I will only point out two caveats:
- We cannot assume that click speakers of the African Southwest are necessarily indigenous to that region, and
- It is possible that, the greater phonemic diversity is due to ancient admixture between quite divergent peoples who possessed two different types of phonemic inventories, while most Africans inherited only the phonemic inventory of one of these peoples, which then decayed as per the author's theory away from Africa.
Science 15 April 2011:
Vol. 332 no. 6027 pp. 346-349
Phonemic Diversity Supports a Serial Founder Effect Model of Language Expansion from Africa
Quentin D. Atkinson
Human genetic and phenotypic diversity declines with distance from Africa, as predicted by a serial founder effect in which successive population bottlenecks during range expansion progressively reduce diversity, underpinning support for an African origin of modern humans. Recent work suggests that a similar founder effect may operate on human culture and language. Here I show that the number of phonemes used in a global sample of 504 languages is also clinal and fits a serial founder–effect model of expansion from an inferred origin in Africa. This result, which is not explained by more recent demographic history, local language diversity, or statistical non-independence within language families, points to parallel mechanisms shaping genetic and linguistic diversity and supports an African origin of modern human languages.