What is life?
The story. It is slightly embarassing that scientists who study the origin of life and the possibility of life on other worlds, like me, cannot actually say what life is. Neither can other scientists, of course, but for them it is not so important. It is clear when a mouse or a fern is alive, because we can see lots of examples of alive and not-alive mice and ferns (Bacteria can be trickier.) But when you want to know whether you have discovered life on an alien planet, it would be nice to be able to say what you were looking for first.
NASA has taken the lead here, because NASA funds the majority of work in astrobiology and the vast majority of the space program looking for life. Their definition is usually quoted as:-
‘Life is a self-sustained chemical system capable of undergoing Darwinian evolution’
My take. Sorry, NASA, but I think this is wrong. Evolution happens, of course - to deny that is to pretty much give up on thinking completely. But it is not helpful as a definition, and almost useless as a way of detecting life. For reasons that I summarise in this paper, trying to 'define' life is probably useless. We know life when we see it, so what is it we are recognizing? I come up with
1. Structure, that is highly improbable in its environment
2. Dynamic maintenance of that structure, through activity that is characteristic of the organism
3. Occurrence of groups of similar organisms that can be distinguished as a natural group.
4. Substrate-independence: living things are determined by an internal code, not (solely) by their external environment.
The open question. Is this enough? If we sent a probe to an alient planet and asked it to detect just those four things, would it only detect life? I think there is at least one other criterion ... but you will have to wait for the next paper for that!
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