A recent paper on the origin of language in Africa was subjected to heavy criticism not long after its publication. Now, the African origin of language hypothesis is championed by a new pair of authors who also base their claim on phonemic diversity of African languages.
First, the paper assumes rather than prove a tree-like divergence of human languages. It is not at all clear to me that language does have a common origin in humans. For one thing, research on Neandertal anatomy has suggested that they had the ability to vocalize, and Neandertals diverged from other humans long before the emergence of H. sapiens or even H. helmei in the African Middle Stone Age.
It could very well be that the ability to talk may have been invented twice or even more times, evolving perhaps from a simpler communication system available to species such as H. heidelbergensis. If that is the case, then a tree model of the accumulation of phonemic diversity need not only involve Out of Africa migration, but also contact with other human groups with languages of their own, possessing separate phonemic inventories.
A second point that needs to be made is that I do not really understand the inference of law-like accumulation of phonemic diversity on which the paper is based. It could well be argued, for example, that migration creates separate language communities that undergo different processes of phonemic evolution, some of them retaining ancestral phonemes, some of them developing new ones. Indo-European is a case in point, where languages such as Greek and German have added fricatives, and dispensed with aspirated stops while Hindi has not. The totality of the phonemic inventory in Indo-European languages today may be greater than it was in PIE.
But, there is also a process of simplification within languages leading to a loss of phonemes (e.g., the simplification of the Greek vowel system and the loss of initial aspiration). It is not clear to me whether simplification or diversification wins out in the end: perhaps our ancestors had a much richer phonemic diversity in the past which was reduced as people started becoming part of larger social units than the ancestral hunting tribe. In such units people might get rid of their ancestral way of speaking and accumulate into a new and simpler way, in the same manner that e.g., the phonemic inventory of the entire world is reduced after a couple generations into the sounds of American English by people who become part of the American social unit.
Finally, the paper assumes Out-of-Africa 60-70ky ago. I have been a vocal critic of that idea for quite some time now, and a bird tells me (or should I say tweets me?) that a new synthesis will soon be published that takes into account some recent and exciting archaeological discoveries, that point to a pre-100ky event and apparently exclude a post-70ky one. But, more on that when it sees the light of day and I read it for myself.
PLoS ONE 7(4): e35289. doi:10.1371/journal.pone.0035289
Dating the Origin of Language Using Phonemic Diversity
Charles Perreault, Sarah Mathew
Language is a key adaptation of our species, yet we do not know when it evolved. Here, we use data on language phonemic diversity to estimate a minimum date for the origin of language. We take advantage of the fact that phonemic diversity evolves slowly and use it as a clock to calculate how long the oldest African languages would have to have been around in order to accumulate the number of phonemes they possess today. We use a natural experiment, the colonization of Southeast Asia and Andaman Islands, to estimate the rate at which phonemic diversity increases through time. Using this rate, we estimate that present-day languages date back to the Middle Stone Age in Africa. Our analysis is consistent with the archaeological evidence suggesting that complex human behavior evolved during the Middle Stone Age in Africa, and does not support the view that language is a recent adaptation that has sparked the dispersal of humans out of Africa. While some of our assumptions require testing and our results rely at present on a single case-study, our analysis constitutes the first estimate of when language evolved that is directly based on linguistic data.