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Thursday, May 18, 2006

The Hybrid Inference: A Da Vinci Code for Biologists?

A team of geneticists and biostatisticians from the Massachusetts Institute of Technology and Harvard have completed a new comparison of the human and chimp genomes that they claim suggests that after the two lineages separated, there may have been interbreeding for more than a million years.

The mapping of the Human Genome Initiative, completed in 2003, has facilitated a new approach to the analysis of human evolution. A key assumption of the analysis is that the rate at which genetic mutations occur is slow and essentially constant. Therefore, counting DNA differences between two species' versions of the same gene serves as an indicator as to how long it has been since they shared genes, or became genetically distinct.

The fossil record suggests that the split between humans and chimpanzees, a pivotal event in human evolutionary theory, occurred about 7 million years ago. The recent report, published in the journal, Nature, suggests a comingling that did not end perhaps until about 5.4 million years ago.

WaPo has the team's explanation, a narrative a Flying Spaghetti Monster would love:

What probably happened was that some of the evolving human ancestors bred with the evolving chimps. This was perhaps not as strange as it seems, for although there were some physical differences between the two groups, "the early humans must have looked pretty much like chimpanzees," said [James] Mallet, [a geneticist at University College] London.

Males have only one X chromosome, which is necessary for reproduction. As is often the case with hybrids, the male offspring from these unions would probably 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-chimp males, some of their male offspring would have received a working X from the chimp side of the family. They would have been fertile -- and with them the hybrid line would have been off and reproducing on its own.

The evolutionary clock indicates this happened no more than 6.3 million years ago, and perhaps as recently as 5.4 million years ago. In that case, the fossils of older species -- such as Toumai, or Sahelanthropus tchadensis, a proto-man from Chad that had a humanlike brow and probably walked on two feet -- must have belonged to descendants of the first human-chimp divergence.

That line must have died out. If it had not, modern man's X chromosome would look as old (or nearly as old) as the other chromosomes.
What we have here, as I am sure the ID-bashers in the crowd are pointing out to everyone who will listen, is not a theory, but a scientific story. Which is fine by me. It is the story and the data that inform all future ground-breaking scientific work, i.e., you do not create a new hypothesis to falsify without having some story behind it that can be consistent with existing data. Here's a truffle quote from NYT:

David Page, a human geneticist at the Whitehead Institute in Cambridge, said the design of the new analysis was "really beautiful, with all the pieces of the puzzle laid out." Whether the hybridization will turn out to be the right solution to the puzzle remains to be seen, "but for the moment I can't think of a better explanation," he said.
Note: "design" = "story" in this context.

Now, sure there are a few paleoanthropologists out there who disagree with the hybrid inference (and question the accuracy of the evolutionary clock). But could that be because they are involved in some millenia-old conspiracy to hide the true feminine-centered nature of the "descent of man?" Hmmmm?

My message to the pro-hybrids is the same as that to the IDers: Go ahead fellas, use your story; try to build some science. And let us know whenever you have something.

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