A team of molecular biologists has taken a stab at defining how we became human. Its answer: We're set apart from other primates not so much by differences in the makeup of our genes but by relatively recent changes in their activity. The heightened bustle is most dramatic in the brain, where its pace has picked up much faster in humans than in other primates.
For 25 years, researchers have known that the genes in humans are virtually identical to the genes in chimpanzees, our closest relatives. That left open the question of how these two species came to be so different. Some researchers suggested that the key might be gene expression, the rate at which messenger RNA and proteins are made from a gene, but the evidence has been scant.
By using gene chips to measure gene activity, Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues have now shown that this may indeed be the case. Pääbo's team collected brain, liver, and blood samples from humans, chimps, macaques, and orangutans that had died of natural causes. They isolated RNA from each sample and passed it over a gene chip with tags for 12,000 human genes. In a second experiment, they used a different assay to look at an additional 6000 genes. In both experiments, they studied RNA from chimps, humans, and one of the other primates.
There was little difference among the species in the liver and blood samples, the team reports [the team reports ] in the 12 April issue of Science. But in the brain, the team detected much more gene expression in humans than in chimps, whereas gene expression in the brains of chimps and the other primates was about the same. By pairing these results with a look at the primate family tree, the team concluded that sometime in the recent evolution of humans, our brains outpaced chimp brains. "This is the first piece of evidence that humans may have a faster rate" of change in the regulation of gene expression, notes Caro-Beth Stewart, a molecular evolutionist at the State University of New York, Albany.