The first life forms had mutation rates 4,000 times faster than we see today, biologists have estimated, meaning astonishingly fast evolution.
Popular culture attributes mutations to radiation and toxic slime, but spontaneous mutations are also important. Most spontaneous genetic mutations are caused by cytosine deamination, the loss of an amine group (simple nitrogen-containing hydrocarbons) from cytosine, one of the four bases that make up DNA's alphabet of life.
The process turns cytosine to thymine, transforming the DNA code and altering the proteins produced. The rate of cytosine deamination rises with temperature. Since life first appeared on Earth at a time when temperatures were higher, mutation should have been more frequent.
Although this reasoning is not new, a team led by Professor Richard Wolfenden of the University of North Carolina has run the numbers in a paper in Proceedings of the National Academy of Sciences, and the figures are extraordinary.
"At the higher temperatures that seem to have prevailed during the early phase of life, evolution was shaking the dice frantically," Wolfenden said in a statement.
Wolfenden combined estimates of Earth's temperature at the time life appeared with measurements the effects of temperature.
"Recent evidence from rock samples in Australia indicates that life forms arose on Earth as early as 4.1 billion years ago – almost in the blink of an eye after the appearance of liquid oceans," Wolfenden said. At the time the Earth was still very hot. Indeed life may well have begun almost as soon as temperatures cooled to the point where its chemistry was possible.
Wolfenden points to species today that grow best at temperatures close to the boiling point of water, and even discoveries of organisms that thrive at 121°C (250°F). Although these could represent extreme modern adaptions, the paper adds, “Reconstructions of ancestral proteins, with amino acid sequences inferred from the sequences of their modern descendants, have been shown to be remarkably thermostable, with melting temperatures ∼30°C (54°F) higher than those of proteins from their modern descendants.”
Previous measurements have been made of the rate at which cytosine deaminates at higher temperatures, but Wolfenden expanded on these, measuring how temperature and acidity combine to affect molecules cytosine and similar molecules. While the pH in which the experiments were done made a difference, temperature proved by far the more important factor.
"Cytosine-based mutations, when the temperature was near 100°C, occurred at more than 4,000 times the modern rate," Wolfenden said. "To me, that was surprising. I thought the ancient rate would be more rapid than the modern rate, but not that rapid." Moreover, some other sources of spontaneous mutation are even more influenced by temperature than cytosine.
It is unclear how organisms, then and now, survive being faced with such forces disrupting their genes, but the work offers insight into how new life forms could have evolved so quickly, having many chances to get it right.
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