By his account, Hawks’s theory of accelerated human evolution owes its genesis to what he could see with his own eyes. But his radical view was also influenced by newly emerging genetic data. Thanks to stunning advances in sequencing and deciphering DNA in recent years, scientists had begun uncovering, one by one, genes that boost evolutionary fitness. These variants, which emerged after the Stone Age, seemed to help populations better combat infectious organisms, survive frigid temperatures, or otherwise adapt to local conditions. And they were popping up with surprising frequency.
Taken together, the skeletal and genetic evidence convinced Hawks that the ruling “static” view of recent human evolution was not only wrong but also quite possibly the opposite of the truth. He discussed his ideas with Harpending, his former postdoc adviser at the University of Utah, and Gregory Cochran, a physicist and adjunct professor of anthropology there. They both agreed with Hawks’s interpretation. But why, they wondered, might evolution be picking up speed? What could be fueling the trend?
Then one day, as Hawks and Cochran mulled over the matter in a phone conversation, inspiration struck. “At exactly the same moment, both of us realized, gee, there’s a lot more people on the planet in recent times,” Hawks recalls. “In a large population you don’t have to wait so long for the rare mutation that boosts brain function or does something else desirable.”
The three scientists reviewed the demographic data. Ten thousand years ago, there were fewer than 10 million people on earth. That figure soared to 200 million by the time of the Roman Empire. Since around 1500 the global population has been rising exponentially, with the total now surpassing 6.7 billion. Since mutations are the fodder on which natural selection acts, it stands to reason that evolution might happen more quickly as our numbers surge. “What we were proposing was nothing new to animal breeders of the 19th century,” Cochran notes. “Darwin himself emphasized the importance of maintaining a large herd for selecting favorable traits.”
The logic behind the notion was undeniably simple, but at first glance it seemed counterintuitive. The genomes of any two individuals on the planet are more than 99.5 percent the same. Put another way, less than 0.5 percent of our DNA varies across the globe. That is often taken to mean that we have not evolved much recently, Cochran says, “but keep in mind that the human and chimp genomes differ by only about 1 to 2 percent—and nobody would call that a minor difference. None of this conflicts with the idea that human evolution might be accelerating.”
CULTURE SHOCK
If their hunch was correct, the scientists wondered a few years back, how could they prove it? As it turned out, it was an opportune time to pose that question.
For decades theories about human evolution had proliferated despite the absence of much, if any, hard evidence. But now there were finally human genetic data banks large enough to allow the scientists to put their assumptions to the test. One of these, the International Haplotype Map, cataloged differences in DNA collected from 270 people of Japanese, Han Chinese, Nigerian, and northern European descent. Moreover, Harpending knew two geneticists—Robert Moyzis of the University of California at Irvine, and Eric Wang of Veracyte Inc. in South San Francisco—who were at the forefront of developing new computational methods for mining this data to estimate the rate of evolution. Harpending contacted them to see if they would be willing to collaborate on a study.
Human races are evolving away from each other. We are getting less alike, not merging into a single mixed humanity.
The West Coast scientists were intrigued. On the basis of their own preliminary data, they, too, suspected that the pace of human evolution was accelerating. But they had arrived at the same crossroads by a different route. “We were focused on cultural shifts as a prime driving force of our evolution,” Moyzis says. As he explains it, an exceptional period in the history of our species occurred about 50,000 years ago. Humans were pouring forth from Africa and fanning out across the globe, eventually taking up residence in niches as diverse as the Arctic Circle, the rain forests of the Amazon, the foothills of the Himalayas, and the Australian outback. Improvements in clothing, shelter, and hunting techniques paved the way for this expansion.
Experts agree on that much but then part ways. These innovations, prominent evolutionary theorists insist, insulated us from the relentless winnowing of natural selection, thereby freeing us from the Darwinian rat race. But Moyzis and Wang looked at the same developments and came to the opposite conclusion. In our far-flung domains, they point out, humans presumably encountered starkly different selective forces as they adjusted to novel foods, predators, climates, and terrains. And as we became more innovative, the pressure to change only intensified. “If you’re a human, what is your environment but culture?” Moyzis asks. “The faster our ingenuity alters our habitat, the quicker we have to adapt in response.”
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