Despite decades of study, geneticists don't know what makes humans human. Humans and our kissing cousins, chimpanzees, share practically all of our DNA. But new research reveals key differences between chimp and human genomes.
Michele Cargill, a geneticist at Celera Diagnostics in Alameda, California, and her colleagues examined 7600 genes shared by chimps, humans, and mice, identifying those that had more than the expected number of base changes, given the normal mutation rates in each species. They concluded that 1547 human genes and 1534 chimp genes had experienced relatively rapid changes that likely endowed a survival advantage. The mouse data helped the team determine how the genes had changed over evolutionary time.
In both chimps and humans, many genes involved in cell signaling and amino acid metabolism have undergone major changes since the time of the species' last common ancestor 5 million years ago. But the genes didn't follow the same track in the two species, suggesting that they faced different pressures from natural selection. In humans, 27 of 48 olfactory proteins and three of 21 hearing proteins showed significant accelerated change, whereas that was not true in the chimp. In contrast, the chimp's genes for mesoderm development and skeletal structure had changed more.
The rapid evolution of olfactory genes in humans is puzzling. Because humans no longer rely strongly on their sense of smell to survive, any changes were assumed to have had no effect or a negative effect. But the new study suggests that some olfaction genes have been evolving in a positive way: Their changes appear to have been selected for during human evolution. The genes may have promoted certain dietary changes or informed sexual selection, says co-author Andrew Clark, a population geneticist at Cornell University in Ithaca, New York.
The work, reported in the 12 December issue of Science, “is a valuable start to the genomic approach to identify ‘humanness,' ” says Ajit Varki, a glycobiologist at the University of California, San Diego, who adds that comparing human and chimp genes to the genome of a closer relative than mice, such as another great ape, would reveal even more genes that defined the branches of the primate family tree.