The DNA divide: chimps, people differ in brain's gene activity.

People and chimpanzees are almost identical when it comes to their DNA sequences, a sure sign of close evolutionary ties. A new study suggests that the distinctive looks and thinking styles of these two primate groups derive from the contrasting productivities of their similar DNA sequences.

The same genes that unleash a cascade of messenger molecules and proteins in human brain tissue yield a relatively modest flow of the same substances in chimp brains, says geneticist Svante Paabo of the Max-Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

During the evolution of Homo sapiens, the brain probably experienced accelerated changes to accommodate so great a surge of genetic products, Paabo and his colleagues assert. Their study appears in the April 12 Science.

The scientists examined the responses of white blood cells and liver and brain tissue to individual DNA sequences isolated from nearly 18,000 human genes. They used blood and tissue samples removed during autopsies from people, chimps, rhesus monkeys, and an orangutan, all of whom had died of natural causes.

In liver and blood samples, humans and chimps exhibited similar concentrations of messenger RNA, molecules copied from DNA that take part in generating proteins. Tissue from monkeys and the orangutan harbored less messenger RNA.

A starkly different pattern emerged in the brain. Humans displayed the greatest concentration of messenger RNA in this tissue. Chimps and monkeys showed lower levels of these molecules.

Considerable individual differences emerged, the researchers note. For example, the messenger RNA production in one of the seven people whose tissue was used in the study more closely resembled the overall measure for chimps than for humans.

Still, the same general trend appeared when Paabo's group measured proteins, the end products of genetic activity. Proteins occurred in far higher concentrations in the brains of people than in those of any other primate. No such protein disparity appeared in white blood cells or liver tissue.

The researchers found no evidence of structural differences in brain proteins of people and chimps.

The scientists also measured messenger RNA and protein in two mouse species that are as genetically similar to each other as people are to chimps. The mouse groups exhibited a smaller disparity in their brain protein levels than that observed between people and chimps. These molecular data bolster the notion that the human lineage experienced evolutionary influences--as yet unspecified--on the brain that other species didn't, Paabo says.

"These are exciting findings," remarks geneticist Evan E. Eichler of Case Western Reserve University in Cleveland. Now, the question is why the same genes in humans and chimps work so differently in the two groups' brains yet so similarly in other tissues, he says.

Although chimps have much in common with people (SN: 3/16/02, p. 166), the report "opens up a new realm of thought" on the nature of differences between the species, comments evolutionary biologist Pascal Gagneux of the University of California, San Diego.