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From: "Dale E. Reddick" <>
Subject: Re: [DNA] New comparison of human and chimp DNA
Date: Wed, 17 May 2006 19:25:13 -0400
References: <20060517230142.39351.qmail@web31504.mail.mud.yahoo.com>
In-Reply-To: <20060517230142.39351.qmail@web31504.mail.mud.yahoo.com>


Hi Folks,

Here's a commentary taken directly from Nature.

Dale E. Reddick
________________________________

Published online: 17 May 2006

Chimpanzee and human ancestors may have interbred

Genetic analysis suggests a messy split between the two lineages.

Michael Hopkin

The evolutionary split between humans and our nearest evolutionary
cousins, chimpanzees, may have occurred more recently than we thought,
according to a new comparison of the respective genetic sequences.
What's more, it might have been a messy divorce rather than a clean
break — leading to the controversial theory that our two sets of
ancestors may have interbred many thousands of years after first parting
company.

The discovery also casts doubt on the status of fossils that were
thought to represent the first flowering of the human branch of the
evolutionary tree — but which now may have to be reclassified as coming
from a time before our split with the rest of the apes.

Previous estimates put the split at as much as 7 million years ago —
meaning that Toumaï, a fossil dating from at least 6.5 million years ago
in Chad and assigned to the species Sahelanthropus tchadensis, was
hailed as the earliest-known member of the line that gave rise to modern
humans.

But researchers led by David Reich of Harvard Medical School in Boston,
Massachusetts, now calculate that the split may have occurred no more
than 6.3 million years ago, and possibly as recently as 5.4 million.
That would make Toumaï older than the time of the split.

-Early days-

The researchers make their claim after comparing the genetic codes of
humans, chimpanzees, gorillas and other primates in unprecedented detail
— more than 20 million DNA 'letters' in all. By checking the differences
between different species' DNA sequences, they were able to estimate the
time since they first diverged.

But the story is not simple, Reich and his team explain in their study,
published online in Nature. Different sections of the genome differ by
different amounts, suggesting that they parted ways at different times.
The divorce period between the two species, the data suggest, could have
lasted a million years.

The region bearing the most similarity is the X chromosome. This is
exactly what one might expect if the two lineages had continued to
interbreed after first starting to separate.

The X chromosome, one of the pair of sex-determining chromosomes, is the
home of many genes that govern fertility, Reich explains. So any two
species that could mate with one another would be expected to have
similar X chromosomes. And natural selection would prevent these
chromosomes from diverging for as long as the hybridization was going on.

-In the family-

If such a hybrid population really did exist, the question remains as to
whether it died out, or whether modern humans or chimpanzees (or both)
are its descendants. It's very difficult to say, admits Reich. "The
fossil data suggest — very tenuously — that it may have been humans who
are descended from the hybrid population."

For some reason, human-like fossils far outnumber chimpanzee-like ones
in the fossil record, making it difficult to see exactly who was
sleeping with whom at the time.

So where does this leave Toumaï and his ilk? They may have sat in an
evolutionary pocket between the initial split and the subsequent
hybridization, Reich suggests, or have been around at the time but not
involved in the inter-species carnality. Reich says that more fossils
and developmental studies will be needed to resolve this tricky question.

________________________________


Jim T wrote:
> This analysis looked at 800 times more DNA than previous ones.
>
> http://www.cnn.com/2006/TECH/science/05/17/human.chimp.split.ap/index.html
>
> Here are a few excerpts from the AP story about the research
> which is to be published in a future issue of Nature:
>
> "The researchers hypothesize that an ancestral ape species split
> into two isolated populations about 10 million years ago, then
> got back together after a few thousand millennia. At that time
> the two groups, though somewhat genetically different, would
> have mated to form a third, hybrid population. That population
> could have interbred with one or both of its parent populations.
> Then, at some point after 6.3 million years ago, two distinct
> lines arose.
>
> "For one thing, the new data suggest the human-chimp split was
> much closer to the present than the 7 million year date that
> fossils and previous studies indicate -- certainly no earlier
> than 6.3 million years ago, and more likely in the neighborhood
> of 5.4 million.
>
> "The data also show that the human-chimp split probably took
> millions of years. That's because in some parts of the DNA
> sequence the genetic difference between humans and chimps is so
> large that those genes must have been isolated from each other
> nearly 10 million years ago. But in other places the human and
> chimp lines are so close that they appear to have still been
> swapping genetic material at least until 6.3 million years ago.
>
> "One of those areas is the X-chromosome, which is intriguing.
>
> "The genes that are a barrier to speciation tend to be on the
> X-chromosome," said David Reich, the main author of the study.
>


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