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 âTwo rabbitsâa male and femaleâare alive but either one alone is dead.â In 1994, a NASA committee deemed that life is âa self-sustaining chemical system capable of Darwinian evolution.â Thatâs 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.
Asking 'What is life?' erroneously implies that the word has an essential meaning independent of how we choose to use it. Like many words, 'life' works fine when its precise definition doesn't matter but becomes problematic when we want to use it to make critical distinctions. ('Species' has the same problem.)
Thus, rather than asking "What is life?", it's better to ask "What definition of 'life' would best capture describe the important scientific distinction we wish to make?"
Once we put the problem that way, the best solution I see is to set the word aside and instead of anguishing about whether or not a phenomenon is 'alive', ask whether it is subject to natural selection. 'Natural selection' is well-defined even at its boundaries, and it captures the real scientific issues much better than 'life' does.
I had a series of posts on the topic a few years ago, Josh:
What is life? It's life if you can kill it! LOL!
I don't like to see "Life" used as a noun, although I use it that way myself sometime. I think it is much better to think of living things--to use living as an adjective. That moves to the question from "What is life?" to, "Is that a living thing over there in the corner?"