Cranial shapes, OOA, Denisovans, Melanesians & Australians

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.

Scatter scoring diagrams of PC2 and PC4 for cranial individuals from nine geographical units (upper), and four geographical units (lower), color maps of cranial surface highly correlating vertices to each PC. Fig 4. in Matsumura et al., 2022

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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Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2026 by Austin Whittall © 

Sunnyvale and Del Mar skulls from California and their age

I was reading about the Del Mar remains that were discovered in 1926 on the California coastline and which were dated by Jeffrey L. Bada at 47,000 years old (Bada, J. L., and Helfman, P. M. 1975, Amino Acid Racemization Dating of Fossil Bones. World Archaeology 7 : 160–183.) as shown below (blue box):

They were doing groundbreaking work with a technique that Bada had developed: "amino acid racemization" or AAR for short.

AAR measures the ratio of right-handed amino acids or (D) amino-acids to the left-handed ones or (L) amino-acids, and uses it to determine the age of a biological sample.

All apha-amino acids except glycine come in two isomers, that are identical except that one bends polarized light to the right (hence the right-hand part of the nam), and the other type bends it to the left.

Proteins are made up exclusively of (L) amino acids. And the moment a living being dies, its (L) amino acids start to degrade by a process called "racemization", and become (D) amino acids. There are many amino acids in our bodies, and scientists use one, aspartic acid because it has a relatively quick racemization speed.

As you can see in the table above, older specimens have (D) amino acids and less (L) amino acids, so the ratio D⁄L is higher than in younger specimens.

Bada calibrated his method against a radiocarbon dated specimen (Laguna Skull) and then extrapolated the dates shown in the last column.

Of course, it was a controversial claim, Del Mar skull at 47 ky and the Sunnyvale skull with 70 ky, were far too old for orthodox scholars.

The novel technique was scrutinized and torn apart, other methods (Uranium and Thorium) used on the samples, which gave a much later date, almost one order of magnitude -ten times less- than Bada's data.

To justify Bada, we must admit that despite his claims that "The effects which other physical factors (pH, humidity, leaching) have on racemization rates are discussed. Because of the close correlation between temperatures calculated from in situ racemization rates (under diverse environmental conditions) and actual mean annual temperatures at various sites throughout the world, we conclude that factors other than temperature have very small effects on the reaction rate.", there are many factors that affect racemization.

Bada pointed out in 1984 that radiocarbon dates and AAR didn't seem to correlate in American specimens, while in other parts of the world, the correlation was very good.

Apparently the cause of this discrepancy was the degree of preservation of protein (and therefore amino acids) in the bones (see Stafford, 1990.), meaning that the same bone can give a Holocene or recent dating or a Pleistocene or older date!

So we could argue that the age of Del Mar and Sunnyvale bones might (barely) be 48,000 and 70,000 years old, but then we have the issue of the uranium (U) and thorium (Th) dates (Bischoff, 1981), which gave ages of 11,000 and 8,300 years BP respectively for those remains.

Bones can uptake these elements (U and Th) from the environment which can alter the actual age of the specimens, and (Millard, 1996): "are liable to underestimate the true age by at least one-third."

This would make them younger than they really are...

So the age remains an open question -maybe.

Sunnyvale girl skull

But what drew my attention when I was reading about this subject was the photo of the skull of the "Sunnyvale girl" in New Scientist's edition of Jul. 14, 1983 is shown below:

Sunnyvale skull (left) and 4,000 year old Indian skull (right). New Scientist

That skull is really big! I tried to find other photos, but I wasn't able to. But I found a description of her in this paper (Amino Acid dating and early and early man in the New World, a rebuttal, 1981, Bert A. Gerow, page 9):

"The Sunnyvale individual is a female, judged to have been about twentyfive to thirty years of age at the time of death. Morphologically, she is fully modern, with a well developed chin, thin cranial walls, canine fossae, short face, and sub-quadratic orbits. The lateral incisors exhibit "shoveling" on the lingual side. The latter is a common Amerindian physical trait shared with populations of East Asia. "

One source (Biological and Chronometric Applications of Amino Acid Racemization Reactions, Patricia Masters Helfman, 1976, Biology, Univ. of California, pp. 67) says that it was Gerow who found the "Sunnyvale girl" in 1972, but he does not mention this date in the paper cited further up.

Perhaps some photos of the skull would shed some light on the matter of its size. But if I am not mistaken, these bones were reburied in accordance with the US law back in 1996.

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Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2019 by Austin Whittall ©