Last week I wrote about the Age of Dinosaurs, and how great extinctions make way for the emergence and diversification of new species. That's interesting and all, but the reason I was thinking about that stuff is because of that book I read about the RNA World hypothesis.
I was disappointed that the book didn't throw out more concrete examples of what the RNA world might have looked like. I wanted to see a tree that started with an RNA molecule that could self-replicate, branching out to RNA molecules with other interesting functions, and even more branching to complex ribocytes (RNA cells), all leading to a primitive cell containing all the molecules and functions common to all life on earth. I also wanted it to be annotated to include the order in which particular capabilities (like proofreading in replication) were achieved. I wanted a lot from that book, and mostly what it gave me was a description of the experiments people are doing to determine if an RNA world was possible.
But it did talk about a milestone that would have changed the RNA world drastically: the switch from using RNA as a catalyst to using protein (which everyone says is much better at catalysis). Once the genetic code was established, it would have provided such an advantage to any cell that used it to synthesize catalytic proteins, that that cell would have out-competed any ribocyte. And there we have our first great extinction, from which all living beings arose.
The problem (and the reason I didn't get my wish from that book) is that there is no evidence of the extinct RNA cells. Unlike dinosaurs, amphibians, fish, and even multicellular invertebrates, ribocytes leave no fossils. The earliest fossils we have are single celled organisms, colonies of cyanobacteria, which could photosynthesize, and were therefore much smarter than ribocytes.
So what do we do? Could anyone have imagined dinosaurs without the fossils? Could you have hypothesized the Hallucigenia (above, from Dave's Landslide Blog), with its upward pointing spines and downward pointing feet (which, when it was first identified in the fossil record, was presumed to walk on the spines, with the feet up in the air)? Or the Anomalocaris (below)? The lengths to which life will diversify when given the chance are obviously much farther than anyone can imagine.