-Caveat Lector-
Chimpanzees More Genetically Diverse Than Humans
2.00 p.m. ET (1900 GMT) November 4, 1999
By Maggie Fox
WASHINGTON (Reuters) - Chimpanzees are far more genetically diverse than
human beings are, researchers said on Thursday, a finding that has
implications both for efforts to conserve endangered chimps and for learning
more about human origins.
A team of German genetic experts said their findings bolster suggestions
that current human populations date back to some sort of "bottleneck" in
evolution just a few hundred thousand years ago.
Henrik Kaessmann and Svante Paabo of the Max-Planck-Institute for
Evolutionary Anthropology in Leipzig, Germany, compared the genes of both
species of chimpanzees, as well as three subspecies.
"The results show a strikingly lower amount of variation in humans than in
chimps," Paabo said in a statement.
"The simplest explanation for this is that at some rather recent point in
the past, humans have been few in numbers. From a genetic point of view,
this point could be the origin of modern humans."
Scientists make comparisons between humans and chimps because they are our
closest cousins, sharing more than 98 percent of our DNA.
All animals have unique variations in genes from individual to individual.
This variation can be meaningless, or can make the difference in, for
instance, susceptibility to disease.
Writing in the journal Science, Kaessmann's team said they examined genes
from the three recognized subspecies of chimps - central African
chimpanzees, western African chimpanzees and Eastern African chimpanzees.
They also looked at bonobos, commonly known as dwarf chimpanzees.
They looked at one gene called Xq13.3, which is found on the X chromosome,
one of the two chromosomes related to sex. It has no known function, but it
is known to have a low mutation rate and has been well studied in people.
They found Xq13.3 has almost four times as many variants and is three times
as old in chimps as in humans.
This supports other studies that find humans are extremely closely related
to one another genetically despite what geneticists consider to be arbitrary
designations of "race."
The researchers assumed that chimpanzees and humans diverged from a common
ancestor 5 million years ago, and made comparisons.
It is possible to estimate how long ago the animals in a common population
had a common ancestor by looking at the rate of mutations in a gene. This is
one method used to determine the age of the "ancestral Eve" from whom all
humans are theoretically descended.
This has been done with both humans and chimpanzees using a form of DNA
called mitochondrial DNA. The German team's use of Xq13.3 came up with
different estimates.
They determined that the most common ancestor of chimps lived 1.4 million
years ago, while humans can date their genetic Adam and Eve to just 450,000
years ago.
Mitochondrial DNA research suggests the human Eve lived about 200,000 years
ago. Scientists will have to work this all out but Kaessmann and colleagues
said it is the differences between humans and chimpanzees, not the absolute
number of years, that matter in their work.
=======================
Robert F. Tatman
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