By Maggie Fox, Health and Science Editor

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 at http://genetics.plosjournals.org/perlserv/...030007#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. Continued...

…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….

… 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….

… "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 at…

If it takes only 400,000 years or less time to form separate species, there could be far more species than commonly portrayed in most charts of ancient hominids.

That is not news, is it? If you have a common ancestor of 5 million years with chimps, both chimps and humans will necessarily be the same distance from gorillas or about 7 million years removed from the common ancestor. It makes me suspicious of the science background of the writer. Something doesn't seem right.

I have always heard a divergence time of about 12 million years from the orangutan lineage. If that were correct, I wonder if it would make a chimp more like 2.666 million years removed.

The calculations in the genetics study seem a bit over my head but it doesn't seem to fit the fossil record. I suppose it is possible since there are no fossil chimps of that age. I think they found more recent teeth but I am not sure if they would be enough to disqualify the study. If they became quadrupedal from a biped, as some recent evidence suggests, I guess they may have done it later rather then have to have split from the first likely biped.

If it takes only 400,000 years or less time to form separate species, there could be far more species than commonly portrayed in most charts of ancient hominids.