It is highly contested.
But in “Mars Mission Critics Misunderstand Chemistry” (Philly.com, August 5, 2012 ), Faye Flam announces to the great unwashed,
NASA defines life as any self-replicating system that can undergo Darwinian evolution. It’s a broad enough denfinition that it allows life without carbon, but there are reasons that carbon is well-suited to the task.
Characteristically, for legacy media science writers, Flam does not appear to know that NASA’s definition of life is under serious attack within the discipline.
For example, evolutionary biologist Bjørn Østman comments, riffing off a recent talk by “no arsenic” Rosie Redfield, on conventional definitions of life according to which “living things would be those which evolve by natural selection.” He proposes a thought experiment. Noting that languages are not alive but do evolve in various ways, he asks,
Suppose we go to another planet and find one being there, looking exactly like a human being. Everything we can measure about this being confirms that it is just as much alive as you and me. It eats, moves, heals, replenishes, communicates, feels, defecates. Learning more about this being, though, we find that it has no ancestors, and that it does not age. It does not reproduce, and it is the only such being on the planet. Thus, there is no lineage of descent and no population that can evolve. So this being is then not alive? Of course it is. This definition does not work.
Astrobiologist Charley Lineweaver offers,
What is the unit of Darwinian evolution? Is it the gene? Is it the cell? Is it a multicellular organism? Is a city evolving? How about Gaia? Is that a life form?
And so forth. They are only two of many dissenters. At a recent conference, many other definitions were offered.*
But, basically, if you are a legacy media science writer, Darwin is right even when he doesn’ t- or his followers don’t – make any sense.
Just for the record (nil nisi bonum, you know): The actual Darwin – not the current atheist cult figure, but the actual old Brit toff heir of the Wedgwood pottery fortune – prudently stepped around the origin of life mess, as he would have prudently stepped around other messes in the street. And good on him.
*Edward N. Trifonov. Vocabulary of Definitions of Life Suggests a Definition. J. Biomol Struct Dyn 29(2) 259-266 (2011).