DON鈥橳 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鈥檝e 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 (快猫短视频, 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 鈥� for now. It won鈥檛 be possible to define life until biologists have a theoretical explanation of it, they say.
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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 鈥渁n odourless, colourless, thirst-quenching liquid鈥� came unstuck when considering things like acid solutions, muddy water, ice or steam.
鈥淵ou can argue over which characteristics of water are fundamental, but without molecular theory, a precise, unambiguous definition is impossible. That鈥檚 where we are with our definitions of life,鈥� says Chyba. We might never work out a theory that explains life. If so, 鈥渢his argument is going to be interminable鈥�.
In the meantime, the hunt for alien life continues. Jupiter鈥檚 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鈥檚 working definition of life, 鈥渁 self-sustained chemical system capable of undergoing Darwinian evolution鈥�, 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鈥檚 a practical problem 鈥� 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鈥檛 be explained by chemistry alone.