Special to ABCNEWS.com
Pascal Gagneux, an evolutionary biologist
at the University of California at San Diego, and other members of a research
team studied genetic variability among humans and our closest living relatives,
the great apes of Africa.
Humanoids are believed to have
split off from chimpanzees about 5 million to 6 million years ago. With
the passage of all that time, humans should have grown at least as genetically
diverse as our “cousins.” That turns out to be not true.
“We actually found that one
single group of 55 chimpanzees in west Africa has twice the genetic variability
of all humans,” Gagneux says. “In other words, chimps who live in the same
little group on the Ivory Coast are genetically more different from each
other than you are from any human anywhere on the planet.”
The branch lengths
illustrate the number of genetic differences, not only between species,
but among species as well. The pruned bush for humans shows how little
genetic diversity exists. (Marco Doelling/ABCNEWS.com)
“The family tree shows that the human branch has been
pruned,” Gagneux says. “Our ancestors lost much of their original variability.”
“That makes perfectly good sense,”
says Bernard Wood, the Henry R. Luce Professor of Human Origins at George
Washington University and an expert on human evolution.
“The amount of genetic variation
that has accumulated in humans is just nowhere near compatible with the
age” of the species, Wood says. “That means you’ve got to come up with
a hypothesis for an event that wiped out the vast majority of that variation.”
The most plausible explanation,
he adds, is that at least once in our past, something caused the human
population to drop drastically. When or how often that may have happened
is anybody’s guess. Possible culprits include disease, environmental disaster
“The evidence would suggest that we came within a cigarette
paper’s thickness of becoming extinct,” Wood says.
Gagneux, who has spent the last
10 years studying chimpanzees in Africa, says the implications are profound.
“If you have a big bag full
of marbles of different colors, and you lose most of them, then you will
probably end up with a small bag that won’t have all the colors that you
had in the big bag,” he says.
Similarly, if the size of the
human population was severely reduced some time in the past, or several
times, the “colors” that make up our genetic variability will also be reduced.
If that is indeed what happened,
then we should be more like each other, genetically speaking, than the
chimps and gorillas of Africa. And that’s just what the research shows.
“We all have this view in our
minds that we [humans] started precariously as sort of an ape-like creature”
and our numbers grew continuously, adds Wood. “We’re so used to the population
increasing inexorably over the past few hundred years that we think it
has always been like that.”
But if it had, Gagneux notes,
our genetic variability should be at least as great as that of apes.
A Stormy Past
Gagneux is the lead author of a report that appeared
in the April 27 issue of the Proceedings of the National Academy of
Sciences. The study, carried out with researchers in Germany, Switzerland
and the United States, is the first to examine large numbers of all four
ape species in Africa.
“We can do that now because
new technology allows us to non-invasively take some hair, or even some
fruit that these apes chew, and then we get their DNA from a couple of
cells that stick to a hair or a piece of fruit they chewed.”
Then they compared the DNA variability
of apes and chimps to that of 1,070 DNA sequences collected by other researchers
from humans around the world. They also added the DNA from a bone of a
Neanderthal in a German museum. The results, the researchers say, are very
“We show that these taxa [or
species] have very different amounts and patterns of genetic variation,
with humans being the least variable,” they state.
Yet humans have prevailed, even
though low genetic variability leaves us more susceptible to disease.
“Humans, with what little variation
they have, seem to maximize their genetic diversity,” Gagneux says.
“It’s ironic,” he notes, that
after all these years the biggest threat to chimpanzees is human intrusion
into their habitats. When he returned to Africa to study a group of chimps
he had researched earlier, Gagneux found them gone.
“They were dead,” he says, “and
I mean the whole population had disappeared in five years.”
Yet as our closest living relatives,
chimps still have much to teach us about ourselves.
Lee Dye’s column appears Wednesdays on ABCNEWS.com.
A former science writer for the Los Angeles Times, he now lives
in Juneau, Alaska.
evidence would suggest that we came within a cigarette paper’s thickness
of becoming extinct.”
Bernard Wood, George Washington University
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