A Tale of Two Lineages

[Hey, I’ll bet I’m the first person to make that allusion…]
In a paper published in JHE today, M. Schillaci posits that human facial anatomy suggests the existence of two human lineages in the late Pleistocene. Schillaci’s analysis reveals that faces of early Australasian crania (40-8 ka) are very similar to those of Levantine specimens Skhul 5, Qafzeh 6 and Qafzeh (100-90 ka). The overall results of the study suggest that the Australasian and Levantine populations share an earlier common ancestor than modern humans, including Upper Paleolithic Europeans. Schillaci interprets this to mean that modern humans first dispersed from Africa around 100 ka, long before the supposed “revolution” of Paleolithic Europe, and made it as far as Australia; second dispersal then occurred some 50 ka later.

The article brings up the issue of Out-of-Africa (Replacement) vs. Multiregional models, but does not clearly come out directly in favor of either one. But by setting up a scenario in which two human lineages are present throughout the Old World in the last 100 ka, the possibility is opened up for these lineages to accrue genetic differences simply by drift or even by selection, then to come into contact again and admix, and for the “archaic” genes to be incorporated into the newer (and modern) genome (introgressive hybridization; cf Evans et al. 2006, Garrigan and Kingan 2007, Hawks and Cochran 2006).

Schillaci does note that the Levantine sample

exhibits a slightly closer genetic relationship to Neandertals (d=0.7318) than to Upper Paleolithic Europeans (d=0.7483). . . . This observed relationship is probably not the result of phenotypic convergence, and likely reflects a slightly more recent common ancestry and/or perhaps hybridization between early modern humans and Neandertals (Trinkaus, 2007). (p. 6)

However, he later notes,

In the present study, the relationship between early modern humans from the Levant and early Australasians (d=0.348) is more than 2.1 times closer than between early modern humans and Neandertals (d=0.7318), and Neandertals do not show a close relationship with early Australasians (d=1.2615). If the observed relationship between Neandertals and early modern humans is the product of hybridization, there is no craniometric evidence indicating that there was substantial introgression of Neandertal alleles into the dispersing modern human population… (p. 7)

Can Schillaci make these claims about “genetic relationship[s]” based on his data? Let’s look at the opening line of the abstract: “This study examines the genetic affinities of various modern human groupings using a multivariate analysis of morphometric data.” A rewording might be: This study examines the craniofacial affinities of various modern human groupings, and to thereby infer genetic relationships. Basically, Schillaci assumes that genetic relationships between populations are accurately reflected in facial anatomy, a bold statement to make. Indeed, he acknowledges the problems with this assumption, but also cites studies (which I haven’t yet read) suggest that craniometric variation in humans throughout the world fits a neutral model of genetic variation. So, Schillaci talks about genetic relationships throughout the paper, but these aren’t based on actual genetic data, but rather inferences from craniometric data–quite confusing. His aforementioned lack of evidence for introgression between neandertals and early modern humans does not preclude real genetic evidence for introgression (cf. what I cf-ed above.)

What I care about: the study allows for, and possibly corroborates, a Multiregional model of human evolution (of course, what I really care about is the possibility of such a model prior to, and in the early stages of, the genus Homo). Hey, I guess old-school craniometrics hasn’t outlived its usefulness in physical anthropology.

References
Evans PD et al. 2006. Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage. Proc Nat Acad Sci 103(48): 18178-18183.

Garrigan D and SB Kingan. 2007. Archaic Human Admixture. Curr Anthropol 48(6): 895-902.

Hawks J and GM Cochran. 2006. Dynamics of adaptive introgression from archaic to modern humans. Paleoanthropol 4: 101-115.

Schillaci M, in press. Human cranial diversity and evidence for an ancient lineage of modern humans. J Hum Evol xx: 1-13.

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