As a result, it may be difficult to determine which human genes have genuinely been affected by recent natural selection. “It's hard to be confident about individual signals beyond the top 10 or so,” says Pritchard.

Some dispute these conclusions. “It depends on the model of population history that you use,” says Hawks. “We believe that populations were larger in the past, which means that there was more selection.”

Sabeti, however, finds Pritchard's conclusions convincing. “There has to be a false-positive threshold,” she says. “We don't really understand the demography of these populations, and there are lots of question marks.”

New data will help answer some questions. The 1000 Genomes Project will soon begin delivering full DNA sequences—not just the most common DNA differences—of more than a thousand people from around the world. Geneticists also will learn more about the genes that have been selected, which will help them separate true examples of selection from misleading signals. “As functional studies go forward, people will start figuring out the phenotypes that are associated with selective signals,” Coop says. “That will be very important, because then we can figure out what the selection pressures were on these phenotypes.”

Pritchard remains cautious about whether new results will answer every question. “For lots of these historical questions, some things are fundamentally unknowable,” he says. But he acknowledges that geneticists may be on the verge of answering a historic question: To what extent has selection shaped both our bodies and our minds? If Darwin were alive today, he would be eagerly awaiting the answer.

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