As for the role of population size in spurring our evolution, he and Wang had not given it much thought, but they saw the idea as complementary to their own view, since cultural innovations allowed more people to survive. So when Harpending’s group came calling, Moyzis says, “we were happy to combine ideas and work together.”
To study natural selection, the team combed the International Haplotype Map for long stretches of DNA flanked by a single nucleotide polymorphism (SNP, or “snip”)—that is, an altered base, or “letter,” in the genetic alphabet. When the exact same genetic block is present in at least 20 percent of a population, according to the scientists, it indicates that something about that block has conferred a survival advantage; otherwise, it would not have become so prevalent. Because genes are reshuffled with each generation, Moyzis adds, the presence of large unchanged blocks of DNA means they were probably inherited recently. In the parlance of scientists, it is “a signature of natural selection.”
Scanning genomes in the haplotype map for these clues, the researchers discovered that 7 percent of human genes fit the profile of a recent adaptation, with most of the change happening from 40,000 years ago to the present. As predicted, these apparent adaptations occurred at a rate that jumped almost exponentially in prevalence as the human population exploded. To rule out the prevailing view—that our evolution has proceeded at a steady rate all along—the scientists ran an additional check. They performed a computer simulation to see what would have happened if humans had evolved at modern rates ever since we diverged from chimpanzees 6 million years ago. The steady-state test led to a nonsensical result: The difference between the two species today would be 160 times greater than it actually is. To Moyzis and the others, the results confirmed that human evolution had only recently hit the accelerator.
MORPHING AT HIGH SPEED
All of these findings mesh beautifully with the notion that cultural and demographic shifts sparked our transformation. Our exodus out of Africa, for example, paved the way for one of the most obvious markers of race, skin hue. As scientists widely recognize, paler complexions are a genetic adjustment to low light: People with dark skin have trouble manufacturing vitamin D from ultraviolet radiation in northern latitudes, which makes them more susceptible to serious bone deformities. Consequently, Europeans and Asians over the last 20,000 years evolved lighter skin through two dozen different mutations that decrease production of the skin pigment melanin.
Similarly, the gene for blue eyes codes for paler skin coloring in many vertebrates and hence might have piggybacked along with lighter skin. Clearly something made blue eyes evolutionarily advantageous in some environments. “No one on earth had blue eyes 10,000 years ago,” Hawks says.
The transition to an agrarian existence after hundreds of thousands of years of hunting and gathering was another key catalyst of evolution. Once people began keeping cattle herds, for example, it became an advantage to derive nutrient calories from milk throughout life rather than only as an infant or toddler suckling at its mother’s breast. A mutation that arose about 8,000 years ago in northern Europe, Hawks says, allowed adults to digest lactose (the main sugar in milk), and it propagated rapidly, allowing the rise of the modern dairy industry. Today the gene for lactose digestion is present in 80 percent of Europeans but in just 20 percent of Asians and Africans.
Agriculture may have opened up other pathways for evolution by supporting an ever-growing population that eventually began to congregate in the first cities. In crowded, filthy quarters, pathogens spread like wildfire. Suddenly there were epidemics of smallpox, cholera, typhus, and malaria, diseases unknown to hunter-gatherers, and so began an evolutionary arms race to fend off the assault through superior immunity.
“The clearest example of that is malaria,” Hawks says. “The disease is about 35,000 years old, with the most lethal form of it just 5,000 years old.” Yet in sub-Saharan Africa and other regions where it is endemic, “people have already developed 25 new genes that protect against malaria, including the Duffy blood type, an entirely new blood group,” he notes. More recently, HIV resistance has appeared due to a genetic mutation now found in 10 percent of Europeans. Scientists speculate that the variant may have originally evolved as a protection against smallpox.
Paralleling the constant war against pathogens, human sperm may also be evolving at high speed, driven by the race to get to the egg before another man’s sperm. “It could be that cities create more sexual partners, which means fiercer competition among males,” Hawks says. Because sperm can fertilize an egg up to 24 hours after being ejaculated in the vagina, a woman who copulates with two or more partners in close succession is setting up the very conditions that pit one man’s sperm against another’s. Hawks infers that “sperm today is very different from sperm even 5,000 years ago.” Newly selected mutations in genes controlling sperm production show up in every ethnic group he and his team have studied; those genes may affect characteristics including abundance, motility, and viability. The selection for “super sperm,” Hawks says, provides further corroboration that our species is not particularly monogamous—a view widely shared by other anthropologists.
At the other end of the human life span, “genes that help us live longer get selected,” Hawks reports. This may seem counterintuitive, since evolutionary biologists long assumed that the elderly do not contribute to the gene pool and hence are invisible to natural selection. But as studies of the Hadza people of Tanzania and other groups suggest, children doted on by their grandmothers—receiving extra provisions and care—are more likely to survive and pass on their grandmothers’ genes for longevity. (Grandfathers were less involved with their grandchildren in the cultures studied, so the phenomenon is known as the “grandmother effect.”) Old men can also pass on their genes by mating with younger women.
As agriculture became established and started creating a reliable food supply, Hawks says, more men and women would have begun living into their forties and beyond—jump-starting the selection pressure for increased life span. In support of that claim, Moyzis is currently performing a genetic analysis of men and women in their nineties who are of European ancestry. He has traced many early-onset forms of cancer, heart disease, and Alzheimer’s to older human gene variants. “The idea is that people with more modern variants tend to have greater resistance to these chronic illnesses of old age and should be overrepresented in the age 90-plus population,” Moyzis says.
EVOLUTION AND THE BRAIN
Perhaps the most incendiary aspect of the fast-evolution research is evidence that the brain may be evolving just as quickly as the rest of the body. Some genes that appear to have been recently selected, Moyzis and his collaborators suggest, influence the function and development of the brain. Other fast-changing genes—roughly 100—are associated with neurotransmitters, including serotonin (a mood regulator), glutamate (involved in general arousal), and dopamine (which regulates attention). According to estimates, fully 40 percent of these neurotransmitter genes seem to have been selected in the past 50,000 years, with the majority emerging in just the past 10,000 years.
Addressing the hot-potato question—What might these changes signify?—Moyzis and Wang theorize that natural selection probably favored different abilities and dispositions as modern groups adapted to the increasingly complex social order ushered in by the first human settlements.
