With our big brains, capacity for speech, and upright stance, humans have long assumed that our species must have hit the genetic jackpot. But a controversial new study challenges the idea that we sprinted along on the evolutionary fast track while our chimp brethren were left swinging in the trees.
A comparison of thousands of human and chimpanzee genes suggests that chimps have actually evolved more since the two species parted from a common ancestor approximately five million years ago, according to Jianzhi Zhang, an evolutionary biologist at the University of Michigan in Ann Arbor, who led the research.
Mutations happen spontaneously, and most are neutral or bad, says Zhang. But sometimes a beneficial mutation occurs in an individual and spreads throughout the population over time, a process known as positive selection: the genes carrying these good mutations confer evolutionary advantages that allow organisms to adapt and thrive. The changes thus become “fixed” in the genome.
Scientists generally believed that traits like higher cognitive skills were due to bursts of adaptive evolution, in which key genes accumulated beneficial mutations that contributed to the evolution of the human species.
To test that idea, Zhang and his colleagues analyzed sequences of approximately 14,000 genes from the chimp and human genomes. They compared rates of two types of mutations–those that alter the shape of the gene’s protein product and those that leave the structure of the protein unchanged. Genes that have been changed by positive selection have significantly more protein-altering mutations.
The results, published today in the Proceedings of the National Academy of Sciences, were surprising. Chimps had 233 positively selected genes while humans had just 154, implying that chimps have adapted more to their environment than humans have to theirs.
“It’s human egotism to put us on a pedestal,” says molecular anthropologist Morris Goodman of Wayne State University School of Medicine in Detroit. “I was attracted to the paper because it seemed to be chipping away at this desire to make us all that extra-special. At the molecular level, humans are not necessarily exceptional in terms of the adaptive changes.”
To Zhang’s surprise and disappointment, the positively selected genes were not related to brain or cognitive function but to more mundane cellular housekeeping duties. “One explanation might be that the number of genes responsible for evolution of the human brain may be very small,” Zhang speculates.
The Michigan team also discovered that a higher percentage of positively selected genes were associated with disease in humans than in chimps. According to the laws of population genetics, natural selection tends to be more efficient at spreading good genes and tossing bad ones in large populations than in smaller ones. Until recently, the chimpanzee population was much larger than the human population, which may have allowed natural selection to eliminate the deleterious chimp genes.
The other explanation, says Zhang, is that human genes that may have been advantageous in the past may now trigger disease because our environment and way of life have changed.
Not everyone is convinced that Zhang’s team has drawn the correct conclusion from the gene analysis. Humans and chimps are so similar that it is difficult to determine whether the genes are the product of positive selection, says Bruce Lahn, an evolutionary geneticist at the University of Chicago who studies the genetic basis of brain evolution.
“It is very rare that there will be enough changes in such a short lineage to tell us there is positive selection,” says Lahn. “I’m very surprised that they claim these are positively selected genes. I would guess if they tried to publish each of these genes as an example of positive selection, there wouldn’t be enough supporting data for the majority of them.”
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