Virginia Hughes, once the benevolent overlord here at Scienceblogs, asks the Question of the Year: What is Life, Anyway? She notes that many of the major scientific discoveries or advances of the year hinged on that question, and this month’s Astrobiology has a series of essays on the state of our understanding. She explains:
Is life simply the ability to reproduce? Well, no. If that were true, as one scientist famously noted, then “Two rabbits—a male and female—are alive but either one alone is dead.” In 1994, a NASA committee deemed that life is “a self-sustaining chemical system capable of Darwinian evolution.” That’s not quite right, either: certain non-living crystals, for example, can seed the growth of new crystals and even pass down traits such as molecular orientation. Some scientists have much broader definitions: life as a triad of a program, a metabolism and a container (the PMC model), for example, or life as a series of negative feedback loops.
It’s remarkable how hard it is to nail down the definition of life, and fascinating to see folks trying to work through the issue. A big part of the problem is that we only have one example to work from. It’s easy to say that life has to involve nucleic acids and proteins and lipids and a cell membrane and so forth, because life on earth works that way. But it doesn’t have to. Life elsewhere might not even have cells, and life on earth might not have started that way. If that first life still exists on earth, we probably couldn’t even recognize it.
So whether we’re looking on other planets or on earth, it matters a lot what we think life means. It also matters in thinking about synthetic biology, bioethics, and a range of other issues. And if nothing else, it matters because biology is the study of life, and it’d be nice to define what that means.
This is, by the way, another reason that philosophy of science is really important. John Wilkins has a nice post exploring the general importance of philosophy for scientists.