-Caveat Lector-
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From: [EMAIL PROTECTED]
Date: March 5, 2007 8:51:24 PM PST
To: [EMAIL PROTECTED]
Cc: [EMAIL PROTECTED], [EMAIL PROTECTED]
Subject: Fwd: A Recent Split of Humans and Chimps?
The findings also support Reich's controversial proposal last year
(Science, May 2006:) that the ancestors of chimps and humans might
have interbred more recently than previously believed, even after
they had begun to head down separate evolutionary paths
Finally, the roots of Bush's family tree!
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free from AOL at AOL.com.
From: "Jim S." <[EMAIL PROTECTED]>
Date: March 5, 2007 2:56:23 PM PST
To: undisclosed-recipients:;
Subject: A Recent Split of Humans and Chimps?
Reply-To: [EMAIL PROTECTED]
http://sciencenow.sciencemag.org/cgi/content/full/2007/227/2?
etoc *A Recent Split of Humans and Chimps?*
By Ann Gibbons
ScienceNOW Daily News
27 February 2007
In recent years, paleoanthropologists have been closing in on the
exact time and place where the ancestors of humans and chimpanzees
went their separate ways. As they uncovered several types of
fossils from the dawn of humankind, they proposed that these early
hominids lived between 5 million and 7 million years ago -- dates
that match up nicely with molecular studies. Now, however, this
satisfying consensus is being challenged by a new study that
proposes a surprisingly recent separation.
In a report published online in the February issue of PLoS
Genetics, Danish postdoctoral researcher Asger Hobolth of North
Carolina State University in Raleigh and his colleagues compared
1.9 million base pairs of DNA in four regions of the genomes of
humans, chimpanzees, gorillas, and orangutans. They then used a
well-known statistical method called the hidden Markov model, which
was developed in the 1960s for speech recognition, to help them
identify subtle patterns in the genomes of apes and humans. The
researchers used the method to quantify how closely humans are
related to chimpanzees, gorillas, and orangutans. They also used
it to spot how humans and chimps inherited different segments of
noncoding DNA, such as tracing stretches of the genome that humans
inherited from their last common ancestor with chimps or from
earlier ancestors shared with gorillas or orangutans.
More to the point, the researchers could then calculate the order
-- and relative timing -- in which various lineages split apart on
the primate family tree, with orangutans appearing first, followed
by gorillas, chimps, and then humans. They dated the branching
points by using fossils of orangutan ancestors, which were 18
million years old, to set a starting time at the base of the tree
for a "molecular clock." Although molecules mutate at various
rates, the average is relatively constant if enough time passes --
and those mutations can be used like a clock to date how long ago
two species split. The team ended up with a date of 4.1 million
plus or minus 400,000 years for the human-chimp split. It was so
recent it even surprised the authors, says Hobolth.
Some researchers say the date is so recent, something must be wrong
with this application of the Markov methodology. It would bump all
the earliest fossils out of the human tree -- including a 4.1-
million-year-old fossil from Kenya called Australopithecus
anamensis, which was already well on its way toward becoming human;
it already walked upright, which is a defining character of being
ancestral to humans, but not apes. "A 4.1-million-year split for
humans and chimps ... is hard to defend because fossils practically
reject it," says evolutionary biologist Blair Hedges of
Pennsylvania State University in State College.
Others, such as geneticist David Reich of Harvard Medical School in
Boston, Massachusetts, say that despite the date, the method shows
that the genetic makeup of the last common ancestor of humans and
chimpanzees -- and the last common ancestor they share with
gorillas -- was more diverse than expected. The findings also
support Reich's controversial proposal last year (Science, 19 May
2006:) that the ancestors of chimps and humans might have interbred
more recently than previously believed, even after they had begun
to head down separate evolutionary paths.
Source: Science
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