When people in hunter-gatherer communities have a conflict, Moyzis reports, usually one of them will just walk away. “There is a great deal of fluidity in these societies,” he says, “so it’s easy to join another group.” But with the establishment of the first farming communities, we put down roots figuratively as well as literally. “You can’t just walk away,” Moyzis notes, a fact that would have created selection pressure to revise the mechanisms regulating aggression, such as the glutamate pathways involved in arousal. “When you domesticate animals, you tend to change genes in that system,” he says.
For decades theories about human evolution proliferated in the absence of hard evidence, but now human genetic data banks are large enough to put assumptions to the test.
The rise of settlements also promoted the breakdown of labor into specialized jobs. That, coupled with food surpluses from farming, led to systems of trade and the need to track the flow of resources, which in turn could have selected for individuals with specific cognitive strengths. “Mathematical ability is very important when it comes to keeping track of crops and bartering,” Wang says. “Certainly your working memory has to be better. You have to remember who owes you what.” The researchers point to China’s Mandarin system, a method of screening individuals for positions as tax collectors and other government administrators. For nearly 2,000 years, starting in A.D. 141, the sons of a broad cross section of Chinese society, including peasants and tradesmen, took the equivalent of standardized tests. “Those who did well on them would get a good job in the civil service and oftentimes had multiple wives, while the other sons remained in a rice field,” Moyzis says. “Probably for thousands of years in some cultures, certain kinds of intellectual ability may have been tied to reproductive success.”
Harpending and Cochran had previously—and controversially—marshaled similar evidence to explain why Ashkenazi Jews (those of northern European descent) are overrepresented among world chess masters, Nobel laureates, and those who score above 140 on IQ tests. In a 2005 article in the Journal of Biosocial Science, the scientists attributed Ashkenazis’ intellectual distinction to a religious and cultural environment that blocked them from working as farm laborers in central and northern Europe for almost a millennium, starting around A.D. 800. As a result, these Jews took jobs as moneylenders and financial administrators of estates. To make a profit, Harpending says, “they had to be good at evaluating properties and market risks, all the while dodging persecution.” Those who prospered in these mentally demanding and hostile environments, the researchers posit, would have left behind the most offspring. Critics note that the association between wealth and intelligence in this interpretation is circumstantial, however.
Stronger evidence that natural selection has continued to shape the brain in recent epochs comes from studies of DRD4, a mutation in a neurotransmitter receptor that Moyzis, Wang, and many others have linked to attention-deficit/hyperactivity disorder (ADHD). Children diagnosed with ADHD are twice as likely to carry the variant gene as those without the diagnosis. DRD4 makes a receptor in the brain less effective in bonding to dopamine, which might explain why Ritalin, which increases the amount of dopamine in the space between neurons, is often helpful in treating the problem.
Sequencing studies suggest that the DRD4 mutation arose 50,000 years ago, just as humans were spreading out of Africa. Its prevalence tends to increase the farther a population is from Africa, leading some investigators to dub it “the migratory gene.” At least one allele (or copy of the gene) is carried by 80 percent of some South American populations. In contrast, the allele is present in 40 percent of indigenous populations living farther north in the Americas and in just 20 percent of Europeans and Africans. Children with the mutation tend to be more restless than other youngsters and to score higher on tests of novelty-seeking and risk-taking, all traits that might have pushed those with the variant to explore new frontiers.
In the context of a modern classroom, it may be hard to understand why kids who appear distractible and disruptive might have a survival advantage. But research shows people with DRD4 do not differ in intelligence from national norms; if anything, they may on average be smarter. Moreover, behavior that may seem like a drawback today may not have been so in ancient environments. When broaching foreign terrain filled with unknown predators, “having the trait of focusing on multiple directions might have been a good thing,” Wang says. “People focused in one direction might get eaten.”
Humans in far-flungdomains encountered starkly different selective forces, adjusting to novel foods, predators, climates, and terrains.
NOT SO FAST
Despite all these clues that human evolution has continued and accelerated into modern times, many evolutionary biologists remain deeply skeptical of the claims. Their resistance comes from several directions.
Some independent experts caution that the tools for studying the human genome remain in their infancy, and reliably detecting genomic regions that have been actively selected is a challenging problem. The hypothesis that human evolution is accelerating “all rests on being able to identify recent areas of the genome under natural selection fairly accurately,” says human geneticist Jonathan Pritchard of the University of Chicago. And that, he warns, is tricky, involving many different assumptions (about population sizes on different continents, for instance) in the poorly documented period before recorded history.
