Whose genome has evolved more, chimpanzee or human? You might answer "human.” After all, with our nearly hairless skin and larger brains, it would surely seem that genetically we've outpaced our closest relatives since parting company 7 million years ago. But that anthropocentric view is mistaken, according to a new study by researchers from the University of Michigan, Ann Arbor.
The team was searching for genetic evidence of adaptive changes. These mutations are positively selected; that is, they confer a benefit that makes survival and reproduction more likely and have spread throughout a population via natural selection. Jianzhi Zhang, a population geneticist, and his colleagues compared nearly 14,000 protein-coding genes in humans and chimpanzees, which have about the same size genome. Using a statistical analysis, they identified 154 human genes that have been positively selected. In contrast, they found 233 such genes in chimpanzees, a 51% increase, they report online this week in the Proceedings of the National Academy of Sciences.
What explains that discrepancy? The chimpanzees' larger past population, says Zhang. Other genetic data have shown that the human population remained smaller than that of the chimpanzee for most of the last 1 million to 2 million years, he notes, and as population genetic theory predicts, recombination will produce more adaptive genetic changes in larger groups.
Intriguingly, the fast-evolving genes in humans and chimpanzees do not readily account for the obvious physical differences between the two species, Zhang points out. In the chimp, genes that have outpaced those in humans include ones involved in protein metabolism, gene transcription, and stress response. "You wouldn't immediately notice if the chimpanzee has a better stress response than a human," says Zhang. In the human, too, the differences appear to be subtle, with selection working rapidly on genes concerned with fatty acid metabolism and phosphate transport.
The study "challenges the idea that there was a great burst of adaptive change in humans, one that was more profound than in other primates or mammals," says Morris Goodman, an evolutionary biologist at Wayne State University School of Medicine in Detroit, Michigan.
But that's not the whole story, argues Ajit Varki, a physician-scientist at the University of California, San Diego. "It's a terrific paper, but they're only looking at one mechanism, the changing amino acids in proteins. Other mechanisms in gene evolution--such as gene expression, duplication, conversion, and inactivation--are likely to be equally important." Further, Varki adds, these types of genomewide analyses are limited, because they do not address the issue of gene function. "It could be that the deletion of a specific gene or a single amino acid change could have more biological significance than a large number of genes that seem to have undergone many changes." And that means we're still a long way from explaining what makes us human--or them chimpanzee, he says.