In past experiments, researchers have examined the same human gene's effect on the brain sizes of mice and ferrets. The brains did grow larger — but in those experiments, the team used an over-expression of the gene, rather than the typical human levels. So the effect could have been due to another hidden factor rather than the gene itself. Plus, crucially, mice and ferrets aren't primates — so they can't tell us how the introduction of the gene may have changed the brains of our modern ancestors. Primates are particularly interesting to researchers because — before now, they didn't know...