human ancestors not in Africa 4-3 Ma?

From: Marc Verhaegen (fa204466_at_skynet.be)
Date: 03/03/05 

Date: Fri, 4 Mar 2005 00:46:43 +0100

28.2.05

RETROVIRUS STRUCK ANCESTORS OF CHIMPS AND GORILLAS MILLIONS OF YEARS AGO,
BUT NOT ANCESTRAL HUMANS

Retrovirus struck ancestors of chimps and gorillas millions of years ago,
but not ancestral humans. The ancestors of chimpanzees and gorillas were
infected with a deadly retrovirus about three to four
million years ago, but there is no evidence it infected ancestors of
modern-day humans, according to research by genome scientists. The virus
struck after humans had split off the evolutionary tree from
primates, researchers said. The infection may have played a role in the
evolution of such great apes as chimps and gorillas. The research appears in
the April issue of the journal Public Library of Science-Biology, which is
available online on March 1.
Researchers studying portions of the genome containing 'retroelements,' also
known as junk DNA, found many copies of a gene sequence in the chimp and
gorilla genome that didn't appear anywhere
in the human genome. They translated that genome sequence into its
corresponding protein, and discovered that it was the remnant of a
retrovirus, a type of virus that copies its genetic information into
the host's genome. Evidence suggests that the 'retroelement' originated from
an external retrovirus that actively infected ape species in the past.
"The reason retroviruses are so deadly, at the genetic level, is that they
have a tremendous potential to mess up a gene and interfere with its
expressio," explained Dr. Evan Eichler, UW associate professor of
genome sciences and co-author of the study. "That can have negative effects.
It's a double-whammy: the virus infected and possibly killed off some of the
population, but also caused genetic errors in
survivors. Those errors would have later eliminated more of the population."
The virus had invaded the genome in the germline - in sperm or egg cells -
allowing the sequence to be passed on to future generations In those animals
in which the virus was taken up next to or inside a gene - in the part of
the genome that codes for the most important biological functions - the
virus had an even stronger effect.
What researchers don't understand is why the virus affected the ancestors of
chimps, gorillas, and Old World monkeys, but didn't affect the ancestors of
humans or of Asian apes like orangutans and gibbons. The infections took
place independently, and did not originate in a common ancestor of humans
and apes. The event also took place between three and four million years
ago, well after the
separation of humans from apes. That split is estimated to have occurred
five to seven million years ago. During that period, ancestral humans were
likely to be living in the same area of Africa
as great apes. African apes may have been susceptibile to the virus, or
ancestral humans and Asian apes may have been resistant to it. Another
possibility is that some early humans may have carried the
virus, but eventually died off.
Researchers also don't know the impact the virus had on the primate species
it did affect. They found many copies of the virus in the genomes of both
species, but only a tiny fraction of those copies
landed in or near a gene, where it would have the greatest impact. Other
studies have shown that most retroviruses typically land near or within
genes. This difference may mean the animals that had the virus
taken up in or near a gene didn't survive long. Because of that natural
selection, researchers believe that the virus may have had major impacts on
the formation of the species we now call chimps and
gorillas. The virus struck when each of the primate groups was still an
incipient species with widely varying populations.
If the virus had killed off much of the population of both species, it may
have created what evolutionary biologists call a population bottleneck. This
much smaller group of surviving animals would then sort out most of its
genetic variation in relatively fewer generations than would a larger group.
This would lead to a higher probability of rare genetic variants becoming
fixed in a short time. Before long, a
genetically disparate population, possibly with wide variations in
morphology, would have emerged, leading to today's chimps and gorillas.

The study's lead authors are Zhaoshi Jiang, a Ph.D. student in Eichler's lab
at the UW, and Chris Yohn, a technician at Case Western Reserve University
in Cleveland.

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