And people thought we were the ones who should be insulted at the idea of being descended from apelike ancestors, as science concluded 150 years ago. Those who object to having a monkey for an uncle regularly harrumph that humans are so much more evolved than other of today's primates such as chimpanzees and gorillas. Anyone can see that humans have evolved much more than chimps did since the last common ancestor lived about 6 million years ago. After all, it is humans and not chimps who speak, do math, walk upright, get by with little body hair, and do evolutionary biology.
Now, however, scientists at the University of Michigan find that more chimp genes than human genes have changed since our last common ancestor, undergoing what's called positive selection "that is, advantageous genetic mutations that become fixed, or set, being passed on from one generation to the next over thousands of millennia. "We often think that we're unique and superior to other species, so there must be a lot of Darwinian selection behind our origin," said Michigan's Jianzhi (George) Zhang, associate professor of ecology and evolutionary biology.
In their analysis of nearly 14,000 chimp and human genes, he and colleagues tried to determine which of the differences between a chimp gene and a human genes were the result of genetic changes in the human lineage and which reflected changes in the chimp lineage.
"If we only compare human and chimp, we can see differences, but we can't tell whether a particular difference is due to a change in the human or a change in the chimp," Zhang said. "But if we compare both to another species "say, the monkey "and if chimp and monkey are identical for a particular trait such as brain size, but human is different, then we can infer that something must have changed during human evolution." For most of the comparisons, it was chimp genes that had changed from that of the common ancestor, not human genes.
Even more surprising than the finding that chimps have substantially more positively-selected genes than people is which genes evolution has been tinkering with. You'd expect that to be genes for traits that make us distinctively human, such as those that function in the brain. But no. "We didn't see that," Zhang said. There were no detectable trends in where positively-selected genes were expressed. "I believe that human brain evolution is due to changes in a small number of genes, not large numbers, and that is why we do not see a genome-wide signal," he said. However you slice it, the findings are one more blow to the idea that humans are evolution's darlings, the species that was specially favored by natural selection.
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