by Andre on 10 September 2007
Life is a great thing to think about. Not only can it be bewilderingly (and satisfyingly) complex, it’s quite relevant to us humans and for whatever reason we feel a certain harmony with it. We have an intuitive feeling for what is alive and what is not. Mice, cod, and pines—alive. Rocks, tables, and air—not. But when it comes down to giving a precise definition, it can be surprisingly difficult. All sorts of fringe cases refuse to simultaneously fit with our intuition and our definition. For an interesting review of some of the modern definitions that have been proposed, see John Wilkins’ recent post. Sean Carroll also comments and includes his candidate for a tricky case: the Milky Way itself.
Defining life can be a fun puzzle, but I don’t normally get too excited about it. In some ways it’s like defining scientific disciplines: some things are clearly physics or biology or chemistry, but lots of interesting things happen at the boundaries and it’s more interesting and productive to go after interesting problems than to worry too much about whether what you’re doing fits into a particular category. Similarly, it’s more interesting to study simple life, try to make it in the lab, or look for it on other worlds than it is to give a precise definition of what it is. On the other hand, the things that people decide to include in their definitions are a nice reflection of the state of our understanding and sense of fashion. There’s been a nice progression in sophistication and subtlety even if the result isn’t totally satisfactory.
When life started might seem to be a more concrete question but it suffers from the same kind of blurry boundaries as the question of what life is, and for the same reasons. If we limit ourselves to life on Earth (but why should we?), we can say that 5 billion years ago, there was no life, 3 billion years ago, there was. On this topic, I’d recommend that you watch this video of an informal talk by Craig Venter hosted by Edge. He has some interesting things to say about the diversity of life, but also discusses the possibility of panspermia and mentions that even if we discover evidence of life on Mars we won’t know if it had a unique origin there until we have a better understanding of the gene repertoire here on Earth (unless of course it turns out to be a completely different form of life). In any case, at some point, a transition was made between non-living and living and deciding exactly when that was relies on having a good definition of life, but again, deciding when it happened is not nearly as interesting as learning how it happened.
On this, there are also some interesting contributions at Edge. Freeman Dyson speculates about a possible “garbage bag model” for the first living (or almost living) things and Robert Shapiro gives perhaps the most important contribution to the “how” portion by discussing the implausibility of life starting with RNA (although his framework doesn’t really argue against the RNA world, it simply excludes it from being the first step in prebiotic chemistry). I tend to agree with Seth Lloyd that we probably won’t know for sure what conditions gave rise to life on Earth, but we might be able to demonstrate convincingly that complex chemical processes of the kind that would be required to create life are possible or even likely. I think that would already be very exciting, because it might not get directly to our distant roots, but instead might give us a glimpse of the roots of our distant cousins that might exist elsewhere. This approach also has the advantage that even if you don’t discover plausible life origins you might discover interesting complex chemistry and that’s almost sure to have its own rewards, practical or otherwise.
For some interesting further reading, check out Carl Zimmer’s latest article in Seed.