Chimps and Humans: Closer Than We Thought?

Chimpanzees and humans shared a common ancestor millions of years ago. Then they split into two separate branches, with one leading to modern human. Biologists have always assumed that the split was clean. But new DNA evidence suggests the two populations intermingled for some time after the split.

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STEVE INSKEEP, host:

Some other news now. Scientists have some new information about one of our ancestors.

Consider this a bit of Darwinian genealogy. It's the story of an ape-like creature that existed long ago. Eventually it split into two new species. One branch would lead to chimpanzees, and another would lead to us.

Evolutionary theory holds that such a split ultimately separates the two new species. They do not normally reproduce with each other. But scientists studying DNA say that common ancestors of humans and chimps may have spent millions of years hovering between the two lines, mating and swapping genes.

NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE reporting:

Some scientists who study chimp and human evolution use the term chuman to refer to this common ancestor, which lived at least five million years ago. But let's make it clear that this new research does not suggest that early chimps were mating with pre-humans.

These were two new species, just beginning to emerge from an ape-like common ancestor. As Harvard University's David Reich points out, it would be millions of years before the two lines evolved into what we know now as chimp and human.

Professor DAVID REICH (Harvard University): These species were much more similar to the ancestral populations than they were to either us or to chimpanzees today. So thinking about it in terms of a mixture between modern humans and modern chimpanzees is a completely inappropriate way to think about it.

JOYCE: But during the process of splitting, members of the two new species, well, it seems they wavered.

The two populations kept crossing back.

Prof. REICH: That's right. And that leaves an impact on the pattern of genetic variation that we see when we compare the human and chimpanzee genomes.

JOYCE: Reich is a geneticist, not a fossil hunter. He and colleagues at Harvard and MIT looked at all the genes in chimps and humans, their genomes. The difference between the two is only about three or four percent. But you can tell a lot from the pattern of those differences.

Prof. REICH: Instead of a kind of homogeneous pattern across the genome, which would be expected, you see a very unusual pattern with some sections very much more recently diverged, and very much less different from each other than in other sections where they're much more diverged. And we're using that to really learn about the history at this time, five or six million years ago.

JOYCE: When the protochimp line split from the protohuman line, the genes of each new line slowly accumulated changes. They diverged. Think of how the Model T has slowly diverged into Mustangs and Tauruses and Pintos. The longer it's been since the split, the more differences you'll see in the two genomes.

Writing in the journal Nature, Reich says it's the patterns he sees in regions of the human and chimp genomes that suggests this persistent hybridizing. Some genes indicate a very old split, others a much more recent divergence. In fact, it apparently took at least four million years for the species that would be man and the one that would be chimp to finally break it off and go their own ways.

Scientists who study evolution say this hypothesis makes sense. It takes a long time to make a new species. But it took the sequencing of human and chimp genomes to see the evidence, and it may take similar studies of other closely related species to prove it.

INSKEEP: Christopher Joyce, NPR News.

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