A little hot monkey lovin'

Humans, chimps may have bred after split

By Gareth Cook, Globe Staff | May 18, 2006

Boston scientists released a provocative report yesterday that challenges the timeline of human evolution and suggests that human ancestors bred with chimpanzee ancestors long after they had initially separated into two species.

The researchers, working at the Cambridge-based Broad Institute of Harvard and MIT, used a wealth of newly available genetic data to estimate the time when the first human ancestors split from the chimpanzees. The team arrived at an answer that is at least 1 million years later than paleontologists had believed, based on fossils of early, humanlike creatures.

The lead scientist said that this jarring conflict with the fossil record, combined with a number of other strange genetic patterns the team uncovered, led him to a startling explanation: that human ancestors evolved apart from the chimpanzees for hundreds of thousands of years, and then started breeding with them again before a final break.

''Something very unusual happened," said David Reich, one of the report's authors and a geneticist at the Broad and Harvard Medical School.

The suggestion of interbreeding was met with skepticism by paleontologists, who said they had trouble imagining a successful breeding between early human ancestors, which walked upright, and the chimpanzee ancestors, which walked on all fours. But other scientists said the work is impressive and will probably force a reappraisal of the story of human origins. And one leading paleontologist said he welcomed the research as a sign that new genetic information will yield more clues to our deep history than once thought.

''I find this terrifically exciting and important work," said David Pilbeam, a Harvard paleontologist who was not part of the Broad team.

Pilbeam helped discover an early human ancestor known as Toumai, which walked on two legs and is thought to have lived in present-day Chad 6.5 million to 7.4 million years ago. The new report, published in today's issue of the journal Nature, estimates that final break between the human and chimpanzee species did not come until 6.3 million years ago at the earliest, and probably less than 5.4 million years ago.

This contradiction could be resolved, Reich said, if early creatures like Toumai then interbred with chimpanzee ancestors, leaving a population of hybrids that developed into today's humans. (In this scenario, the line of Toumai creatures then went extinct.) But it is also possible, he said, that the dating of the early human fossils is wrong, or that the dating of other, older fossils used in his calculations is wrong, which would partially undercut the interbreeding theory. Scientists said that the report will probably bring intense scrutiny, as researchers look for potential flaws in the work or other explanations for its findings.

The work will also probably inspire biologists to devote more attention to hybrids, the term for offspring with parents of different species, and the role that they may play in fueling evolution. Biologists have long known about hybrids -- a half-grizzly bear, half-polar bear was recently discovered in Canada -- but it has been assumed that these were generally lone animals that had had little impact on the story of evolution. The Nature paper joins a wave of work showing that the lines between species are hazy, according to James Mallet, a biologist who studies hybrids at University College London.

As two species evolve, they can develop new abilities. Some hybrids could combine the best of both species, Mallet said, though the biological barriers to the creation of hybrids increase the longer the species are apart. It is thought that human ancestors were adapting to life on the savannah instead of the forest, where chimpanzees still live today. It is not known why human ancestors would have begun mating with chimpanzee ancestors again, or why they would have stopped.

To understand how long ago humans split from chimpanzees, Reich and his colleagues did a close study of DNA from the two. This technique rests on the idea that once the populations separate, the DNA will slowly drift apart as natural mutations accumulate. If they can count the number of changes, and determine how quickly the changes happened, then they can calculate how long the two populations have been separate, according to Nick Patterson, a scientist who was part of the Broad team.

Previous studies have used this idea and found that the two species split between about 5 million and 8 million years ago.

The Broad team sought to get a more precise answer by looking at how different the DNA of chimps and humans is at many locations, instead of calculating an average difference. The DNA of humans and chimpanzees is quite similar, meaning that scientists can readily identify many segments of DNA that are so similar they must have been handed down by a common ancestor, deep in the past. Scientists can then use a computer to put the segments of human and chimp DNA into alignment, placing side by side the segments that are very similar.

For each pair of segments, they then calculated how long it would have taken to accumulate all the differences. The team used sophisticated statistical techniques to calculate these ''divergence times."

This analysis brought surprises that the team could explain only by suggesting human ancestors and chimpanzee ancestors interbred. First, they found that the divergence times varied widely. Some parts of the DNA seemed to indicate the human and chimpanzee species had been apart much longer than others, by millions of years. If humans split from chimps and then interbred before splitting again, the more divergent DNA sequences could date to before the first split, while the less divergent sequences could date to just before the second split.

The other surprise was that sequences from the X chromosome, one of two chromosomes that determine gender, gave consistently more recent divergence times, instead of the range seen on other chromosomes. This, too, would be explained by the idea of interbreeding, according to the report. The X chromosome is thought to be the focus of fertility problems in hybrids, and population models suggest that all of the X chromosomes in a hybrid population would quickly come to match those of one of the parent species. This would explain why the human and chimpanzee X chromosomes are so similar.

Although the idea is controversial, there will soon be a wealth of more information to test it. Part of the Broad team's analysis relied on using DNA sequences from the gorilla and other primates as a kind of baseline to interpret their results. Only a relatively small amount of DNA has been sequenced from gorillas, limiting the amount of data the team could use. By the end of 2007, there should be a full sequence of the gorilla, allowing the scientists to do a much fuller analysis, Reich said.

The team also plans on looking at genetic data for other groups of closely related species to try to determine whether those species split apart fairly abruptly, or whether there is evidence that hybridization is a common part of evolution, bringing together the best of two species.

 

You know what? I realized this right after I posted.

I think that while I was posting, I was just so amused that I could post a thread titled "A little hot monkey lovin'" that I sort of lost sight of where the discussion would head.

Sorry