Messy break on evolutionary tree

Geneticists say DNA suggests humans' split from chimpanzees was a long process


PHILADELPHIA -- As we branched off from each other on the evolutionary tree, our ancestors look to have made a messy break from those of chimpanzees. By comparing samples of chimp, gorilla and human DNA, scientists from MIT and Harvard say they see possible evidence of interspecies sex.

But there's a problem with this finding, say paleontologists who study human origins. The geneticists are proposing that our ancestors were still mixing it up with those of the chimps until 6 million years ago -- a time when one lineage was on all fours, the other walking upright.

And you thought your relationship had compatibility problems.

"We see a complex speciation with a long, drawn-out gene flow," said David Reich, a geneticist from Harvard Medical School and lead author of the paper, published in yesterday's issue of the journal Nature.

Would creatures that different want to mate, let alone be able to create viable offspring? The geneticists conceded this might be a stretch but suggest that the fossil finders are off on the dates for upright walking. Paleontologists say it's the geneticists' dates that are in error.

The geneticists get their dates by relying on something called a molecular clock -- a technique that estimates when two organisms shared a common ancestor by counting the number of genetic differences between them.

With much more of the human and chimp genetic codes now available for comparison, Reich and his colleagues examined several thousand short stretches side by side and a few from the gorilla as well.

To their surprise, Reich and his colleagues found some parts of the genetic code look like they diverged more recently than others -- falling into a range of several million years. This alone was not enough to conclude there was any intermixing -- these different pieces of the genetic code simply originated at different times in the ancestral population.

What led them to their more shocking conclusion was the X chromosome. There, our DNA appears to have diverged from chimps more than a million years later than the DNA on the other chromosomes.

In other creatures, sharing a similar X with another species can be a sign -- a scarlet letter -- pointing to interbreeding. This has to do with a connection between the X chromosome and infertility in hybrid offspring, said co-author Nick Patterson of the Broad Institute in Cambridge, Mass.

Most animals from different species can't reproduce together, he said. Some that are closely related can produce infertile offspring -- as in the well-known pairings of donkeys and horses to create mules. But in other cases, some offspring might be fertile and not others, said Patterson. For reasons they don't completely understand, he said, hybrid infertility in mammals seems to come from a combination of incompatible genes on the X chromosome.

Only hybrids with certain combinations of genes on the X would have the fertility necessary to pass them down, and that would dampen diversity in the X chromosome until the mixing stopped.

Patterson said these hybrid ape-creatures may have mated with each other or with members of either the human ancestral line or that of the chimps. They can't say who ended up getting the hybrid genes -- us or the chimps.

It's a complicated picture but the separation of one species from another is more than a simple matter of reproductive compatibility, said Montgomery Slatkin, a biologist from the University of California at Berkeley.

Generally, he said, speciation starts with some kind of geographic separation between two different groups of the same species. They acquire different traits as they adapt to different environments and at the same time also grow apart through random genetic changes.

Eventually the two groups get labeled as different species though if reunited soon enough, they sometimes still mate and reproduce. Not long ago, he said, someone discovered the progeny of a polar bear and a grizzly bear, for example.

So human ancestors mixing with chimp ones is plausible, he said, and has already been proposed by other scientists. More questionable, he said, are the dates.

Reich and colleagues say their data point to some genetic mixing about 5 million years ago, or at the very outside limit, 6.3 million.

But the fossil record says we were walking upright well before that. Several years ago scientists discovered a bipedal ape called Toumai, found in Chad, that's estimated to have lived 7 million years ago.

There are two other examples in Africa of early upright walking hominids, said Tim White, a University of California Berkeley paleontologist. "At 6 million years ago it is clear there are animals in Africa that have already evolved anatomical characteristics placing them on our branch of the family," he said. Those characteristics include bipedality and the loss of large canine teeth.

And if these hybrids existed, we should have found their remains, he said.

But in general, he said, genetic analysis will become an ever more important tool for digging into our past. Genes and fossils point to the same place for humanity in the tree of life. "These are two very different ways of trying to figure out what happened in evolution," he said. "The difference is over the details."

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