Drs Villa and Roebroeks,

I read with interest your paper on Neandetal demise[1], which concluded that "complex processes of interbreeding and assimilation may have been responsible for the disappearance of the specific Neandertal morphology from the fossil record".

In 2002, I published a paper [2] in Current Anthropology that proposed an Out-of-Africa assimilation theory, akin to the Assimilation model of FH Smith [3]. However, my theory differed from the latter in postulating that the spread of anatomically modern humans (AMH) outside Africa was not a migration in the traditional sense but rather was a "diffusion-wave" of advance of a gene-complex ("genotype") of several genes that together conferred an adaptive advantage to the AMH phenotype. That is, it proposed the AMH phenotype spread though demic diffusion, interbreeding and hybridization, as natural selection for the AMH phenotype ensured that it alone survived the interaction, while the archaic phenotype disappeared. The paper also showed how wide-spread hybridization could yet be accompanied by a low rate of assimilation of archaic human alleles into modern populations, which has also now been empirically confirmed.

In the commentaries accompanying the paper, one eminent biological anthropologist wrote that my theory explained "the available genetic and morphological data better than anything we have had until now", while two eminent archaeologists, who are also cited extensively in your paper, commended it for its compatibility with the archaeological data from Europe. So I am surprised this paper is not cited by you, as it presented the most detailed working-out, till date, of a "disappearance by hybridization" scenario for the vanishing of the archaic human morphology.

A follow-up paper [4] looked at data from 313 nuclear DNA loci and concluded that assimilation of genetic material from archaic humans had occurred --- some five years before the Neanderthal contribution to present-day populations was directly confirmed [5]. I believe that the diffusion-wave theory still gives the best explanation for the archaeological and genetic data, and find its neglect by the modern human origins research community inexplicable.


Coming to a more precise point, my 2002 paper submitted that the differences between archaic and modern humans, namely the "congruence of the specific changes in robusticity, facial projection, skull shape, thickness, and density, and pelvis structure powerfully suggest adaptive modifications shaped by childbirth”, and proposed a purely anatomical genetic advantage --- a decrease in child-birth mortality due to the AMH phenotype--- had caused the disappearance of the archaic morphology[2, p762c1, p770c2]. It is well-known that human childbirth is one of the most difficult among mammals, and may have been even more difficult among archaic humans [6,7]. Unfortunately, this suggestion does not seem to have been taken up yet by physical anthropologists, who have a long tradition of treating modern-archaic craniofacial differences as selectively neutral. However, recent work [8,9] concluded that Neandertals (and other archaic humans?) had a non-rotational birth pattern, compared to the modern human rotational one. This adds great force to my earlier argument, for it is inconceivable that such a major and complex change would have occurred without impetus from some significant evolutionary advantage. So there is every likelihood that the new birth pattern and concomitant changes in the head and pelvic structures combined to lower birth mortality among modern humans.

To conclude, while I congratulate you on eliminating so many of the "usual suspects" in the search for modern human "superiority", I point out that the very comprehensiveness of your debunking leaves us begging the question as to why the AMH phenotype spread across the world. Your paper, while not directly addressing my solution, indirectly strengthens it --- by suggesting that Neanderthals and modern humans could have coexisted for several thousand years at the same locations, which would be sufficient for a hybridization and disappearance scenario to play out. I believe it is now time for my hypothesis, that the AMH phenotype resulted in decreased child-birth mortality, to be examined thoroughly, as it is quite possibly the cause of the disappearance by hybridisation of the archaic morphology.

Vinayak Eswaran
IIT Hyderabad, India. eswar@iith.ac.in

[1] Villa, P., and Roebroeks W. (2014). Neandertal Demise: An Archaeological Analysis of the Modern Human Superiority Complex. PLoS ONE, 9 (4), e96424.

[2] Eswaran, V., (2002). “A Diffusion Wave Out of Africa: The mechanism of the modern human revolution?”, Current Anthropology, 43, No 5, (2002).

[3] Smith, F. H. (1985). Continuity and change in the origin of modern Homo sapiens. Zeitschrift fur Morphologie and Anthropologie 75:197–222.

[4] Eswaran, V., Harpending, H.C., and Rogers, A.R., (2005) “Genomics refutes an exclusively African origin of humans”, Journal of Human Evolution, 49(1):1-18.

[5] Green RE, Krause J, Briggs AW, Maricic T, Stenzek U, et al. (2010) A Draft Sequence of the Neandertal Genome. Science 328: 710–722.

[6] Rosenberg, K. (1992). The evolution of modern human childbirth. Year book of Physical Anthropology 35:89–124.

[7] Tague, R. G. (1992). Sexual dimorphism in the human bony pelvis with a consideration of the Neandertal pelvis from Kebara Cave, Israel. American Journal of Physical Anthropology 88:1–21.

[8] Weaver, T.D. and J.-J. Hublin (2009). Neandertal birth canal shape and the evolution of human childbirth. PNAS. 106(20):8151-8156.

[9] Franciscus, R.G. (2009). When did the modern human pattern of childbirth arise? New insights from an old Neandertal pelvis. PNAS. 106(23):9125-9126.