“The drunk looking for keys under the street lamp…”

Today, the Proceedings of the National Academy of Sciences published in its Early Edition section a new paper entitled “Why did moden human populations disperse from Africa ca. 60,000 years ago? A new model.” by Paul Mellars, of Cambridge University. This, incidentally, is the same title as the paper which Mellars was originally scheduled to present at the Paleoanthropology Society meetings in San Juan, Puerto Rico this past April.

The post’s title is a quote of Mellars’ name for what he calls the ‘syndrome’ of the tendency to assume that something must have developed in a given area simply because most of the currently available evidence for it comes from that area.

The paper is a short overview of Mellars’ now well-known stance on what the mtDNA evidence means for the origins and dispersals of modern humans out of African. He summarizes a number of recent genetic studies, concluding that

“.. it is clear that some significant demographic or cultural factors must have promoted these lineage expansions at roughly the same time as the mtDNA mismatch analyses point to a rapid increase in total population numbers from some localized geographical source.” (Mellars 2006:9382)

In a nutshell, Mellars argues that the genetic data demonstrate a population boom in some African population of anatomically modern humans roughly 80,000 years ago, which would then have genetically swamped the rest of Africa before subsequently swamping the rest of the Old World. He suggests that this genetic expansion resulted from the concomitant development of a number of behavioral traits at roughly that time, which explains why the ancestors of all modern humans today could spread out of their homeland and ultimately replace all other archaic populations starting only 60,000 years ago, despite the modern human form having been in existence since maybe 200,000 BP.

Mellars argues that these behavioral changes that would have enabled this population expansion are:
1) new technology that would have made hunting more efficient and productive;
2) some forms of plant food management strategies;
3) the exploitation of marine fish “and perhaps sea birds” (Mellars 2006:9383);
4) more reliable and extensive exchange networks reflected by “large-scale movements of high-quality stone and imported shell ornament” (Mellars 2006:9383).

Basically, it is his typical list of the diagnostic features of the Upper Paleolithic “revolution” (see Mellars 2004, 2005), only projected back in time. Except that in this case, these changes are based at most on a handful of sites (and in certain cases only one site) distributed over the largest continent on earth over a 20,000 year window. He suggests in passing that these populations might have possessed bow and arrow technology despite providing no empirical proof whatsoever for a statement which, if true, would have profound implications for Late Pleistocene technological evolution. Likewise, the case for plant food management is based on only two documented instances while the case for marine fish and sea bird exploitation is based solely on one level from Blombos. As for the empirical support for long-distance transfers (and by extension the existence of extensive exchange networks), it is important to note that it has been argued elsewhere (e.g., Féblot-Augustins 1997, 1999) that even Neanderthals were capable of procuring fine-grained raw material from considerable distances away (up to 300 km), if there was a need to do so, imposed more than anything by contextual factors. That this capacity is also evident in the MSA is therefore not surprising. In fact, Negash and Shackley (2006) have recently shown that the MSA site of Porc Epic in Ethiopia contains obsidian from sources located over 250 linear kilometers from the site. This is in spite of the site containing no other evidence of what Mellars believes is “advanced” behavior and the assemblage being typically MSA rather than either Howiessons’ Poort or Still Bay in typological designation.

Mellars insists that he is highlighting “the crucial evolutionary and adaptive developments that allowed these populations to colonize” the rest of the world and extirpate all archaic populations they encountered. However, this paper is simply a list of select archaeological finds from a very limited number of sites which are then used to weave together a grand synthesis of recent human prehistory. The “evolutionary and adaptive” nature of any of these finds is assumed rather than demonstrated. In other words, they are only adaptive by association. It is the old (and logically fallacious) scenario of “If modern humans spread from Africa, and if they are associated by behavioral traits x and y, then behavioral traits x and y must have enabled this spread.” Nowhere is there an explicit demonstration of the large-scale adaptive features of these behavioral innovations relative to previous ways of doing things in the Middle and Late Pleistocene.

While there is nothing wrong with that, Mellars’ argument here suffers from one major limitation, namely, that – archaeologically speaking at least – it is all grand synthesis with such a small amount of empirical evidence to back it up that it amounts to little more than a possible, though far from probable, scenario that injects little new blood into the modern human origins debate beyond a few ‘textbook’ generalizations.

References:

Féblot-Augustins, J. 1997. La Circulation des Matières Premières au Paléolithique: Synthèse des Données Perspectives Comportementales (2 Volumes). ERAUL 75, Liège.

Féblot-Augustins, J. 1999. Raw material transport patterns and settlement systems in the European Lower and Middle Paleolithic: continuity, change and variability. In The Middle Paleolithic Occupation of Europe (W. Roebroeks & C. Gamble, eds.), pp. 193-214. University of Leiden Press, Leiden.

Mellars, P. 2004. Neanderthals and the modern human colonization of Europe. Nature 432:461-465.

Mellars, P. 2005. The impossible coincidence. A single-species model for the origins of modern human behavior in Europe. Evolutionary Anthropology 14:12-27.

Mellars, P. 2006. Why did modern human populations disperse from Africa ca. 60,000 years ago? A new model. Proceedings of the National Academy of Sciences 103:9381-9386.

Negash, A., and M. S. Shackley 2006. Geochemical provenance of obsidian artefacts from the MSA site of Porc Epic, Ethiopia. Archaeometry 48:1-12.