Human, Chimp Lineages Interbred After Splitting, Study Suggests
 

Probably the most shocking aspect of Darwin’s theory of evolution has always been its implication that we descend from ape-like ancestors. But that idea may be easy to stomach compared with new findings.

A study has concluded that human and chimp ancestors may have interbred for a long time after their two lineages began to split apart evolutionarily.

The research also found the final separation was more recent than previous research suggested.

“The study gave unexpected results about how we separated from our closest relatives, the chimpanzees,” said David Reich of the Broad Institute of Harvard University and the Massachusetts Institute of Technology in Cambridge, Mass.

Reich is senior author of a paper detailing the findings, published in the May 17 online edition of the research journal Nature.

“Something very unusual happened at the time of speciation,” he added.

Speciation, the evolutionary branching off of new species from existing ones, is the key mechanism that creates new species, according to evolutionary theory. Since chimps are our closest relatives, our speciation from them would be the pivotal event that put us on the road to humanhood.

Previous genetic studies have focused on the average genetic difference between human and chimpanzee across their genomes.

By contrast, the new study scrutinized the variation in evolutionary history across the whole human genome. In theory, some regions of the genome should be “older” than others, the researchers explained. That is, different regions should have characteristics traceable to different times in the evolutionary history of the common ancestors of humans and chimps.

This analysis led to three surprising conclusions, the scientists said:

• The time from the beginning to the end of the splitup ranges over more than 4 million years across of the genome. In other words, the date of the divergence seems different depending on where in the genome you look—suggesting the process may have been gradual, and marked by interbreeding.
• The youngest genomic regions are surprisingly recent, no more than 6.3 million and probably no more than 5.4 million years old. This would suggest the final speciation itself occurred on the same time frame, more recently than scientists previously thought.
• The X chromosome, which contributes to sexual characteristics, falls almost completely at the more recent end of the time frame.

Chromosome X’s young age is a “smoking gun” for interbreeding, said Eric Lander, a co-author of the paper and director of the Broad Institute.

Interbreeding is known to produce strong pressure for evolutionary change—called selective pressure—in sexual characteristics, the scientists said. That, they added, could explain the chromosome’s young age.

The researchers said their estimate for the time of the final splitup is more recent than previous figures based on studies of the famous Toumaï fossils, widely thought to be the oldest from the human family. Those previous estimates put the divergence time at between 6.5 million and 7.4 million years.


Skull of "Toumaï" or Sahelanthropus tchadensis, thought to be the earliest fossil from the human family tree. If the results of a new study are correct, it could have come from a time when the chimp and human lineages had begun to split, but were still interbreeding. (courtesy M.P.F.T.)

The Toumaï fossil may itself be “more recent than previously thought,” said the institute’s Nick Patterson, one of the authors of the new study. “But if the dating is correct, the Toumaï fossil would precede the human-chimp split. The fact that it has human-like features suggests that human-chimp speciation may have occurred over a long period with episodes of hybridization between the emerging species.”

Hybridization, or interbreeding, is thought to play a common role in plant speciation, but not usually in animals. However, the apparent lack of such events among animal species, Reich said, “may simply be due to the fact that we have not been looking for them.”

- Courtesy The Broad Institute of MIT and Harvard
and World Science staff