Given such uncertainties, researchers are more likely to be persuaded that a mutation has been recently selected if they understand its function and if its rise in prevalence meshes well with known human migratory routes. Genetic variants fitting that description include those coding for lighter skin coloring, resistance to diseases such as malaria, and metabolic changes related to the digestion of novel foods. There is broad consensus that these represent genuine examples of recent adaptations.
Question marks surround many other recent genetic changes. We know almost nothing about most regions of the genome that have been identified as potential targets of natural selection, observes Sarah Tishkoff, a geneticist at the University of Pennsylvania School of Medicine. Until scientists understand more of the landscape of the human genome, she says, she will have a hard time believing that adaptive genetic differences between ethnic groups have mushroomed over the past 20,000 years. She is particularly wary of claims that selective pressures recently played a role in shaping different cognitive abilities and temperaments among ethnic groups. “We have no strong evidence of that,” Tishkoff says.
Francis Collins, who until last year headed the National Human Genome Research Institute at the National Institutes of Health, concurs. “This is not a place to idly speculate about possibilities,” he says. “When it comes to brain functioning, let’s be honest: That is a tinderbox of possible explosive reactions based upon a very unpretty history of discrimination and of demagogues using information that they claimed came from biology in order to put down some groups that they didn’t like.” Even when it comes to the ADHD connection, Collins is a skeptic. “I want to see DRD4 replicated by independent investigators on an independent sample of children,” he says.
In some circles, Moyzis says, to suggest that natural selection is acting on the human brain is tantamount to heresy—an incredible hypothesis that demands extraordinary proof. Harpending, Cochran, and their collaborators are mystified as to what it is that makes their theory so incredible. “I would turn that statement on its head,” Moyzis says. “The extraordinary claim is that evolution somehow stopped once we developed culture.” Cochran says, “You’re allowed to change, but only if it’s below the neck. Many people think the brain has to be immune to natural selection; if it isn’t, they don’t want to hear it.”
Harvard University evolutionary biologist Pardis Sebati defends that view. “The immune system and skin interact directly with the outside world,” she says. “They are our first line of defense.” Based on the current evidence, she concludes, sunlight and pathogens were among the strongest selective forces, and skin and the immune system underwent the most dramatic change; evolutionary pressures on the brain are not nearly as clear-cut. As Harvard geneticist David Altshuler wrote in response to one of Sebati’s articles, “It’s reassuring that differences between the races seem to be mostly skin deep.”
The “reassuring” quality of that belief makes those in the opposing camp wonder if some of the logic of skeptics is tinged with wishful thinking. Harvard’s Steven Pinker, the celebrated author of The Blank Slate and an expert on the evolution of language and the mind, addressed that point in an interview in New Scientist magazine: “People, including me, would rather believe that significant human biological evolution stopped between 50,000 and 100,000 years ago, before the races diverged, which would ensure that racial and ethnic groups are biologically equivalent.”
Many scientists apparently worry that proof of divergent brain evolution could be so racially polarizing that we, as a society, would almost be better off in the dark. Hawks responds that the best safeguard against bigotry is educating the public. He thinks we understand enough about human genetics to know that the notion of racial superiority is absurd. Intelligence, he argues, is not a single trait but a vast suite of abilities, and each ancestral environment may have favored a different set of talents. What is sorely needed, he says, is “an ecological framework” to interpret the results. “Groups are best adapted to their own environment, which eliminates the question of superiority.” Even he concedes, though, that communicating the nuances will be no easy task.
“Whatever we find,” Wang says, “it would never be justification for abandoning the egalitarian value that all individuals, regardless of their ethnicity, are deserving of the same rights and opportunities.” Moyzis expands on that line of reasoning, putting a sunny spin on the group’s findings. “It would be boring if all the races were fundamentally the same,” he argues. “It’s exciting to think that they bring different strengths and talents to the table. That is part of what makes melting-pot cultures like our own so invigorating and creative.”
Of course, in melting-pot cultures all kinds of ethnic groups intermingle freely, and the children who result literally meld our DNA together. Even if those groups were diverging, international travel is now causing the diversity to get lost in the genetic reshuffling. “That’s the ultimate irony,” Moyzis says. “By the time we finally settle this debate, we’ll all be such a mixture of genes that we won’t care.”