By David Brown
Washington Post Staff Writer
Thursday, May 18, 2006
When the ancestors of human beings and the ancestors of chimpanzees parted ways 6.3 million years ago, it was probably a very long goodbye. Some of their descendants may even have gone back for a final tryst.
That is the conclusion a group of scientists has reached, using a comparison of the genes of humans and their closest animal relatives to sketch a picture of human origins far more detailed than what fossil bones have revealed.
According to the new theory, chimps and humans shared a common apelike ancestor much more recently than was thought. Furthermore, when the two emerging species split from each other, it was not a clean break. Some members of the two groups seem to have interbred about 1.2 million years after they first diverged -- before going their separate ways for good.
If this theory proves correct, it will mean modern people are descended from something akin to chimp-human hybrids. That is a new idea, and it challenges the prevailing view that hybrids tend to die out.
It also strongly suggests that some of the oldest bones of "proto-humans" -- including the 7 million-year-old Toumai skull unearthed in Chad in 2001 -- may have belonged to a line of non-hybrids that died out, and were not human ancestors at all.
This narrative, by a team of geneticists and biostatisticians from the Broad Institute of the Massachusetts Institute of Technology and Harvard, not only casts new light on the origin of humans, but also raises questions about how all new species arise.
"This is contributing to the idea that species are kind of fuzzy. They become real over time, but it takes millions of years," said James Mallet, a geneticist at University College London who was not involved in the new research. "We probably had a bit of a messy origin."
The research is the latest fruit of the Human Genome Initiative, the effort to transcribe and read out the entire genetic message of human chromosomes, which was completed in 2003.
The evidence of ancestral chimp and human interbreeding emerged from comparing parts of their genomes to each other and to those of gorillas, orangutans and macaques. The scientists now want to know whether similar "hybridization events" happened between other emerging species.
The separation into two species "left a footprint on our genome that we can go back and read," said Eric S. Lander of MIT. "We were never able to look at things like this before. What we need to do now is to collect more data and look for other smoking guns."
Humans have 23 pairs of chromosomes that contain about 30,000 genes. Each gene is made of strands of DNA "letters" in a specific order, and the letters can change, by mutation, over time. The rate at which changes occur is fairly constant -- and very slow.
As a result, genetic mutations can be used as a kind of evolutionary clock. The number of DNA differences between two species' versions of the same gene is an indication of how long the species have been separate -- how long since individuals were last interbreeding and sharing genes.
When Nick Patterson of MIT and his colleagues at the Broad Institute compared the genes of humans and chimps, they found that one of the chromosomes -- the female sex chromosome X -- was 1.2 million years younger than the others. It appeared the two species shared a common ancestor who gave them both their X chromosomes, and did so more recently than the ancestors who gave them all the other chromosomes.
The best explanation, the scientists think, is that ancient humans and chimps broke away from each other not once, but twice. The first time was more than 6.3 million years ago. The second time was at least a million years later.
What probably happened was that some of the evolving human ancestors bred with the evolving chimps. This was perhaps not as strange as it seems, for although there were some physical differences between the two groups, "the early humans must have looked pretty much like chimpanzees," said Mallet, the London geneticist.
Males have only one X chromosome, which is necessary for reproduction. As is often the case with hybrids, the male offspring from these unions would probably have been infertile.
But the females, which have two X chromosomes, would have been fertile. If some of those hybrid females then bred with proto-chimp males, some of their male offspring would have received a working X from the chimp side of the family. They would have been fertile -- and with them the hybrid line would have been off and reproducing on its own.
The evolutionary clock indicates this happened no more than 6.3 million years ago, and perhaps as recently as 5.4 million years ago. In that case, the fossils of older species -- such as Toumai, or Sahelanthropus tchadensis, a proto-man from Chad that had a humanlike brow and probably walked on two feet -- must have belonged to descendants of the first human-chimp divergence.
That line must have died out. If it had not, modern man's X chromosome would look as old (or nearly as old) as the other chromosomes.
"I think the most interesting thing [is] this idea that long, extended gene flow seems to have occurred and that this might be a creative mode of evolution," said David Reich, a geneticist at Harvard Medical School. He is one of the authors of the study, which appears in today's issue of the journal Nature.
The idea that new species emerge in a slow and stuttering fashion was favored by Charles Darwin, Mallet said. But in the early part of the 20th century, biologists came to favor the idea of clean breaks, with the "pure" lines of emerging species being stronger and fitter than hybrids.
In fact, Mallet said, about 10 percent of animal species are capable of interbreeding with related species, even though the number that do so in any population is very small.