On primate extinctions...
We Dodged Extinction
?Pruned? Family Tree Leaves
Little Genetic Variety
Just one group of chimpanzees can have
more genetic diversity than all 6 billion
humans on the planet. (Corel)
Special to ABCNEWS.com
A worldwide research program has
come up with astonishing evidence
that humans have come so close to
extinction in the past that it?s
surprising we?re here at all.
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 Family Bush
?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 and conflict.
Almost Extinct
?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
convincing.
?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. |
