Humans and chimpanzees reportedly share 98.7 percent of the same DNA, but differ greatly in appearance, behavior, and ability. Now scientists may have discovered why this is. According to a report published in the current issue of the journal Science, there is more genetic activity in the human brain than there is in the chimpanzee brain. Although there is little difference in the structure of the underlying genes, what they can produce varies substantially, scientists say. The findings suggest that this acceleration of gene expressionthe rate at which a gene generates the messenger RNA and proteins that carry out its orders could be the most significant change to occur during the evolution of humans from simpler primates. The study found that gene expression levels evolved 5.5 times faster in humans than in chimps. The identification of such differences between humans and our closest living relatives, the researchers note, could explain why humans are susceptible to some diseases that chimpanzees can resist.

Svante Pbo of the Max Planck Institute for Evolutionary Anthropology in Germany and colleagues compared the expression levels of genes in tissue and blood samples from humans and several primate relatives: chimpanzees, orangutans and rhesus macaque monkeys. The scientists isolated RNA from each sample and used two techniques to study how this genetic material interacted with a collection of nearly 18,000 human genes. As expected, expression patterns in humans and chimpanzees were similar in the liver and blood samples. Brain tissue, in contrast, showed significant differences among the groups. Expression levels in chimpanzees and other primates remained within the same range, but the human samples registered much more activity. According to the group, these results indicate that the rate of evolutionary change of genetic activity in the brain is more pronounced in humans than in chimpanzees.

The next step, Pbo says, is to isolate the genes that have changed between chimps and humans and to analyze them in greater detail. This could lead to a better understanding of why chimpanzees resist diseases such as AIDS, Alzheimer's, cancer and malaria. With this approach, co-author Ajit Varki of the University of California, San Diego, notes, "we may be able to learn more about the genetics underlying diseases that seem to harm humans but not chimpanzees."