[Paleopsych] NYT: In Chimpanzee DNA, Signs of Y Chromosome's Evolution
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Wed Sep 14 01:30:53 UTC 2005
In Chimpanzee DNA, Signs of Y Chromosome's Evolution
New York Times, 5.9.1
By NICHOLAS WADE
Scientists have decoded the chimp genome and compared it with that of
humans, a major step toward defining what makes people human and
developing a deep insight into the evolution of human sexual behavior.
The comparison pinpoints the genetic differences that have arisen in
the two species since they split from a common ancestor some six
million years ago.
The realization that chimpanzees hold a trove of information about
human evolution and nature comes at a time when they and other great
apes are under harsh pressures in their native habitat. Their
populations are dwindling fast as forests are cut down and people
shoot them for meat. They may soon disappear from the wild altogether,
primatologists fear, except in the few sanctuaries that have been
Chimpanzees and people possess almost identical sets of genes, so the
genes that have changed down the human lineage should hold the key to
what makes people human.
Biologists suspect that only a handful of genes are responsible for
the major changes that reshaped the apelike ancestor of both species
into a human and that these genes should be identifiable by having
evolved at a particularly rapid rate.
The comparison of the human and chimp genomes, reported in today's
issue of Nature, takes a first step in this direction but has not yet
tracked down the critical handful of genes responsible for human
One problem is the vast number of differences - some 40 million - in
the sequence of DNA units in the chimp and human genomes. Most are
caused by a random process known as genetic drift and have little
effect. For now, their large numbers make it difficult for scientists
to find the changes caused by natural selection.
But another aspect of the comparison has yielded insights into a
different question, the evolution of the human Y chromosome. The new
finding implies that humans have led sexually virtuous lives for the
last six million years, at least in comparison with the flamboyant
promiscuity of chimpanzees.
Some 300 million years ago, the Y chromosome used to carry the same
1,000 or so genes as its partner, the X chromosome. But because the Y
cannot exchange DNA with the X and update its genes, in humans it has
lost all but 16 of its X-related genes through mutation or failure to
stay relevant to their owner's survival. However, the Y has gained
some genes from other chromosomes because it is a safe haven for genes
that benefit only men, since it never enters a woman's body. These
added genes, not surprisingly, all have functions involved in making
The scientific world's leading student of the Y chromosome, David Page
of the Whitehead Institute in Cambridge, Mass., has been seeking to
understand whether the Y will lose yet more genes and lapse into
terminal decay, taking men with it.
The idea of the Y's extinction "was so delicious from the perspective
of gender politics," Dr. Page said. "But many of my colleagues became
confused with this blending of gender politics with scientific
Two years ago, he discovered a surprising mechanism that protects the
sperm-making genes. Those genes exist in pairs, arranged so that when
the DNA of the chromosome is folded back on itself, the two copies of
the gene are aligned. If one copy of the gene has been hit by a
mutation, the cell can repair it by correcting the mismatch in DNA
The 16 X-related genes are present in only single copies. Dr. Page and
his colleagues thought the chimpanzee genome might show how they were
protected. To their surprise, they report in Nature, the protection
was not there.
The chimp Y chromosome has lost the use of 5 of its 16 X-related
genes. The genes are there, but have been inactivated by mutation. The
explanation, in his view, lies in the chimpanzee's high-spirited
sexual behavior. Female chimps mate with all males around, so as to
make each refrain from killing a child that might be his.
The alpha male nonetheless scores most of the paternities, according
to DNA tests. This must be because of sperm competition,
primatologists believe - the alpha male produces more and better
sperm, which outcompete those of rival males.
This mating system puts such intense pressure on the sperm-making
genes that any improved version will be favored by natural selection.
All the other genes will be dragged along with it, Dr. Page believes,
even if an X-related gene has been inactivated.
If chimps have lost five of their X-related genes in the last six
million years because of sperm competition, and humans have lost none,
humans presumably had a much less promiscuous mating system. But
experts who study fossil human remains believe that the human mating
system of long-term bonds between a man and woman evolved only some
1.7 million years ago.
Males in the human lineage became much smaller at this time, a sign of
The new result implies that even before that time, during the first
four million years after the chimp-human split, the human mating
system did not rely on sperm competition.
Dr. Page said his finding did not reach to the nature of the joint
chimp-human ancestor, but that "it's a reasonable inference" that the
ancestor might have been gorillalike rather than chimplike, as
supposed by some primatologists.
The gorilla mating system has no sperm competition because the
silverback maintains exclusive access to his harem.
Frans B. M. de Waal of the Yerkes National Primate Research Center in
Atlanta said he agreed with fossil experts that the human pair bonding
system probably evolved 1.7 million years ago but that the joint
ancestor could have resembled a chimp, a bonobo, a gorilla, or
something else entirely.
The scientists who have compared the whole genomes of the two species
say they have found 35 million sites on the aligned genomes where
there are different DNA units, and another five million where units
have been added or deleted. Each genome is about three billion units
The chimp genome was completed in draft form in December 2003 by the
Broad Institute in Cambridge and Washington University in St. Louis.
Statistical tests for accelerated evolution are not yet powerful
enough to identify the major genes that have shaped humans. "We knew
that this was only a beginning, but from a general standpoint we have
captured the vast majority of the differences between human and
chimps," said Robert H. Waterston of the University of Washington,
Seattle, the senior author of the report. The genome of a third
primate, the orangutan, is now in progress and will help identify the
genes special to human evolution, he said.
At the level of the whole animal, primatologists have uncovered
copious similarities between the social behavior of chimpanzees,
bonobos and humans, some of which may eventually be linked to genes.
But this rich vein of discovery may be choked off if the great apes
can no longer be studied in the wild.
"The situation is very bad, and our feeling is that by 2040 most of
the habitat will be gone, except for those little regions we have set
aside," Dr. de Waal said.
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