Cannibals in the Closet?

Did human cannibalism leave its signature in our DNA? The debate is heating up again over that controversial 2003 claim by British researcher Simon Mead. The evidence, he wrote, lies in the gene that produces the prion protein, the one that in its abnormal form causes mad-cow and kuru--a degenerative brain disease that was endemic among cannibals in New Guinea. The gene, and thus the protein, exists in two slightly different forms, which both work equally well in the body. But people who inherit two copies of the same form--either one--are at risk for prion disease, while those with the two different versions are protected. In a paper in Science, Mead argued that the prevalence of these two forms, and a mathematical analysis of other mutations on the same gene, showed there was strong evolutionary pressure for defense against prion disease for much of human history. But how were people exposed to it? The spontaneous form is very rare, and mad-cow outbreaks are an artifact of industrial agriculture. That left cannibalism.

From the start, the assertion that cannibalism may have been widespread in human history was controversial among geneticists, although some anthropologists seized on it eagerly. Now a new study of prion genes, to be published this week in Genome Research, disputes Mead's findings. Mead acknowledges some problems with his analysis but stands by his conclusions, and is working on another paper that will contain new evidence. "We're having a hard time finding genes for the things we know are important in human evolution--like language," says Jaume Bertranpetit, a Spanish geneticist who was one of the new study's authors. "But cannibalism? We're not saying it didn't happen, but we'll have to look somewhere else for the evidence."