“If a favorable mutation appears, then the number of copies of that chromosome will increase rapidly” in the population because people with the mutation are more likely to survive and reproduce, Harpending says.
“And if it increases rapidly, it becomes common in the population in a short time,” he adds.
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The increase in human population from millions to billions in the last 10,000 years accelerated the rate of evolution because “we were in new environments to which we needed to adapt,” Harpending adds. “And with a larger population, more mutations occurred.”
Study co-author Gregory M. Cochran says: “History looks more and more like a science fiction novel in which mutants repeatedly arose and displaced normal humans – sometimes quietly, by surviving starvation and disease better, sometimes as a conquering horde. And we are those mutants.”
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A key finding: 7 percent of human genes are undergoing rapid, recent evolution.
The researchers built a case that human evolution has accelerated by comparing genetic data with what the data should look like if human evolution had been constant:
-- The study found much more genetic diversity in the SNPs than would be expected if human evolution had remained constant.
-- If the rate at which new genes evolve in Africans was extrapolated back to 6 million years ago when humans and chimpanzees diverged, the genetic difference between modern chimps and humans would be 160 times greater than it really is. So the evolution rate of Africans represents a recent speedup in evolution.
-- If evolution had been fast and constant for a long time, there should be many recently evolved genes that have spread to everyone. Yet, the study revealed many genes still becoming more frequent in the population, indicating a recent evolutionary speedup.
Next, the researchers examined the history of human population size on each continent. They found that mutation patterns seen in the genome data were consistent with the hypothesis that evolution is faster in larger populations.
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“Rapid population growth has been coupled with vast changes in cultures and ecology, creating new opportunities for adaptation,” the study says. “The past 10,000 years have seen rapid skeletal and dental evolution in human populations, as well as the appearance of many new genetic responses to diet and disease.”
The researchers note that human migrations into new Eurasian environments created selective pressures favoring less skin pigmentation (so more sunlight could be absorbed by skin to make vitamin D), adaptation to cold weather and dietary changes.
Because human population grew from several million at the end of the Ice Age to 6 billion now, more favored new genes have emerged and evolution has speeded up, both globally and among continental groups of people, Harpending says.
"We have to understand genetic change in order to understand history,” he adds.
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- “We aren’t the same as people even 1,000 or 2,000 years ago,” he says, which may explain, for example, part of the difference between Viking invaders and their peaceful Swedish descendants.
“The dogma has been these are cultural fluctuations, but almost any temperament trait you look at is under strong genetic influence.”
We show that humans are changing relatively rapidly on a scale of centuries to millennia (....)
