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Human, Chimp Ancestors May Have Mated, DNA Suggests

Brian Handwerk
for National Geographic News
May 17, 2006
 
Early human ancestors and chimpanzee ancestors may have mated and produced offspring, according to a new DNA study.

The study suggests that the human and chimp lineages initially split off from a single ape species about ten million years ago. Later, early chimps and early human ancestors may have begun interbreeding, creating hybrids—and complicating and prolonging the evolutionary separation of the two lineages.

The second and final split occurred some four million years after the first one, the report proposes.

"One thing that emerges [from the data] is a reestimate of the date when humans and chimps last exchanged genes," said David Reich, a professor at Harvard Medical School's Department of Genetics in Boston.

"Our data strongly suggest that [the last gene exchange] occurred more recently than 6.3 million years ago and probably more recently than 5.4 million years ago," said Reich, senior author of the study, to be published tomorrow in the journal Nature.

"This paper is very interesting, because it provides a hypothesis that is outside of the currently accepted dogma," said Kateryna Makova, a professor at Pennsylvania State University's Center for Comparative Genomics and Bioinformatics who is unaffiliated with the study.

(Related news: "Chimps, Humans 96 Percent the Same, Gene Study Finds" [2005].)

Did Human, Chimp Ancestors Hybridize?

"The genome analysis revealed big surprises, with major implications for human evolution," biologist Eric Lander said in a release announcing the findings. Lander is director of the Broad Institute, a cooperative institute for genomic medicine in Cambridge, Massachusetts.

A genome is an organism's complete set of DNA. The human genome, for example, contains some 3 billion base pairs, which code for the approximately 30,000 genes that define a person's unique traits. (See our quick overview of human genetics.)

"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," Lander said.

This sex-determining chromosome typically occurs in pairs in cells of females and combined with a Y chromosome in cells of males.

"The young age of chromosome X is an evolutionary smoking gun."

Different regions of the human and chimp genomes were found to have diverged at widely different times, and the two species' X chromosomes show a surprisingly recent divergence time.

This genetic evidence boosts the theory that the two species may have hybridized, because interbreeding causes strong selective pressure on the X chromosome and could have resulted in that chromosome's very young age in both humans and chimpanzees.

Different species can, and sometimes do, mate to produce hybrid offspring. Horses and donkeys produce mules, for example. Likewise, "rheboons" are the offspring of female baboons and male rhesus macaques.

Because most hybrid offspring can't reproduce, evolutionary biologists don't believe hybridization plays a significant role in the long-term evolutionary success of new animal species.

But recent research has found some evidence of the process in the development of unique fly and fish species.

"That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them," Harvard's Reich said.

Reich also explained that the new study doesn't prove that hybridization occurred.

"It's the only explanation that we could imagine," he said. "But there may be others that we can't imagine."

Penn State's Makova notes that a chimpanzee genome sequencing project she took part in pointed to "male mutation bias" as a possible cause of the X chromosome's young appearance. "Male mutation bias" is the term for a higher mutation rate in males than in females.

"To obtain a final answer … analysis of complete sequences of orangutan, macaque, and marmoset will be very helpful in obtaining better understanding of this question."

Researchers are currently working to generate those sequences, and studies of the Y chromosome in primates may also help to paint a clearer picture of the chimp-human split.

DNA and Fossil Evidence

If the new evolutionary timeline estimates prove correct they will raise interesting new questions about the status of notable fossils such as Sahelanthropus tchadensis, a species believed to be an early human precursor.

S. tchadensis, or Toumaï fossils have been dated to the proposed interval period between the initial divergence and the final human-chimp genetic split.

The Toumaï species has what are considered distinctive human features and consequently has been regarded as evidence that the lineages must have split before Toumaï's era, estimated at 6.5 to 7.4 million years ago.

"It is possible that the Toumaï fossil is more recent than previously thought," Nick Patterson, a senior research scientist at the Broad Institute, said in a statement.

"But if the dating is correct, the Toumaï fossil would precede the human-chimp split. The fact that it has humanlike features suggests that human-chimp speciation may have occurred over a long period, with episodes of hybridization between the emerging species."

Such a possibility raises some interesting questions about the relations of humans, chimps, and other primates.

"Are we the hybrids, or are chimps the hybrids—or are we both the hybrids?" Reich pondered.

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