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Lialei
Knowflake

Posts: 453
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Registered: Jul 2005

posted May 19, 2006 10:20 AM     Click Here to See the Profile for Lialei     Edit/Delete Message
Chimps and humans might have bred, then
redivided

New genome study suggests the species
didn't split cleanly


The split between the human and chimpanzee lineages, a pivotal event in evolution, may have occurred millions of years later than fossil bones suggest, and the break may not have been as clean as humans might like.

A new comparison of the human and chimp genomes suggests that after the two lineages separated, they may have begun interbreeding.

The analysis,by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass., sets up a serious conflict between the date of the split as indicated by fossil skulls, about 7 million years ago, and the more recent date implied by genetic analysis, as late as 5.4 million years ago.

The conflict can be resolved, Reich's team suggests in an article published in Thursday's issue of Nature, if there were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split.

The suggestion of a hybridization has startled scientists, who nonetheless are treating the new genetic data seriously.

The earliest human-lineage fossil remains, such as Sahelanthropus, seem clearly to have been bipeds, walking on two feet, but the ancestors of chimps presumably walked on their two feet and the knuckles of their hands, as do modern chimps.

"If the earliest hominids are bipedal, it's hard to think of them interbreeding with the knuckle-walking chimps -- it's not what we had in mind," said Daniel Lieberman, a biological anthropologist at Harvard.

Beautiful -- and animal

Hybrid populations often go extinct because the males are sterile, Reich noted, so hybrid females may have mated with male chimps to produce viable offspring. The human lineage finally re-emerged from this hybrid population, Reich suggests, explaining the younger genetic dates, while the very early fossils with humanlike features may come from the earlier period before the hybridization.

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.

These crucial events in early human evolution are hard to judge dispassionately, Page said. "We'd like to have a more Victorian view of our genome, and this reminds us that we are really animals and gives us a glimpse of our past and of a story that we might like to have told in a different way."

The earlier view

Geneticists' previous estimates of the human-chimp lineage split were based on the number of DNA differences in a small section of the genome, matched against some concrete date in the fossil record, like the split between orangutans and the other primates.

One such approach, in 2001, suggested the human and chimp leneages split somewhere between 4.6 and 6.2 million years ago, but recent discoveries of fossil skulls have pushed the likely date backward. The Sahelanthropus fossil, found in Chad in 2002, had upright gait, human-like teeth and lived around 7 million years ago.

The Reich team's study is a far more detailed analysis of the human and chimp genomes, and also draws on DNA sequences from the gorilla, orangutan and macaque to iron out amiguities.

Overall, the team calculates that the human and chimp lines must have finally split apart at the earliest 6.3 million years ago and more probably 5.4 million years ago, a sharp discrepancy with the Sahelanthropus date.

But besides averaging genome-wide differences in DNA to get an overall fix on when the two species divided, the team has also been able to scroll along the genome and estimate the relative age of each small section.

A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes. Females have two X chromosomes, males one X and one Y chromosome.


*Detroit Free Press--May 19, 2006

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fayte.m
Knowflake

Posts: 4174
From: ~out looking for Schrodinger's cat~
Registered: Mar 2005

posted May 19, 2006 06:29 PM     Click Here to See the Profile for fayte.m     Edit/Delete Message
EXCELLENT ARTICLE!

------------------
~I intend to continue learning forever~"Fayte"
~I am still learning~ Michangelo
The Door to Gnosis is never permanently locked...one only needs the correct keys and passwords.
The pious man with closed eyes can often hold more ego than a proud man with open eyes.
Out of the mouth of babes commeth wisdom that can rival that of sages.
In the rough, or cut and polished..a diamond is still a precious gem.
-NEXUS-

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