Although humans come in many shapes and sizes, from the compact Inuit of the Arctic to the willowy Masai warriors of Africa, any two people are a lot more alike genetically than any pair of chimpanzees or gorillas. The reason may be our advanced culture, according to a new study. Our ancestors' different tools, eating habits, and even body decorations limited their mate choices to individuals of a similar culture, the work suggests, reducing the spread of new mutations across many groups. Because only a few of these ancient groups survived, humans are much less genetically diverse than other primates, even though there are many more of us on the planet.
Ever since researchers discovered in the 1970s that humans lacked the genetic variation expected of our population size, they have proposed that our ancestors went through a big squeeze: Volcanic eruptions, disease, or climate change created a population "bottleneck" that reduced the number of breeding adults to about 10,000 sometime in the past 100,000 years. But new genetic studies of ancient DNA from Neandertals have found that they and the last ancestor they shared with humans, about 600,000 years ago, also lacked much genetic variation, which would require at least three dramatic bottlenecks--an improbable scenario. Meanwhile, other studies have found that language differences restricted gene flow in recent times in Europe, suggesting that cultural barriers might have limited genetic diversity more consistently than occasional local bottlenecks.
Paleoanthropologists Jean-Jacques Hublin and Luke Premo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, tested this hypothesis by simulating how mating preferences alter gene flow between individuals in different groups. Genetic variability plunged when individuals required mates with the highest degree of cultural similarity, the team reports this week in The Proceedings of the National Academy of Sciences. Conversely, genetic diversity increased when individuals were less selective about their mates--as is the case in chimpanzees or gorillas, which mate whenever possible with individuals from other groups.
Hublin and Premo propose that if human ancestors selected mates from similar backgrounds, there would have been a lot of inbreeding within different populations, restricting the flow of new mutations to other groups. "If these guys on the other side of the river spoke a different language and had different weapons, you would not try to mate with them or they might kill you," says Hublin. Over time, most populations went extinct, allowing the genes of only a few groups to proliferate, further erasing genetic diversity.
Researchers who have long worked on this problem are eager to test the new hypothesis in living hunter-gatherers. Paleoanthropologist John Hawks of the University of Wisconsin, Madison, for example, plans to ask his students to determine whether there's more intermarriage between hunter-gatherer groups that live close together and, therefore, are likely to have similar cultures. Biological anthropologist Henry Harpending of the University of Utah in Salt Lake City also likes the new explanation for the missing mutations: "It is time that human population geneticists recover from waving the magic wand of 'bottleneck' to try to explain everything."