Humans, chimps split 4 million years ago: study

WASHINGTON (Reuters) - A new study, certain to be controversial, maintains that chimpanzees and humans split from a common ancestor just 4 million years ago -- a much shorter time than current estimates of 5 million to 7 million years ago.

The researchers compared the DNA of chimpanzees, humans and our next-closest ancestor, the gorilla, as well as orangutans.

They used a well-known type of calculation that had not been previously applied to genetics to come up with their own "molecular clock" estimate of when humans became uniquely human.

"Assuming orangutan divergence 18 million years ago, speciation time of human and chimpanzee is consistently around 4 million years ago," they wrote in their study, published in the Public Library of Science journal PLoS Genetics, available online here#toclink4.

"Primate evolution is a central topic in biology and much information can be obtained from DNA sequence data," Dr. Asger Hobolth of North Carolina State University said in a statement.

The theory of a molecular clock is based on the premise that all DNA mutates at a certain rate. It is not always a steady rate but it evens out over the millennia and can be used to track evolution.

Experts agree that humans split off from a common ancestor with chimpanzees several million years ago and that gorillas and orangutans split off much earlier. But it is difficult to date precisely when, although most recent studies have put the date at somewhere around 5 million to 7 million years ago.

Hobolth and colleagues from the University of Aarhus in Denmark and the University of Oxford in Britain looked at four regions of the human, chimpanzee, and gorilla genomes.

They used a statistical technique called the hidden Markov model, developed in the 1960s and originally applied to speech recognition.

What they found directly contradicts some other recent research. They found evidence that it took only 400,000 years for humans to become a separate species from the common chimp-human ancestor.

Just last May, David Reich of the Broad Institute at the Massachusetts Institute of Technology and Harvard Medical School's Department of Genetics found evidence that the split probably took 4 million years to occur, although his team put the final divergence at just 5.4 million years ago.

Reich's study of chimpanzee and human DNA suggested that the early ancestors of humans and the ancestors of chimpanzees may have interbred for a long time before they separated.

Experts have long known that humans and chimpanzees share much DNA, and are in fact 96 percent identical on the genetic level.

And one year ago, Soojin Yi and colleagues at the Georgia Institute of Technology said they found genetic evidence that chimpanzees may be more closely related to humans than to gorillas and orangutans.

Their look at the molecular clock showed humans evolved one unique trait just a million years ago -- our longer life span and our long childhood that means humans reach sexual maturity very late in life compared to other animals.

Other experts in genetics were not immediately available to comment on Hobolth's report.