In Tanzania, the Hadzabe people communicate in part through an unusual vocabulary of clicking sounds. So do the Ju'hoansi of southwestern Africa, located about 1,600 miles away. "It's been a mystery why these widely separated populations, who have different languages, should share these clicks," says geneticist Alec Knight of Stanford University.
He and his colleague Joanna Mountain studied the two populations, examining a series of genetic markers on both the Y chromosome, which is inherited unchanged from father to son, and the mitochondrial DNA, which is passed on from mother to child. To their surprise, the researchers found that the two groups were not related at all. In fact, the genetic distance between them was as wide as or wider than that between any other pair of African peoples. The result implies that click languages arose among an ancestral people common to both the Hadzabe and the Ju'hoansi before humans left Africa, at least 40,000 to 50,000 years ago. Knight suggests that click languages survived because they serve a practical purpose. The Ju'hoansi communicate by signs and clicks when stalking prey such as antelope and giraffe. Unlike vocal speech, clicks do not scare game, perhaps because the sounds resemble the rustling of dry brush on the savanna. "So even though their languages changed over time, they kept the clicks," Knight says.