14 March 2011

Out of southern Africa: the ancestral click

In a previous post I talked about the Out of Africa (OOA) theory, which is the most generally accepted explanation about the origin and dispersal of our species. There are also some alternative views, the so called multirregional theories, and in recent times the original OOA proposal has become more complex, with the addition of possible hybridization between H. Sapiens and other hominid groups, e.g. the Neanderthals, and the possibility of various migration routes from Africa, also with varying chronologies. The debate is still open, but the core of the theory remains the same: our species emerged in Africa some time between 100,000 and 200,000 years ago. Now, whereabouts in Africa exactly?

In a very recent article (Henn et al, 2011, available here) it has been suggested that South Africa is the most likely location of the emergence of Homo Sapiens. The authors used a very large sample of genetic material from various sub-Saharan populations, especially hunter-gatherers. They concluded that the highest level of genetic variability in Africa, and therefore in the world, can be found in some areas of southern Africa. The models of geographic dispersal clearly suggest the same conclusion: (p. 3) "A point of origin in southwestern Africa was approximately 300-1,000 times more likely than in eastern Africa". On the right you can see a map, taken from the article, with a representation of geographic distance deduced from genetic material.

The authors also mention the fact that the suggested original area coincides roughly with the aera where the so-called Khoisan languages are spoken. We would have a very deep chronology based on the genetic diversity of those human populations and the presence of a very peculiar group of languages. The authors don't make any further comments or proposals about the population-language correlation, but the idea is tempting, and it has of course been proposed by other researchers, as we will see later. Now, what is it that makes the Khoisan languages so peculiar? No doubt, the presence of click phonemes, which are not found anywhere else in the world, with only some rare isolated cases in eastern Africa. What does a click sound like? Let's take a first lesson:

The correlation between population divergence and linguistic distribution has led some researchers to think that the Khoisan languages may date to a very early period in our history as a species. In their 2003 article (available here), Knight et al studied the genetic divergence between some of the Khoisan-speaking groups, especially between the San and the Hadzabe. This divergence is so deep that it can only be understood as a very old phenomenon, maybe more than 40,000 yeras old. The authors suggest that the linguistic divergence between those populations could correlate with the genetic variation, and be equally old. According to them , the possibility of an independent 'invention' of a click sound repertoire must be ruled out; therefore, these clicks must have been already present in some kind of proto-Khoisan language that was spoken prior to the San-Hadzabe separation. (p. 471): "The deep genetic divergence between the click-speaking groups is consistent with the hypothesis that clicks are an ancient element of human language", an element which only survived in those original areas and disappeared elsewhere. This hypothesis is in fact a radical proposal of language continuity as it suggests a linguistic survival from beyond the Upper Palaeolithic, but it is nearly impossible to test its validity. Trying to compare hunter-gatherer societies through time, including their languages, is no easy task, and there are other ways of explaining the presence of clicks in those languages (vid. Traunmüller 2003, available here). But it must be admitted that the correlation between the various data (archaeological, genetic and linguistic) is firmly established, which is a good basis for further research and developments. 

- Henn et al (2011). "Hunter-gatherer genomic diversity suggests a southern African origin for modern humans". PNAS, 108 (10).
- Knight et al (2003). "African Y chromosome and mtDNA divergence provides insight into the history of click languages". Current Biology, 13, 464-473.
- Traunmüller (2003). "Clicks and the idea of a human protolanguage". PHONUM, 9, 1-4.


Belenos said...

Interesting article, though there are other genetic explanations for the very high diversity in South-Western Africa. On Dienekes' blog, the author tends towards the theory that the extreme genetic difference of the Khoi-San may be the result of a hybridisation event similar to that which occured with Neanderthals and Denisovans outside of Africa, rather than an early split of this group from the main human lineage.

As to the basal nature of clicks providing evidence for continuity as expressed in your blog, I dont think it really does. After all, almost all linguists argue for the persistance of language from the point of human origin, and the fact one phoneme has survived in one place rather than another is an argument for general linguistic evolution rather than continuity.

Jesús Sanchis said...

Obviously, there are other possible explanations for the genetic diversity of those populations. In any case: is there any other place in Africa which would be a more likely candidate for the original homeland of our species?

As I said in my post, it is probably impossible to test the validity of the ancestral click hypothesis. But I find it interesting.

Of course, it would take much more than a blog post to show the complete details of these proposals. For example, it would be necessary to point out that the classification of the so-called 'Khoisan' languages is far from clear, even its atatus as a language group. A lot of work is needed in order to fully understand the areal distribution of those languages, and also to establish more detailed patterns of genetic variation.