Chimps Belong on Human Branch of Family Tree, Study Says

John Pickrell in England
for National Geographic News
May 20, 2003

A new report argues that chimpanzees are so closely related to humans that they should be included in our branch of the tree of life. Chimpanzees and other apes have historically been separated from humans in classification schemes, with humans deemed the only living members of the hominid family of species.

Now, biologists at Wayne State University School of Medicine in Detroit, Michigan, provide new genetic evidence that lineages of chimps (currently Pan troglodytes) and humans (Homo sapiens) diverged so recently that chimps should be reclassed as Homo troglodytes. The move would make chimps full members of our genus Homo, along with Neandertals, and all other human-like fossil species. "We humans appear as only slightly remodeled chimpanzee-like apes," says the study.

"The loss of the [wild] chimp and gorilla seems imminent," said Morris Goodman, a study co-author. "Moving chimps into the human genus might help us to realize our very great likeness, and therefore treasure more and treat humanely our closest relative," he said.

However, experts say many scientists are likely to resist the reclassification, especially in the emotionally-charged and often disputed field of anthropology.

Knowing Me, Knowing You

The term genus describes a very closely related group of similar species, thought to have diverged from one another relatively recently, and is the first grouping above the species level. Common chimpanzees and bonobos have until now been classified into their own genus, Pan.

Historical classification schemes, based on physical similarities such as bones, argued that chimps and gorillas were each other's closest relatives, and that both were closely related to orangutans to the exclusion of humans.

However, with the advent of molecular techniques to compare similarities in our DNA starting in the 1960s, most experts have come to accept the fact that humans and chimps are most closely related. Studies indicate that humans and chimps are between 95 and 98.5 percent genetically identical.

Derek E. Wildman, Goodman, and other co-authors at Wayne State argue in their new study, published today in the journal Proceedings of the National Academy of Sciences, that given the evidence, it's somewhat surprising that humans and chimps are still classified into different genera. Other mammalian genera often contain groups of species that diverged much earlier than chimps and humans did, said Goodman. "To be consistent, we need to revise our definition of the human branch of the tree of life," he said.

Historically Flawed

Goodman and colleagues used computer methods to analyze the amount of similarity between 97 important human and chimp genes and as many of the same gene sequences as are currently available for less-studied gorillas, orangutans, and Old World monkeys.

The results suggested that within important sequence stretches of these functionally significant genes, humans and chimps share 99.4 percent identity. (Some previous DNA work remains controversial. It concentrated on genetic sequences that are not parts of genes and are less functionally important, said Goodman.)

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