Friday, October 22, 2010

rules for complex life

There was an article in yesterday's The New Scientist which lends support to claims I made in an earlier series of posts on evolution. According to the article, once complex organisms come into existence, they're freed from a host of constraints. This might allow their subsequent development to seem meteoric by comparison. The problem is crossing that initial hurdle from simplicity to complexity in the first place.
Once freed from energy restraints, genomes could expand dramatically and cells capable of complex functions – such as communicating with each other and having specialised jobs – could evolve. Complex life was born.

So if Lane and Martin are right, the textbook idea that complex cells evolved first and only later gained mitochondria is completely wrong: cells could not become complex until they acquired mitochondria.

Simple cells hardly ever engulf other cells, however – and therein lies the catch. Acquiring mitochondria, it seems, was a one-off event. This leads Lane and Martin to their most striking conclusion: simple cells on other planets might thrive for aeons without complex life ever arising. Or, as Lane puts it: "The underlying principles are universal. Even aliens need mitochondria."
It's a bit different from what I was saying. I was siding with Peter Ward and his hypothesis that life has a self-destructive tendency. It's not deliberately suicidal of course, but it has had a tendency to go in directions which were "unsustainable", i.e., which led to drastic decreases in genetic diversity and productivity over the long haul. I suggested that if the Medea hypothesis were true, it might explain why it took so long (about 3 billion years) for multicellular life to form on Earth.

Lane and Martin are suggesting something different though complimentary. Putting aside the fact that life often goes off in directions which are unsustainable, there's an energy threshold that has to be reckoned with in order to move from simplicity to complexity in living things. To put their idea in my own words, it's not enough to grow larger; things have to become more internally complex. And it turns out that transition from external complexity (concatenating cells) to internal complexity (adding intracellular parts like the mitochondria) is relatively difficult. Or at least it seems to be, given how long it took to happen after the initial abiogenesis.

One of the intriguing things about Lane and Martin's idea is that it's generalizable. It's just a function of the relationship between surface area and total volume. So as long as the life form in question has to occupy good old 3-dimensional Euclidean space, the same rules are going to apply there. Anything we find in the cosmos above the level of simple organisms like bacteria or archea will have to have something analogous to mitochondria. Though what the odds are of encountering anything that is, is anyone's guess.

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