From this article:
Because words change so rapidly, many linguists think that languages cannot be traced very far back in time. The oldest language tree so far reconstructed, that of the Indo-European family, which includes English, goes back 9,000 years at most.
Quentin D. Atkinson, a biologist at the University of Auckland in New Zealand, has shattered this time barrier, if his claim is correct, by looking not at words but at phonemes — the consonants, vowels and tones that are the simplest elements of language. Dr. Atkinson, an expert at applying mathematical methods to linguistics, has found a simple but striking pattern in some 500 languages spoken throughout the world: A language area uses fewer phonemes the farther that early humans had to travel from Africa to reach it.
Some of the click-using languages of Africa have more than 100 phonemes, whereas Hawaiian, toward the far end of the human migration route out of Africa, has only 13. English has about 45 phonemes.
This pattern of decreasing diversity with distance, similar to the well-established decrease in genetic diversity with distance from Africa, implies that the origin of modern human language is in the region of southwestern Africa, Dr. Atkinson says in an article published on Thursday in the journal Science. ...
His study was prompted by a recent finding that the number of phonemes in a language increases with the number of people who speak it. This gave him the idea that phoneme diversity would increase as a population grew, but would fall again when a small group split off and migrated away from the parent group.
Such a continual budding process, which is the way the first modern humans expanded around the world, is known to produce what biologists call a serial founder effect. Each time a smaller group moves away, there is a reduction in its genetic diversity. The reduction in phonemic diversity over increasing distances from Africa, as seen by Dr. Atkinson, parallels the reduction in genetic diversity already recorded by biologists.
For either kind of reduction in diversity to occur, the population budding process must be rapid, or diversity will build up again. This implies that the human expansion out of Africa was very rapid at each stage. The acquisition of modern language, or the technology it made possible, may have prompted the expansion, Dr. Atkinson said.
via Marius Comper