Chimpanzees and humans 'split' later than thought

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, according to a study published on Friday.

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 www.genetics.plosjournals.org

"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.

Molecular clock

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 human, chimpanzee, and gorilla genomes.

What they found may contradict some other recent research. They found evidence that it took only 4 million years for humans to become a separate species from the common chimp-human ancestor. Experts have long known that humans and chimpanzees share much DNA, and are in fact 96 per cent identical on the genetic level.