The lineages of humans and chimpanzees, our closest relatives, diverged from one another about 4.1 million years ago, according to a new estimate that is said to be far more precise than previous ranges for this critical evolutionary moment.
However, the claim is a bad match with previous estimates based on fossil evidence and other genetic work.
Asger Hobolth of North Carolina State University and his colleagues arrived at the new estimate of " the time we became human," or the time in the past when descendents of the human-chimp ancestor split into human and chimp, by statistically comparing DNA from four regions of the human, chimp and gorilla genomes.
The new divergence date is considered fairly recent, maybe too much so. Previous estimates, based on fossil evidence, put the most recent common ancestor of humans and chimps on Earth anywhere from 2 million to 10 million years ago, Hobolth said.
"Primate evolution is a central topic in biology and much information can be obtained from DNA sequence data," Hobolth said in a prepared statement.
The team also came up with a fairly large estimate for the size of the ancestral population of the primates just before human and chimp species evolved from it—about 65,000 individuals. Other primates would have existed at the time, too, but not all were ancestors of ours.
The new estimate supports claims that recently discovered primate fossils, the Millennium man (Orrorin tugenesis) and Sahelanthropus, are not on the human lineage but belong rather to an ancestral lineage from which both humans and chimps evolved.
The results are detailed in the February issue of the journal PLoS Genetics.
The new estimate fails to square up with previous molecular estimates for the divergence date, not to mention the fossil evidence on hominids, said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History.
"You have bipedal hominids by 4 million years ago, and 4.2 million years ago you have definite fossil evidence of bipeds around the place, part of a knee joint and a lower ankle joint that are pretty good indicators," Tattersall told LiveScience. "And then you have more arguable fragmentary back 6 million years ago."
"Chimps are knuckle-walkers and hominids are bipeds," he said, "and it's inconceivable that you could have a common ancestor to both at 4 million years ago when you already have evidence in the hominid lineage that there were bipeds already around at that time."
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Robin Lloyd was a senior editor at Space.com and Live Science from 2007 to 2009. She holds a B.A. degree in sociology from Smith College and a Ph.D. and M.A. degree in sociology from the University of California at Santa Barbara. She is currently a freelance science writer based in New York City and a contributing editor at Scientific American, as well as an adjunct professor at New York University's Science, Health and Environmental Reporting Program.