DNA hints at origin of all language. (Biology: from San Diego, at a meeting of the American Society of Human Genetics).

Some tribes in Africa speak to each other with a vocabulary that includes sharp clicking sounds. Genetic comparison of two such tribes suggests that the unusual click languages, known as Khoisan languages, could resemble the ancestral tongue of all humankind.

For more than a century, linguists have debated the origins of these click languages, notes Alec Knight of Stanford University. These tongues are most prevalent in southwestern Africa where many tribes, including the San and !Kung tribes (the ! represents a click sound), speak them. The Hadzabe people and several other tribes in the East African country of Tanzania also talk with clicks. The geographic diversity of Khoisan tongues, Africa's apparent role as the birthplace of humanity, and other clues have led some scientists to propose that all living humans descended from speakers of a click language.

"It's been very difficult to test that hypothesis," says Knight.

He, Joanna Mountain at Stanford, and their colleagues compared the Y chromosomes of the San tribe with those of the Hadzabe. They conclude that the genetic differences between the San and the Hadzabe are greater than any other pair of populations in the world, indicating that the two groups diverged at the dawn of humanity. Knight says this, in turn, implies that click languages date back that far, as well.

The origin of click languages isn't quite settled. They could have been invented by different tribes at different times or transmitted from one tribe to another, Knight notes. --J.T.