Scientists Complete Genetic Map of the Chimpanzee

The DNA analysis -- the first of a non-human primate and the fourth of a mammal (after human, mouse and rat) -- was done on blood drawn from a chimp named Clint, who lived at a research center in Atlanta until dying in January from causes unrelated to the project. Key scientific findings and related commentaries fill about 100 pages in today's Nature and today's online version of the journal Science.

The human and chimpanzee genomes are distinguished by 35 million differences in individual DNA "letters" -- each the result of a tiny, random mutation -- and an additional 5 million larger differences in which entire chunks of DNA were either added to or deleted from one genome or the other.

All told, the two sequences differ by 4 percent. But three-quarters of the differences seem to be in non-functional parts of the genome, suggesting that a mere 1 percent variation makes all the difference.

Put another way, the difference between the human and chimp genomes is 10 times as great as the difference between any two humans.

Among the genes that appear unique to humans are some involved in brain development and body plan, and one that has been postulated as being crucial to the development of language. But most of the differences between chimpanzees and humans seem attributable not so much to the genes themselves but to how genes that both species share are regulated -- that is, the timing and level of intensity under which those shared genes operate.

"The class of genes that has changed the fastest in humans compared to chimps are the genes that control other genes," said Tarjei S. Mikkelsen of the Broad Institute.

Developmental changes are behind many of the differences between human and chimp brains. Human brain cells divide several more times than chimp brain cells during fetal development, a fact that contributes to the adult human brain's growth to three times the size of the chimpanzee's. Much of that increase is in the cerebral cortex, home to higher cognition.

But scientists confess to knowing little about how such changes might add up to differences in intellect and behavior.

"We are woefully ignorant about how genes build brains, and how the electrical activity of the brain builds thoughts and emotions," wrote Marc D. Hauser, co-director of Harvard's Mind, Brain and Behavior Program, in Nature.

Chimpanzees have repeatedly toppled conceptions about the ways in which humans are purportedly unique. They fashion and use tools, including hammers, anvils, probes for fishing termites from the ground and seats to rest on, though unlike humans, they make all their tools by modifying found objects and never by putting complementary pieces together.

Chimps also medicate themselves, swallowing rough leaves and chewing on bitter stems to treat a type of intestinal infection.

And in perhaps their cheekiest aping of humanity, chimpanzees display remarkable political acumen. They form complex alliances and trade grooming services, sex and food. Like many denizens of the world's great cities, they lobby, demand bribes, repay favors and, when crossed, exact revenge.

Yet precisely because chimpanzees are so similar to humans (most medicines are absorbed, metabolized and excreted by chimps just as they are in people, for example), they make excellent stand-ins for humans in medical labs.

Medical studies on chimpanzees are no longer done in most countries other than the United States, where about 1,100 are now in research labs. Several scientists yesterday predicted that release of the chimp genome would escalate a debate as to whether U.S. research restrictions -- including an eight-year-old federal moratorium on breeding chimps for research -- should be tightened or loosened.

Pascal Gagneux of the Zoological Society of San Diego and two colleagues wrote in a Nature commentary that a stricter code of ethics for chimpanzee research is needed. They recommend rules similar to those now in place for research on humans who cannot give meaningful informed consent because of their age or mental status.

Others, recalling the initial importance of chimpanzees as research tools when AIDS first emerged, argue that newly emerging medical challenges demand renewed breeding for research.

Acknowledging recent challenges by proponents of "intelligent design," a proposition that posits the need for an intelligent creator, several scientists said the genome study offered elegant confirmation of Darwin's vision of evolution.

One analysis, for example, showed that the accumulation of deleterious mutations in the human and chimp genomes is greater than in the mouse and rat genomes in just the proportion predicted by one of the mathematical corollaries of the theory of evolution.

"I can't imagine Darwin hoping for a stronger confirmation of his ideas," said Robert H. Waterston, who led the Washington University team.