Meanwhile, a team led by Bruce Lahn of the University of Chicago has been investigating a human gene called microcephalin. A statistical analysis of mutations in this gene indicates that its most common form (or allele) evolved as long as 1.1 million years ago, was carried for most of that time by a different hominid species and then was reintroduced into the human population–conceivably even by a single mating–some 37,000 years ago. That’s about the time that modern humans, coming from Africa, were replacing Neanderthals in Europe. Whatever that allele does, it must have conveyed a very strong evolutionary advantage, because from that single event of what geneticists politely call “introgression” it spread to 70 percent of the human population today.
And what might that advantage have been? We don’t know yet. The gene is known to control brain growth, and Lahn says the crucial factor could have been anything from changing head size to make childbirth less risky, to improving energy efficiency in the brain. But one obvious possibility is that, perhaps in combination with genes that humans already possessed, it made them smarter. “I don’t buy the stereotype that Neanderthals were dumb,” says Henry Harpending, a leading researcher on the genetics of intelligence. “Modern humans came into Europe and encountered Neanderthals and within a few thousand years were making glorious cave paintings, figurines and art. About the same time they show up in Australia [where there were no Neanderthals], and there’s not a trace of that.” Lahn has no direct evidence that it was Neanderthals who carried the crucial gene, and Paabo and Rubin haven’t found it yet in their fossil DNA. So it’s still “far out,” Harpending admits, but civilization could have gotten its start in an act of “introgression” with another species.
