When we ask where life came from and what it looks like, we are asking which qualities define the contours of life – pattern or structure, simplicity or complexity, form or information, metabolism or reproduction. This means that life is a concept – it bears meaning and the force of signification; it carries historical baggage, philosophical expectations and scientific framings. Life is something about which people – life scientists and others – must be persuaded. But of course, to say that life is up for debate certainly does not damn the scientists who seek it and sometimes stumble upon its wily facsimiles. On the contrary, it means that our theories of life are spectacularly lively.

The Mars 2020 Mission will collect new rocks and might instigate renewed debate about extraterrestrial lifelike fossils. We don’t yet know what fantastic forms these might take, but history suggests that many Martians will be, at best, dubious.