DON’T even bother asking what life is. Coming up with a definition right now is impossible, according to US researchers.
In recent decades, scientists have been worrying about this question more than ever. Without a definition, how will astronomers looking for life on other planets know if they’ve found it? If we created life artificially, would we even know? Last week, news that scientists had made a polio virus from scratch sparked renewed discussions about what is alive (91av, 20 July, p 6).
But in an upcoming issue of Origins of Life and Evolution of the Biosphere, philosopher Carol Cleland of the University of Colorado at Boulder, and biologist Chris Chyba of the SETI Institute, California, argue that we should admit defeat – for now. It won’t be possible to define life until biologists have a theoretical explanation of it, they say.
Advertisement
They use the historic struggle to define water to support their argument. Before the theory of atoms and molecules was developed in the 18th and 19th centuries, water had scientists stumped. Definitions such as “an odourless, colourless, thirst-quenching liquid” came unstuck when considering things like acid solutions, muddy water, ice or steam.
“You can argue over which characteristics of water are fundamental, but without molecular theory, a precise, unambiguous definition is impossible. That’s where we are with our definitions of life,” says Chyba. We might never work out a theory that explains life. If so, “this argument is going to be interminable”.
In the meantime, the hunt for alien life continues. Jupiter’s moon Europa, one of the most likely homes for extraterrestrial life in our Solar System, was named earlier this month as a priority for exploration by the US National Research Council. But NASA’s working definition of life, “a self-sustained chemical system capable of undergoing Darwinian evolution”, is far from ideal. It risks excluding some novel life forms that could exist, such as ones that replicate so haphazardly that natural selection is not an option. And there’s a practical problem – just how long are you going to hang around on Europa to see whether potential life forms are evolving?
Instead, the search will have to rely on a list of expected characteristics, such as the presence of complex organic molecules, or entities with a morphology that can’t be explained by chemistry alone.