WHEN life began in the hostile conditions of early Earth, so many random mutations and errors would have plagued the first molecules struggling to copy themselves that explaining how longer or more complex forms ever evolved has been tough. But it seems that the right combination of random events or “biological noise” counteracted the high mutation rate, speeding up evolution.
A certain level of mutation is vital to provide the variation on which natural selection acts. But high levels of mutation limit the maximum length a molecule such as RNA can reach, because too many mutations will be introduced in each generation for the molecule to keep functioning. To get longer molecules, you need a protein that assists copying and lowers the mutation rate.
But how could such a protein have evolved? “It’s a chicken and egg situation,” says David Krakauer of the Santa Fe Institute in New Mexico. “To have a protein, you’ve got to have a really long string [of RNA]. But you can’t have a long string to build that protein, because mutation is so high.”
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Another source of biological noise called drift only makes matters worse. Natural selection is not very good at picking out small differences in fitness. So in a pool of quite similar molecules, chance can ultimately dictate which individuals replicate and which ones don’t, and the gene pool drifts. “Drift can get rid of the best individuals,” says Lauren Ancel Meyers of the University of Texas at Austin.
How did self-replicating systems ever get going at all? Krakauer and his colleague Akira Sasaki of Kyushu University in Fukuoka, Japan, realised that there was one more source of error that had not been included in the models – developmental noise. These are random errors that occur during an organism’s development, but are not passed on to the next generation. For example, even if the DNA of a cell is copied perfectly, mistakes can still occur in RNA transcription, or in protein-folding.
When Krakauer and Sasaki included such errors in their models they found that they actually made the fittest self-replicating molecules seem fitter (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2002.2127). “Here comes developmental noise to the rescue,” says Krakauer. A developmental error that wouldn’t cause a problem for a robust individual, for example, could be a serious handicap for another individual already weakened by mutation. That means developmental noise can exaggerate the differences in fitness between individuals in a population, allowing natural selection to act more quickly to improve replication.
The higher the level of developmental noise, the more mutations a population can tolerate, so replicators can evolve longer lengths than allowed by theories that only consider mutation and drift. “The analysis provides us with additional insights,” says evolutionary theorist Peter Schuster. But he says it does not fully explain how early life could have evolved.
“We are still in trouble,” admits Krakauer. “But it’s a lot better than it would have been.” He thinks developmental noise will be even more critical in the evolution of multicellular organisms. “Once we move to that next level of complexity it becomes even more crucial to consider it,” he says.