Scientists Explore Early Hybrids

The giant gorilla King Kong was never able to consumate his affection for his beloved Ann Darrow because it is not biologically feasible, but things might have turned out differently if he had been a chimpanzee.

After early human and chimpanzee ancestors branched off from a common ancestor, they may have mated to create a hybrid species, according to a new study by researchers at the Broad Institute of MIT and Harvard published in the journal Nature last week.

The study also found that the divergence between humans and chimps occurred nearly a million years later than previously estimated.

“The thing that we’ve shown very solidly is there was a complex speciation between humans and chimpanzees,” said David Reich, an assistant professor of genetics at Harvard Medical School and the senior author of the Nature paper.

A comparison of the DNA sequences of humans and chimps revealed that the complete divergence of the two species took over four million years, Reich said.

The evidence indicated that the decisive split between humans and chimpanzees “must have occurred more recently than 6.3 million years, and probably [even] more recently than 5.4 million years,” Reich said.

These figures differ from previous estimates that the human-chimp split occurred 6.5 to 7.4 million years ago, a figure that was based on hominid fossils with distinctive human features dating from that period.

“It suggests that there was gene flow after the development of human-like features, like upright walking,” Reich said.

To explain the discrepancy between the appearance of human-like features and the later divergence date, the Broad researchers hypothesize that interbreeding and hybridization may have occurred.

In short, early humans and chimps may have mated and produced offspring before the two species permanently split.

These hybrid species may have developed into either the modern humans or chimpanzees, although fossils seem to point to humans, Reich said.

The interbreeding hypothesis might also explain why the X chromosome shared by humans and chimps is about 1.2 million years more recent than any other chromosome, Reich added.

When humans and chimpanzees diverged into distinct evolutionary branches, the two species hardly had the cleanest of break-ups.

So far, scientists have believed that “speciation” occurs when an isolation barrier such as a geographic impediment divides a population and mating stops, Reich said.

However, that theory is not applicable to the drawn-out separation of humans and chimps discovered by the researchers, according to Reich.

Reich said he hopes to conduct further research looking into the question of whether speciation processes involving hybridization might have been more common in the animal kingdom.

—Staff writer David Zhou can be reached at dzhou@fas.harvard.edu.