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8-Humans: What makes the difference between chimps and humans?



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TITLE:  Chimps are human, gene study implies
SOURCE: The New Scientist, UK, by Jeff Hecht
        http://www.newscientist.com/news/news.jsp?id=ns99993744
DATE:   May 19, 2003

------------------ archive: http://www.gene.ch/genet.html ------------------


Chimps are human, gene study implies

The latest twist in the debate over how much DNA separates humans from
chimpanzees suggests we are so closely related that chimps should not
only be part of the same taxonomic family, but also the same genus. The
new study found that 99.4 percent of the most critical DNA sites are
identical in the corresponding human and chimp genes. With that close a
relationship, the two living chimp species belong in the genus Homo, says
Morris Goodman of Wayne State University in Detroit. The closeness of
relationship between chimps and humans has become an important issue
outside taxonomy, becoming part of the debate over the use of chimps in
laboratory experiments and over their conservation in the wild.
Traditionally chimps are classified with the other great apes, gorillas
and orangutans, in the family Pongidae, separated from the human family
Hominidae. Within Hominidae, most paleoanthropologists now class
virtually all hominid fossils in three genera, Homo, Australopithecus, or
Ardipithecus. On the basis of the new study, Goodman would not only put
modern humans and all fossils back to the human-chimp divergence into
Homo, but would also include the common chimp (Pan troglodytes) and the
bonobo (Pan paniscus).


"The third chimpanzee"

It is not the first time such a suggestion has been made - in 1991
physiologist and ecologist Jared Diamond called humans "the third
chimpanzee". But subsequent genetic comparisons have yielded varying
results, depending on how the genotypes are compared. Goodman compared
published sequences of 97 genes on six species, including humans, chimps,
gorillas, orangutans, and Old World monkeys. He looked only at what he
considered the most functional DNA, bases which cannot be changed without
a consequent change in the amino acid coded for by the gene. Among these,
he found that 99.4 percent were identical in humans and chimps. He found
a lower correspondence for bases that could be changed without affecting
the amino acid, with 98.4 percent identical for chimps and humans and the
same for the "junk" DNA outside coding regions. Goodman believes the
differences are larger for non-coding DNA because their sequences are not
biologically critical.


Split date

His correlations are much higher than the 95 per cent similarity reported
in 2002 by Roy Britten of the California Institute of Technology. Goodman
does not disagree with those results, he told New Scientist, but points
out that the differences analysed by Britten are not important to gene
function because 98 percent of the DNA did not code for proteins. The
small difference between genotypes reflects the recent split between
chimps and humans, says Goodman, who dates the divergence to between five
and six million years ago. But Sandy Harcourt, an anthropologist at the
University of California at Davis, believes chimps and humans split six
to 10 million years ago. "That's an awful long time to be in the same
genus," he told New Scientist. Classifying chimps as human might raise
their conservation profile, but Harcourt hopes that is not the only way
to get people to worry about them. "I'd prefer to go the other way, and
consider more things that aren't human" as important for conservation, he
says.

Journal reference: Proceedings of the National Academy of Sciences (DOI:
10.1073/pnas.1232172100)