One way to find such genetic signatures is to search for genes that reveal signs of positive natural selection. The assumption is that genes touched by positive selection will show more functionally significant molecular changes than unaffected regions. In the new study, Rasmus Nielsen, Michele Cargill, and their colleagues compared 13,731 genes in humans to their equivalents in chimps to find positively selected genes in both species.
Nielsen et al. identified many genes involved in sensory perception, as well as spermatogenesis, but found the strongest evidence for positive selection in genes related to immune defense. Immunity genes, the authors explain, were likely targeted throughout mammalian evolution, while the perception and olfactory genes probably reflect primate-specific adaptations.
Nielsen et al. also found a "surprising number" of tumor-suppressor and apoptosis genes. The factors behind this pattern are unclear, but the authors suggest that studying the genes' other functions, in immunity or spermatogenesis, may offer clues to selective pressures. It could be, for example, that there is a battle between the interests of sperm cells (favoring genetic changes that increase cell number and decrease cell death), and the need for the organism to avoid cancer.
Future studies will have to determine whether these explanations - of an evolutionary arms race - prove p lausible. We're a long way from understanding why we're so different from our closest primate cousins, but this study provides plenty of tools, and hypotheses, to mine the tiny differences in our DNA for more clues.
Citation: Nielsen R, Bustamante C, Clark AG, Glanowski S, Sackton TB, et al. (2005) A scan for positively selected genes in the genomes of humans and chimpanzees. PLoS Biol 3(6): e170.