By Rebecca Wilbanks
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 recognise 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 neighbour’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. As the origins-of-life researcher Pier Luigi Luisi at Roma Tre University puts it, there is life-as-it-is-now, life-as-it-could-be and life-as-it-once-was. These categories point to a dilemma that medieval mystical philosophers addressed. Life, they noticed, is always more than the living, making it, paradoxically, permanently inaccessible to the living. Because of this gap between actual life and potential life, many definitions of life focus on its capacity to change and evolve rather than trying to pin down fixed characteristics.
In the early 1990s while advising NASA on the possibilities of life on other planets, the biologist Gerald Joyce, now at the Salk Institute for Biological Studies in California, helped to come up with one of the most widely used definitions of life. It’s known as the chemical Darwinian definition: ‘Life is a self-sustained chemical system capable of undergoing Darwinian evolution.’ In 2009, after decades of work, Joyce’s group published a paper in which they described an RNA molecule that could catalyse its own synthesis reaction to make more copies of itself. This chemical system met Joyce’s definition of life. But nobody wanted to claim that it was alive. The problem was, it hadn’t done anything new or exciting yet. A New York Times article put it this way: ‘Someday their genome may surprise their creator with a word – a trick or a new move in the game of almost life – that he has not anticipated. “If it would happen, if it would do it for me, I would be happy,” Dr Joyce said, adding, “I won’t say it out loud, but it’s alive.”’
Picture: National Human Genome Research Institute [Public domain], via Wikimedia Commons