A research paper published back in 2022 (Matsumura, H., Tanijiri, T., Kouchi, M. et al. Global patterns of the cranial form of modern human populations described by analysis of a 3D surface homologous model. Sci Rep 12, 13826 (2022). https://doi.org/10.1038/s41598-022-15883-3), compared the skulls of contemporary human populations from Africa, Europe, Asia, Americas, Melanesia-Australia, etc., finding some interesting traits.
Modern human skulls differ
The paper relies heavily on PCA data, which, as per my last post, may not be the most suitable tool to emply in studies like this one, which is, basically, morphometric, as it compares skull dimensions and reduces these different data values into several Principal Components).
The PC4 and PC2 (principal components 2 and 4) analysis shows that there are distinct groups: Africans overlapping Australo-Melanesians (see the lower image below, pink and red ovals on the right side), Northeastern Asians (green oval, bottom left), and Europeans (blue oval, top left). There are other populations as you can see in the upper image (Middle East, South Asians and Americas), but they are not shown as distinct groups in the second, lower, image.
Australo-Melanesian skulls are similar to African ones
The grouping toghether of two populations separated by an Out of Africa (OO) walking distance of 20,000 to 24,000 km (12,000 to 15,000 mi.) is indeed surprisin. But the authors point it out: "Australo-Melanesian (including Papuan) samples exhibited a closer resemblance to African samples than any other geographic series in the PCA of the homologous model." And they also explain it as if it was a normal consequence of such a vast trek: "This finding supports the hypothesis that the first dispersal group of AMH along the southern rim of Eurasia was of direct African origin."
But, if, as we now know, that Papuans and Melanesians have the highest levels of Denisovan admixture of all the global populations. Why would they carry 5% of Denisovan genes and still display a skull shaped like an African?
The paper states that Australo-Melanesians have a "large" AUC, comparable to Sub-Saharan Africans which also have large AUCs. They also share (this is the dimension marked by the PC2 axis) a "relatively longer neurocrania than the others... which describes the close affinity between the proportionally elongated crania of Australo-Melanesians and Africans and dissimilarities with the globular crania of some Europeans and Northeast Asians." Note that AUC is a metric (Area under the curve —AUC, in an ROC Receiver Operating Characteristics Analysis. A lot of jargon!)
The paper includes many comments on the differences between modern human skull shapes, and when it comes to the Australo-Melanesians explains it as follows. The authors mention the possibility of two OOA events, an ancient one, some 177,000 years ago, which led to anatomically modern humans out of Africa and into the Levant and Mediterranean basin, but whether these people expanded into Eastern or Southern Asia is still unknown. The other option is a recent wave (60,000 years ago) out of Africa along the Indian Ocean coastal regions towards Southeast Asia, Papua, and Australia. This was followed by other waves into North-Central Asia and Europe.
The paper adds that the similar appearance of African and Papuan-Australian crania was the result of this first migration along the Indian Ocean, which "...was of direct African origin, without significant morphological alterations in accordance with a particular climate or other substantial conditions" (they note the climate, becaus the attribute the shape differences of other humans to adaptation to colder climes).
Denisovans
Denisovans admixed with Melanesians, and Papuans exhibit a high proportion of Denisovan alleles. Denisovan skulls are very different from ours (Source) but they are do have a long skull like Austro-Melanesians: "...long and low cranial vault, a wide and low face, large and almost square orbits, gently curved but massively developed supraorbital torus, flat and low cheekbones with a shallow canine fossa, and a shallow palate with thick alveolar bone supporting very large molars."
But, Denisovans are Asian hominins, they never entered Africa. So, why do Africans share this long skull too?
Controversial pro-China, sinocentric scholar Shi Huang argues that the OOA theory is wrong, ant that "An alternative worth testing is my proposal that Australo-Melanesians, including Papuans, retain unusually high Denisovan ancestry, and that Denisovans ultimately trace to an African-related archaic lineage. Under that model, the otherwise striking African–Australo-Melanesian cranial affinity could reflect shared archaic ancestry rather than simply an early modern-human migration out of Africa."
Reasonable. Neanderthals and Denisovans are said to have split from the modern human lineage within Africa, exiting the continent around 600,000 years ago. Denisovans are not supposed to have back-migrated into Africa. Probably the human group that led to Australian and Melanesians carried a higher content of pre-split alleles than other humans who moved north into western Eurasia.
Another possibility is that humans originated in Eurasia, and migrated from there. One group, with the Australian-Melanesian trait moved into that region and also into Africa. Another moved north into Siberia, and west, into Europe. Why should we constrain our theories to the OOA one?
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