"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 hybridsor are we both the hybrids?" Reich pondered.
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