Few taboos are stronger than cannibalism. It's no surprise then that a study published 2 years ago created quite a stir by claiming that modern humans harbor a genetic signature suggesting our ancestors engaged heavily in the practice. A new study, however, questions these findings, concluding that the original analysis was in error.
In the first study, published in Science (25 April 2003, p. 640), a team led by John Collinge of University College London (UCL), looked at a human gene called PRNP which codes for prions (ScienceNOW, 10 April 2003). These misfolded proteins are thought to be responsible for several neurodegenerative diseases including Creutzfeldt-Jacob Disease (CJD) and kuru. Individuals with certain variations in this gene are more resistant to the diseases.
When the team looked at the chromosomes of more than 1000 people from populations around the world, they concluded that the prevalence of two of these variations was due to an evolutionary balancing act that had kept them in the gene pool for as long as 500,000 years. The researchers hypothesized that this "balancing selection" was due to widespread cannibalistic practices that had made early humans susceptible to prion diseases.
Now a second team has tackled the same question and come up with a much different result. Geneticist Jaume Bertranpetit and his coworkers at the Pompeu Fabra University in Barcelona, Spain, completely sequenced the chromosomes of 174 people from around the world. In contrast with the Collinge team, which analyzed only those variations present in 5% or more of the population, the Barcelona team included all 22 known PRNP variations in their study no matter how low their frequencies. As they report in this month's issue of Genome Research, the results were not consistent with balancing selection over the last half million years of human evolution but more likely due to as yet unknown selective pressures. The Collinge team's results, Bertranpetit and his coworkers argue, had been statistically skewed because its study left low frequency variations out of the analysis, an error known as "ascertainment bias."
"I think that this paper clearly rejects the model of selection" posited by Collinge's team, says University of Chicago human geneticist Anna Di Rienzo. The original authors, however, are sticking to their guns. Lead author and prion researcher Simon Mead of UCL argues that his paper's conclusions were based on several different lines of evidence that trump criticisms of ascertainment bias. Nevertheless, Mead says, the team will now try to figure out why there is a discrepancy between the two papers.