abiogenesis looks at how life could begin from non-living matter, while astrobiology searches for life beyond Earth.
Life on Earth is incredibly diverse—from tiny microbes to huge trees. Because of this variety, there is no single simple definition that fits everything.
A common quick definition lists traits like keeping a stable state (homeostasis), growing, reproducing, and eventually dying. But these traits aren’t perfect. Some non-living things show similar behavior (crystals grow), and some living things don’t fit every trait.
Biology offers a practical baseline: all known life on Earth is made of cells, and life passes its information to offspring mainly through DNA and RNA. This makes a useful working definition for science, but not a complete one for life everywhere.
Definitions vary in how strict they are. Four main approaches exist:
- all features must be present,
- one essential feature,
- several necessary features,
- or several life types with different traits.
Many scientists favor the third approach.
Some thinkers, like Carol Cleland, argue that language vagueness makes a strict definition hard. Instead, we may need a broad theory of living systems, especially to recognize alien life when we find it.
Most life we know is carbon-based and uses water as a solvent. Some worry about “carbon chauvinism,” the assumption that life must look like Earth life. We can’t be sure what life elsewhere might be like.
NASA defines life as a self-sustaining chemical system capable of Darwinian evolution. But evolution is a process over generations, not something that happens inside a single organism, and it might not apply the same way to all life.
Viruses and prions blur the line between living and non-living because they don’t reproduce on their own, yet can spread and improvise. They challenge simple definitions of life.
Historically, people debated whether a mysterious “vital force” powered life. Later ideas shifted to chemistry and physics, with Darwin showing that evolution is a natural process. The origin of life is now seen as a chemical, not a separate magical process.
Today, most scientists view life and its origin as problems understood through chemistry. We still don’t know exactly how the first living things began or how to recognize the very first organism.
This page was last edited on 2 February 2026, at 22:12 (CET).