Chimp-human split fuzzy?

When the ancestors of human beings and the ancestors of chimpanzees parted ways 6.3 million years ago, it was probably a long goodbye. Some of their descendants may even have gone back for a final tryst.

That's the conclusion scientists have reached by comparing the genes of human beings and their closest animal relatives to sketch a picture of human origins far more detailed than what fossil bones have revealed.

According to the new theory, chimps and humans shared a common, apelike ancestor much more recently than previously thought. When the two emerging species split, it wasn't a clean break. Some members seem to have interbred 1.2 million years after they first diverged — before finally going their separate ways for good.

If this theory proves correct, it means modern people are descended from something akin to chimp/human hybrids. That's a new idea, and it challenges the prevailing view that hybrids tend to die out.

It also strongly suggests that some of the oldest bones of "proto-humans" — including the 7 million-year-old Toumai skull unearthed in Chad in 2001 — may have belonged to a line of nonhybrids that died out and were not human ancestors at all.

This narrative, by a team of geneticists and biostatisticians from the Broad Institute of Harvard and Massachusetts Institute of Technology (MIT), not only casts new light on the origin of humans, it raises questions about how all new species arise.

"This is contributing to the idea that species are kind of fuzzy. They become real over time, but it takes millions of years," said James Mallet, a geneticist at University College in London not involved in the new research. "We probably had a bit of a messy origin."

The research is the latest fruit of the Human Genome Initiative, which transcribed and read out the entire genetic message of human chromosomes, completed in 2003.

The evidence of ancestral chimp and human interbreeding emerged from comparing parts of their genomes to each other and to those of gorillas, orangutans and macaques. The separation into two species "left a footprint on our genome that we can go back and read," said Eric Lander of MIT. "We were never able to look at things like this before."

Human beings have 23 pairs of chromosomes that contain about 30,000 genes. Each gene is made of strands of DNA "letters" in a specific order, and the letters can change, by mutation, over time. The rate at which changes occur is fairly constant — and very, very slow.

As a result, genetic mutations can be used as a kind of evolutionary clock. The number of DNA differences between two species' versions of the same gene is an indication of how long the species have been separate — how long since individuals were last sharing genes.

When Nick Patterson of MIT and his colleagues at the Broad Institute compared the genes of humans and chimps, they found one of the chromosomes — the female sex chromosome known as X — was 1.2 million years younger than the others. It appeared the two species shared a common ancestor who gave them both their X chromosomes, and did so more recently than the ancestors who gave them the other chromosomes.

How could that be?

The best explanation, the scientists think, is that ancient humans and chimps split not once, but twice. The first time was more than 6.3 million years ago. The second time was at least a million years later.

Some of the evolving human ancestors probably bred with the evolving chimpanzees.

Males have only one X chromosome. As is often the case with hybrids, the male offspring from these unions probably would have been infertile.

But the females, which have two X chromosomes, would have been fertile. If some of those hybrid females then bred with proto-chimpanzee males, some of their male offspring would have gotten a working X from the chimp side of the family.

The evolutionary clock indicates this happened no more than 6.3 million years ago, and perhaps as recently as 5.4 million years ago.