Did ancestral humans, chimps interbreed?

New scientific findings indicate that ancestral humans split from chimpanzee forebears more recently than previously thought and raise the possibility that the two nascent species hybridized before making their final separation.

The surprising findings reveal a much more complicated birth of the human and chimpanzee species than shown by previous research. They also call into question the place on the primate family tree of fossils that scientists had thought were the bones of ancestral humans, but which are older than the newly determined time that the species diverged.

The research, conducted by scientists at Harvard Medical School, the Massachusetts Institute of Technology (MIT), and the Broad Institute of Harvard and MIT, indicates that humans and chimpanzees developed into distinct species less than 6.3 million years ago and probably more recently than 5.4 million years ago. That is about a million years later than the previously accepted range of 6.5 million to 7.4 million years ago.

Researchers made the findings after examining differences between the chimpanzee and human genomes. Because mutations in DNA occur at a steady rate, scientists are able to compare the changes between species and figure out how long ago they last had a common ancestor.

Previous studies have used average figures to get a time that the species split, but scientists have long known that segments of the genetic code are inherited from different ancestors who lived at different times. This study was the first to trace segments back and provide a range of dates within which the common ancestors of humans and chimpanzees lived.

What the results reveal is a surprisingly large range. Different segments of the genome differ in age by about 4 million years, researchers found. That large range of dates could be explained if there had been some genetic exchange between the two developing species over that time.

The parts of the genome traceable to more recent ancestors are clustered on the X chromosome, which could be explained by natural selection working to eliminate unfavorable genes caused by hybridization.

“The data were very, very unexpected and difficult to explain by what we knew,” said David Reich, assistant professor of genetics at Harvard Medical School.

Reich cautioned that though hybridization would answer several questions raised by the research, the research itself does not prove that hybridization occurred. Further work is needed to explore whether that happened.

The research was published in the May 17, 2006 online edition of the journal Nature.