Monday, June 18, 2007

More on the origins of life.

Yesterday, I read a fantastic article in Scientific American about the chemical origins of life. It started off with a review of the RNA world hypothesis, which essentially postulates that life began by the chance formation of a catalytic RNA molecule which had the ability to self-replicate. At this point, however, it has become obvious that the odds of that happening are so extraordinarily astronomical that it's almost certain that something else came first. Even in the famous Stanley Miller experiment, there were only amino acids formed; no sugars or bases. In addition, ribose has proved nearly impossible to synthesize under the prebiotic chemical conditions.

This all led to the metabolism-first hypothesis, the main subject of the article. It's a lot more general than the RNA-first hypothesis, and requires much smaller molecules. What it boils down to is an energetically downhill redox reaction of a mineral fuelling an uphill organic reaction, say A to B. A is eventually regenerated by a stepwise series of exergonic reactions; in other words, a rudimentary cycle. The hypothesis then says that chemical and physical interference with the reactions would cause them to find alternative routes to get back to A, forming more complex networks, and the opportunity to form catalysts. The author goes as far as saying that nucleotides originated as some form of catalyst or energy-producing reaction (after all, ATP is a pretty universal energy-storing compound). The jump to RNA would be made by the chance polymerization of nucleotides, something far far more likely than a de novo formation. And now we've got to the RNA world.

The argument that the article seems to have the most trouble dealing with is how this system of reactions stores information. The author suggests that the chemicals themselves store it in a "compositional genome," which is replicated by the diffusion of chemicals from one physical compartment to another. This theory leaves open the possibility of something like a chemical founder effect - a drastic change in concentration occurs, and a new network is formed. This would in turn allow for the selection of some chemical systems that are better able to grow in concentration than others, and there you have it: evolution. It's a pretty good theory, but it seems a little lacking because it doesn't specify the barriers that would separate these systems, since they would only travel by passive diffusion. How would a new cycle "pinch off" from an old one?

At any rate, it's clear that something that can clearly be defined as life occured before RNA. This sounds like a pretty damn good idea, but it needs a little more data and a little more theory. I'm too lazy right now to look up the actual papers. Maybe if I'm feeling particularly bored I'll read some and post about them.

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