By Dan Vergano

Humanity’s genetic split from an ape-like ancestor came about 13 million years ago, far earlier than the long-supposed era of a common ancestor of early humans and apes, suggests a first study of chimp gene mutations.

Along with shining a new genetic light on human origins, the findings published on Thursday in the journal Science point to the role that evolution plays in fostering mutations, some linked to inherited diseases, in our genes.

On the surface, this and other recent studies contradict the general consensus suggested by the fossil record: that the last common ancestor of the two species, a flat-footed ape, lived some seven million years ago.

But both observations could still be true, said paleoanthropologist John Hawks of the University of Wisconsin, Madison, who was not involved in the new study. The ape-like common ancestor species might have endured until 7 to 10 million years ago, long after the genetic split between chimps and humans, he said.

That would largely explain the difference seen between gene-based and fossil-based estimates of the date the species diverged.

“We also don’t know if mutation rates varied widely in the ancient past; maybe they were different than now,” says study senior author Gil McVean of the United Kingdom’s Wellcome Trust Centre for Human Genetics in Oxford.

That could also change estimates of the age of an ancestral genetic split between men and chimps. “That’s something only looking at more primates might help tell us,” McVean says. (See video: “Curious Chimp.“)

Chimp Mutation Rates

According to the analysis released Thursday, the promiscuity of chimps seems to drive a higher genetic mutation rate in chimp males. Gene mutations in chimp offspring also greatly increases with the age of the ape father. (See video: “Baby Chimp.”)

“You really don’t want an older chimp as a father,” says McVean. “The vast majority of mutations are, at best, neutral,” he says, though some can be “very harmful.”

Chimps and people share about 99 percent of their DNA, making them our closest living genetic relatives. The rate at which human genes are altered, or mutate, has come under wide study in the past five years. Some studies have raised concerns about older fathers passing disease risks on to their children. But in chimps, the generational mutation rate was uncertain, until now.