The study by scientists at the Massachusetts Institute of Technology and Harvard Medical School show the two species split no more than 6.3 million years ago, and probably less than 5.4 million years ago. Moreover, researchers say the speciation process was unusual -- possibly involving an initial split followed by later hybridization before a final separation.
"The study gave unexpected results about how we separated from our closest relatives, the chimpanzees," said David Reich, senior author of the study and an assistant professor in Harvard Medical School's department of genetics. "We found the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time of speciation."
Previous molecular genetic studies focused on the average genetic difference between human and chimpanzee. By contrast, the new study exploits the information in the complete genome sequence to reveal the variation in evolutionary history across the human genome.
The research appears in the online edition of Nature.