The ancestors of humans and chimpanzees may have begun genetically diverging from one another 13 million years ago, more than twice as long ago as had been widely thought, shedding new light on the process of human evolution, researchers say.

Scientists also discovered that male chimps pass on far more genetic mutations to their offspring than male humans do, revealing previously unknown evolutionary differences between the species.

Chimpanzees are the closest living relatives to humans, so studying chimps can help scientists learn more about human evolution. [Unraveling the Human Genome: 6 Molecular Milestones]

The number of genetic differences between two species reveals how closely related the species are. By estimating the rate at which mutations occur, researchers can then determine when the ancestors of species such as humans and chimpanzees may have diverged. Here, estimates of mutation rates act like "molecular clocks" that help scientists pinpoint when key moments in evolution occurred.

But, calibrating how fast these molecular clocks actually tick can be challenging; the molecular clock of one species might conceivably tick faster or slower than that of another species, the scientists said. Researchers usually try to overcome this challenge by comparing molecular clocks to the fossil record to see when species diverged. Yet, ages gleaned from the fossil record are often somewhat imprecise.

One way to directly pin down the rate of mutation in a species is to compare members of that species with their progeny. The genes that children get from their parents may possess mutations caused by factors such as radiation, mutation-triggering chemicals or errors during cell division. By counting the number of genetic changes that accumulate over generations, scientists can estimate the rate at which mutations occur in that species.

Past estimates of when the ancestors of humans diverged from chimps suggested the most recent common ancestor of both species lived about 6 million years ago. However, in the past decade or so, genetic analyses revealed the human mutation rate is actually half as fast as was previously thought, suggesting the most recent common ancestor of humans and chimps actually lived at least 12 million years ago.

The chimp and human split

Now a new study of chimp mutation rates appears to confirm that the most recent common ancestor of humans and chimps lived about 13 million years ago.

"Our results add substance to the idea that the human-chimpanzee split was considerably older than has been recently thought," said study co-author Gil McVean, a geneticist at the Wellcome Trust Centre for Human Genetics in Oxford, England.

In humans, the average mutation rate is about one mutation per 2 billion base pairs per year. (The spiraling double strands of DNA are made of pairs of molecules known as bases.) Each person inherits, on average, about 70 new mutations from his or her parents.

To see if chimpanzees have similar patterns of mutation, scientists analyzed nine related western chimpanzees (Pan troglodytes verus) spanning three generations. The researchers found the overall chimpanzee mutation rate was mostly the same as the human one.

"Our results indicate that human and chimp ancestors' genomes would diverge by about 0.1 percent every million years, so when we see divergence of 1.2 percent, we infer that it must have been about 12 million years — 13 million years is our actual estimate," McVean told Live Science.

Paleoanthropologist John Hawks at the University of Wisconsin-Madison, who did not participate in this study, noted that 13 million years is only the average time for when the genes of the ancestors of humans and chimps diverged; it's not necessarily when the ancestors of humans and chimps split into different species.

"A species divergence of 7 million to 10 million years would be just fine with a genetic divergence averaging 13 million years if the common ancestor population was very large in numbers, or the common ancestor population was spread into different subpopulations with reduced mixing between them," Hawks said. [8 Humanlike Behaviors of Primates]

McVean agreed with Hawks' analysis. If the size of the ancestral population of both humans and chimps was very large, then their common gene pool may have begun diversifying long before the ancestors of humans and chimps split into different species, he said.

Male chimps drive mutations faster than humans

Interestingly, researchers found male chimps pass on seven to eight times more mutations to the next generation than do female chimps. In comparison, male humans only pass on three or four times more mutations than female humans. Overall, the offspring of chimpanzees inherit 90 percent of new mutations from their fathers, and just 10 percent from their mothers, the scientists said.

The researchers also discovered that male chimpanzees potentially contribute three more mutations to their offspring with each year of age. Human males potentially add just two new mutations to progeny each year they age, the researchers said.

"We have shown how the mutation process, which determines so many things, can differ, even between closely related species," McBean said.

Male humans and chimps contribute more mutations to offspring than do females because male mammals make sperm all their lives, while females are born with all the eggs they are ever going to have. This means that males can accumulate mutations in their sperm with age, but the females' eggs mostly remain genetically unchanged over time.

One explanation for why male chimps pass on more mutations than male humans may have to do with differences in mating behavior. Male chimps have evolved to produce many more sperm than humans, possessing testes more than three times the relative size of human testes. This greater level of sperm production increases the opportunity for new mutations to emerge.

Since mating behavior could explain why male chimps contribute more mutations than human males, that means gorillas potentially have reduced mutation rates compared to humans. Gorillas encounter less competition for mates and have smaller testes relative to humans.may "It is possible that direct estimates of the mutation rate in gorillas would lead to a re-evaluation of the split times," McVean said. "We'd love to do the experiment in gorillas."

The scientists detailed their findings in the June 13 issue of the journal Science.

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