Human Evolution Has Slowed Down in the Last Million Years, Study Finds

The rate of genetic mutations in humans is significantly slower than our closest relatives—the other great apes—according to a study published in the journal Nature Ecology & Evolution.

Scientists from Aarhus University and Copenhagen Zoo, both in Denmark, sequenced the genes of chimpanzee, gorilla and orangutan families and compared the results to similar studies which were conducted on humans.

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"Genetic mutations are changes to the DNA molecule in a child that are not carried by either of the parents, suggesting that an error has happened during DNA replication in the germline—the cell line that ultimately leads to eggs or sperm—of either the father or the mother," Mikkel Heide Schierup, an author of the study from Aarhus University, told Newsweek.

"Genetic mutations happen very regularly over generations. Most do not have any effect except changing the DNA and can thus be used to time when different species became separate by looking at how different their DNA is (human and chimp DNA is 98.8 percent identical, for example,)" he said. "If we know the mutation rate per year we can then use the differences to time speciation events. For that, we need to know the rate of mutation and whether it has changed over time."

A smaller proportion of the mutations are bad for us and will be weeded out by natural selection over time, since their carriers have fewer children, according to Schierup. An even smaller proportion of new mutations provide adaptations to new environments and will be selected for by natural selection.

Over the past six years, several large studies have calculated the rate of mutation in humans, according to Søren Besenbacher, lead author of the study from Aarhus University.

"Until now, however, there have not been any good estimates of mutation rates in our closest primate relatives," he said in a statement.

Part of the reason the researchers conducted the study was because they thought the mutation rates in humans today are at odds with what we would expect further back in time.

"If we extrapolate the human rate, we estimate speciation times with the other great apes that do not fit the fossil record very well," Schierup said. "The obvious thing was, therefore, to determine whether the mutation rate per year in the other great apes is the same as in humans."

By studying families of apes, the researchers were able to look at how many mutations were occurring every year by comparing genetic differences between the generations.

"We sequenced [the genes of] father, mother, children trios very deeply and accurately and then found places in the children's genome where a new DNA variant was found which has not been seen in either the father or the mother," Schierup said. "We then compared the number of mutations acquired per year in humans and in the other great apes and found the latter rate to be larger."

Overall, their findings suggest that the annual mutation rate in humans is about one-third lower than in other great apes.

"[The key finding] is that the human mutation rate per year appears to have been reduced by 33 percent (i.e. 1.5 times larger in chimps, gorillas and orangutans) very recently—likely within the last one million years," he said. "Our evolution has thus slowed down, we do not know why."

"This means that timing speciation events among great apes likely has to be reconsidered," Schierup said. "For example, using the previous human rate, it is 10 million years for human-chimp speciation. With our new rate it is 6.6 million years. It might also have implications for the timing of the split between modern humans and Neanderthals. Presently, this was believed to be around 500 thousand years ago, but if the slowdown in mutation in humans that we observe is very recent then this implies that the split of humans and Neanderthals is also more recent than previously believed."

The latest paper could also have implications for conservation efforts, the authors say.

"All species of great apes are endangered in the wild," Christina Hvilsom from Copenhagen Zoo said. "With more accurate dating of how populations have changed in relation to climate over time, we can get a picture of how species could cope with future climate change."

This article was updated to include comments from Mikkel Heide Schierup and the headline was changed.

File photo: The rate of human genetic mutation has slowed down over the last million years. iStock