Via GNXP, I hear there's been a new study on North African genetics: Genomic Ancestry of North Africans Supports Back-to-Africa Migrations. It provides an interesting cross-check on linguistic hypotheses.
In brief, the story these geneticists propose is: the main ancestors of modern North Africans, in particular Berbers, migrated into North Africa at least 12,000 and perhaps as much as 40,000 years ago; this "Maghrebi" component is close to Western Eurasian populations, and is dominant in most of their Moroccan and Algerian samples (and prominent in Libya). Arabs migrated in more recently starting 1,400 years ago, and Near Eastern influence is prominent throughout, especially in Libya, and dominant in Egypt. The Sub-Saharan African component seems to have arrived even later (~1,200 years ago in southern Morocco) and thus probably reflects the trans-Saharan slave trade; in Morocco it looks West African, while in Egypt it appears more diverse. Some European admixture is visible in Algeria and northern Morocco as well, but its nature is not clear. The data set is a bit small: a better coverage of Sahelian populations would be highly desirable, as would more Near Eastern populations, and one wonders where the ancient Egyptians fit in. However, the overall picture seems reasonable.
The more recent stages fit trivially with the detailed linguistic and historical data available (see my earlier post on linguistic traces of sub-Saharan immigration into North Africa), but the genetic divergence between Maghrebis and western Eurasian populations takes us into a realm where both fields offer much less certainty. Linguistically, we know that Berber, Semitic, and Egyptian are all distantly related to one another (and to Chadic and Cushitic, though that doesn't show up in the genetic data here); but we don't know when they split apart. There is no generally agreed upon method for dating linguistic divergences, and Swadesh's original "radioactive decay" glottochronological formula has proved too poor an approximation to be relied upon. However, a much-modified glottochronological formula was more recently proposed by Sergei Starostin in an attempt to fit a curve of attested data points. As it happens, two of his followers, George Starostin and Alexander Militarev, have ventured to offer estimates for Afroasiatic; for the split between Semitic and Berber, they respectively estimate 9,700 or 11,000 years ago. This seems strikingly close to the lower limit of the geneticists' estimate here. But even if this estimate is rejected, if the divergence date is anywhere near what the genetics is suggesting, then we have to conclude that genetic relationships older than 10,000 years can be discerned, contrary to some claims in the literature.
There is a way around this: one could propose a pre-Phoenician immigration that changed the language but had relatively little impact on the gene pool. In fact, such an event may have to be postulated for Afroasiatic's history in at least some areas anyway: speakers of one Chadic language are represented in this paper - Hausa - and their genes look nothing like North Africans or Near Easterners. However, it hardly seems like a parsimonious hypothesis in this case, given the split dates suggested. So... is this a corroboration of Starostin's method, or just a lucky guess?