DETROIT, May 19 (UPI) -- New genetic evidence suggests chimpanzees belong in the genus Homo, of which the most prominent member is Homo sapiens -- humans -- scientists reported Monday.
By redrawing the human genetic family tree, researchers hope to unearth more about what makes us human and how human diseases develop.
"In the last 30 million years, we knew our brains grew. If we can find the genetic underpinnings for it, I wouldn't be surprised if we could find variations in those genes involved in widespread mental illnesses," researcher Morris Goodman, a molecular evolutionist at Wayne State University in Detroit, told United Press International.
"So many disease genes haven't been discovered yet," Goodman said, "and you can't get a molecular therapy for a disease unless you know what to direct the therapeutic agent against."
Biology classifies all living things into species, or groups of organisms that interbreed regularly and successfully. Closely related species such as wolves, dogs and coyotes are in turn lumped together into a genus, which is Latin for family. The plural of genus is genera.
Scientists traditionally have set chimpanzees, gorillas and other apes on separate evolutionary branches of humanity's family tree. In this genomic scheme, humans are considered the only living members of the genus Homo, which is Latin for man.
"The traditional view is a hangover from a philosophy Aristotle is responsible for starting called 'The Great Chain of Being,' where living creatures are arranged in a series progressing from what are considered the most perfect to the least perfect, with humans on top. This is of course a very subjective bias," Goodman said.
Accumulating DNA evidence suggests humans and chimps are extremely similar genetically, although prior analyses of DNA showed the two species had divergent proteins, which left open the question of whether changes during evolution left chimpanzees more functionally related to gorillas than to humans.
Goodman and his team compared 97 genes from six different species -- humans, chimpanzees, gorillas, orangutans, Old World monkeys and mice. The genes examined regulated proteins known to be crucial to survival, including biological processes such as DNA repair and oxygen transport.
The genetic similarities and differences found were used to develop an evolutionary tree to measure relatedness between species. In findings made public Monday and appearing this week in the Proceedings of the National Academy of Sciences, humans and chimpanzees are 99.4 percent genetically similar. Next on the family tree are gorillas, followed by orangutans and then Old World monkeys. None of the primates was closely related to mice.
"It is astonishing that there is so little difference in actual functional genes between humans and chimpanzees," biological anthropologist Colin Groves of the Australian National University in Canberra told UPI.
The longer species evolve apart, the more DNA mutations build up. When these numbers of mutations are compared against well-dated points in the fossil record -- such as the separation between Old and New World monkeys about 25 million years ago -- scientists find the mutation rate is more or less constant.
Using such "molecular clocks," Goodman and colleagues estimated humans and chimpanzees diverged from a common ancestor relatively recently -- roughly 5 to 6 million years ago. The findings match well against recent fossil evidence that this common ancestor, in turn, diverged genetically from gorillas some 6 to 7 million years ago.
"Most mammalian genera, it turns out, have time depths of 4 to 6 million years, so the human-chimpanzee split lies just on the cusp of this. You could combine them in one genus, or keep them separate in two," Groves said.
Goodman suggested further research should compare more genes between primates. "We applied this method to 97 genes when there are 30,000 or so known in humans," he said.
As soon as scientists have compiled the complete genetic sequences of chimpanzees, gorillas and other close relatives, "we can se what in a genetic sense made us humans and them not. What underlies our ability to use language," Mark Weiss, the National Science Foundation's program director for physical anthropology in Arlington, Va., told UPI. "Was it many genetic changes or a few? Does it have to do with how genes function in certain parts of the brain at certain points of development?"
Scientists largely have completed mapping the human genetic code, but now they have to figure out what each gene does and which ones are important. Goodman said comparing our genes with our relatives could help find genes either unique to humans or largely unchanged throughout primate evolution -- both of which are likely key to our survival.
"Which particular genes changed, and why did they change, and in what manner?" Weiss asked. "Are there implications in the patterns and changes of genes in human health and diseases? These are all sorts of new questions that become open to further investigation."
(Reported by Charles Choi, UPI Science News, in New York)