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)

Jeff Hecht

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