before it's in the papers"
August 03, 2010
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Human, chimp lineages interbred after splitting, researchers say
and World Science staff
Probably the most shocking aspect of Darwin’s theory of evolution, especially to its critics, has always been its implication that we’re descended from ape-like ancestors.
But that idea may be easy to stomach, compared with the findings of a new study.
The research concluded that human and chimp ancestors may have interbred for a long time after their two lineages began to split apart evolutionarily. The study 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 Harvard Medical School in Cambridge, Mass., senior author of a paper detailing the findings. It appears in the May 17 online edition of the research journal Nature.
“Something very unusual happened at the time of speciation,” he added. Speciation is the evolutionary branching off of new species from old ones, the key mechanism that produces new species according to evolutionary theory.
Previous genetic studies had focused on the average genetic difference between human and chimpanzee across their genomes.
By contrast, the new study examined the variation in evolutionary history across the entire human genome. In theory, some regions of the genome should be “older” than others, the researchers explained. In other words, different regions should have characteristics traceable to different times in the evolutionary history of the common ancestors of humans and chimps.
The study, the first to measure the range of these ages, 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 different parts of the genome. That unexpectedly wide time span suggests the splitup may have been gradual, and marked by interbreeding.
· The youngest regions are unexpectedly 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 wholly at the more recent end of the time frame.
“The young age of chromosome X is an evolutionary smoking gun” for interbreeding, said Eric Lander, a co-author of the paper and director of the Massachusetts Institute of Technology’s Broad Institute in Cambridge, Mass.
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, thought to be the earliest remnants from the human family. Those estimates were between 6.5 million and 7.4 million years.
The Toumaï fossil may itself be “more recent than previously thought,” said MIT’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 suggest 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.”
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