What is life? For much of the 20th century, this question did not particularly concern biologists. Life is a term for poets, not scientists, argued the synthetic biologist Andrew Ellington in 2008, who began his career studying how life began.
Despite Ellington’s reservations, the related fields of origins-of-life research and astrobiology have renewed focus on the meaning of life. To recognize the different form that life might have taken 4 billion years ago, or the shape it could take on other planets, researchers need to understand what, in essence, makes something alive.
Life, however, is a moving target, as philosophers have long observed. Aristotle distinguished ‘life’ as a concept from ‘the living’—the collection of existing beings that make up our world, such as the neighbor’s dog, my cousin and the bacteria growing in your sink. To know life, we must study the living; but the living is always changing across time and space. In trying to define life, we must consider the life we know and the life we don’t know.