HARVARD GAZETTE ARCHIVES
Did ancestral humans, chimps interbreed?
Findings cloud human, chimp origins
By Alvin Powell
Harvard News Office
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, conducted by Reich, Nick Patterson, Daniel Richter, and Sante Gnerre of the Broad Institute, and Eric Lander, of the Broad Institute and the Massachusetts Institute of Technology, was published in the May 17 online edition of the journal Nature.
Under the hybridization scenario, the ancestors of humans and chimpanzees split from their common ancestor. After that initial separation, however, members of the two new species interbred an unknown number of times. Fertile hybrid offspring then may have mated back into one or both original populations, bringing in genes from the other species and leaving traces in the genetic code.
"The genome analysis revealed big surprises, with major implications for human evolution," said Lander, who serves as Broad Institute director. "First, human-chimp speciation occurred more recently than previous estimates. Second, the speciation itself occurred in an unusual manner that left a striking impact across chromosome X. The young age of chromosome X is an evolutionary smoking gun."
By pushing human-chimpanzee speciation up by a million years, the research raises questions about several prominent fossils, such as the Toumai fossil, Sahelanthropus tchadensis, which scientists have dated to between 6.5 million and 7.4 million years ago. Though it has humanlike features, the fact that it pre-dates the time when human ancestors finally split from chimpanzee ancestors throws its place on the human family tree into doubt, or suggests that it had a more complex role in the human family tree than previously thought.
In addition to their implications for human evolution, Reich said the findings may cause scientists to re-examine beliefs about speciation and the role of hybridization. Current thinking is that although hybrids do occasionally occur in nature, they are sterile or less fit than the parent populations and so eventually die out.
It may be the case, however, that the rare hybrid is fit enough to survive, which would make hybridization between species a creative process in evolution, rather than a negligible happenstance, as is now thought.
"Maybe hybrids that successfully adapt occur only once every million years," Reich said, adding that if hybridization between human and chimp ancestors did occur, "Either we're the hybrids or the chimpanzees are the hybrids, but we can't tell which.