Unless each group independently invented click languages at some later time, that finding implies that click languages were spoken by the very ancient population from which the Hadzabe and the Ju|'hoansi descended. ''The divergence of those genetic lineages is among the oldest on earth,'' Dr. Knight said. ''So one could certainly make the inference that clicks were present in the mother tongue.''

If so, the modern humans who left Africa some 40,000 years ago and populated the rest of the world might have been click speakers who later lost their clicks. Australia, where the Damin click language used to be spoken, is one of the first places outside Africa known to have been reached by modern humans.

But the antiquity of clicks, if they are indeed extremely ancient, raises a serious puzzle. Joseph Greenberg of Stanford University, the great classifier of the world's languages, put all the click languages in a group he called Khoisan. But Sandawe and Hadzane, the language of the Hadzabe, are what linguists call isolates. They are unlike each other and every other known language. Apart from their clicks, they have very little in common even with the other Khoisan languages.

That the Hadzabe and the Ju|'hoansi differ as much in their language as in their genetics is a reflection of the same fact. They are extremely ancient, and there has been a long time for both their language and their genetics to diverge. The puzzle is why they should have retained their clicks when everything else in their languages has changed.

Dr. Knight suggested that clicks might have survived because in the savanna, where most click speakers live, the sounds allow hunters to coordinate activity without disturbing prey. Whispered speech that uses just clicks sounds more like branches creaking than human talk. Clicks make up more than 40 percent of the language and suffice for hunters to convey their meanings, Dr. Knight said.

Dr. Anthony Traill, an expert on click languages at the University of Witwatersrand in South Africa, said he did not find the hunting idea very plausible.

''Clicks are acoustically high-impact sounds for mammalian ears,'' Dr. Traill said, ''probably the worst sounds to use if you are trying to conceal your presence.''