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Chimp-human split 'recent'
24/02/2007 16:20 - (SA)
Washington - Chimpanzees and humans split
from a common ancestor just four million years ago - a much
shorter time than current estimates of five million to seven million
years ago, accordng to a study published on Friday.
The researchers compared the DNA of chimpanzees, humans and
our next-closest ancestor, the gorilla, as well as orangutans.
They used a well-known type of calculation that had not
been previously applied to genetics to come up with their own
"molecular clock" estimate of when humans became uniquely
human.
"Assuming orangutan divergence 18 million years ago,
speciation time of human and chimpanzee is consistently around
four million years ago," they wrote in their study, published in
the Public Library of Science journal PLoS Genetics.
"Primate evolution is a central topic in biology and much
information can be obtained from DNA sequence data," Dr Asger
Hobolth of North Carolina State University said in a
statement.
The theory of a molecular clock is based on the premise
that all DNA mutates at a certain rate. It is not always a
steady rate but it evens out over the millennia and can be used
to track evolution.
Experts agree that humans split off from a common ancestor
with chimpanzees several million years ago and that gorillas
and orangutans split off much earlier. But it is difficult to
date precisely when, although most recent studies have put the
date at somewhere around five million to seven million years ago.
Quick split
They used a statistical technique called the hidden Markov
model, developed in the 1960s and originally applied to speech
recognition.
They found evidence that it took only 400 000 years for humans
to become a separate species from the common chimp-human
ancestor.
Just last May, David Reich of the Broad Institute at the
Massachusetts Institute of Technology and Harvard Medical
School's Department of Genetics found evidence that the split
probably took four million years to occur, although his team put
the final divergence at just 5.4 million years ago.
"I don't think it really contradicts our paper," Reich said
in an e-mail exchange.
"We were focusing on a maximum time for the common ancestor
of humans and chimpanzees, while they were focusing on a best
estimate," added Reich, who revieved Hobolth's paper before it
as published.
Experts have long known that humans and chimpanzees share
much DNA, and are in fact 96 percent identical on the genetic
level.
And one year ago, Soojin Yi and colleagues at the Georgia
Institute of Technology said they found genetic evidence that
chimpanzees may be more closely related to humans than to
gorillas and orangutans.
Their look at the molecular clock showed humans evolved one
unique trait just a million years ago - our longer life span
and our long childhood that means humans reach sexual maturity
very late in life compared to other animals.
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