Thinking

Population History and European Morphology since the Upper Paleolithic

Christopher Ruff, a paleoanthropologist at the Johns Hopkins University School of Medicine and the director of the Center for Functional Anatomy and Evolution, has been very generous with his time. He has helped me greatly in refining my understanding on human morphology and our population history. Ruff and coworkers have recently published Skeletal variation and adaptation in Europeans: Upper Paleolithic to the Twentieth Century, 2018. The study examines a total of 2,179 individual skeletons since the Upper Paleolithic beginning 33 thousand of years ago (henceforth, Ka). He has been kind enough to share their data with me. What follows is based on my interrogation of this data in light of our population history.

The basics of west Eurasian population history are now well-understood. The following account is based on the scheme presented in David Reich’s Who We Are and How We Got Here, 2018. The picture that emerges from ancient-DNA studies is straightforward. Basically, there are three major departures in west Eurasian population history. (Here we focus specifically on Europe.) The first is the arrival of Homo sapiens during the early Upper Paleolithic around 40 Ka into a continent already populated by Neanderthals. H. sapiens had already mixed with Neanderthal populations in the Near East immediately upon their exit from Africa. There was further admixture in Europe.

Estimates of the precise degree of admixture are quite sensitive to assumptions about the neutrality of genomic sequences acquired from the Neanderthals since even genes acquired in a single admixture event could rapidly get fixed throughout the population if they come under selection; conversely, the prevalence of genomic sequences not under selection contains information on the degree of admixture. Almost all estimates however fall into single digits. This is perhaps not because mating was infrequent despite near-continuous contact but rather because of the large disparity in population sizes. The colonizers were dramatically more populous than the natives so that very high degree of mixing for the latter is consistent with low rates of mixing for the former. (Similar to interracial marriage rates in the present-day United States.)

By the Last Glacial Maximum 26 Ka, the Neanderthals had long vanished. It is not clear if they went extinct or were simply absorbed into H. sapiens populations. Upper Paleolithic populations of Europe were already morphologically-adapted to the macroclimate with more northern populations displaying bigger bodies in accordance with Bergmann’s Rule. This population is basically swamped by a second pulse around 9 Ka when the Neolithic Revolution generates a major population pulse of farmers in the central Eurasian region who explode out in both easterly and westerly directions. The former would go on to found the Dravidian-Harappan Civilization. In Europe, the hunter-gatherers survived in isolated pockets; especially at northern latitudes. Since the Neolithic farmers came from the Near East their morphology was adapted to the much warmer macroclimate of the central region. In accordance with Bergmann’s Rule, we expect them to be smaller than the more cold-adapted populations of the Upper Paleolithic. We’ll presently see what the data has to say about this.

A third major population pulse was triggered by the Secondary Products Revolution in the fourth millenium BCE. In the central region, this dramatic transformation in the material possibility frontier gives rise to the very first state society at Uruk. The introduction of these advanced technologies—especially the wagon, as David Antony has argued—from the core of the Uruk world-system to the periphery, in this case, north of the Caucasus, makes the systematic economic exploitation of the sparsely-endowed steppe possible for the first time. This material revolution in the hitherto very sparsely-populated steppe is in turn responsible for the ethnogenesis of the Yamnaya, the speakers of Proto-Indo-European (the mother tongue whose descendents are spoken by half the world’s population today).

Yamnaya pastoralists explode outward almost immediately from their homeland. By 5 Ka, a massive population pulse reaches Europe, another India, and a third the Altai mountains in Kazakhstan. The Yamnaya are an extremely violent and hierarchical rank-society; obsessed with martial glory, competitive feasting, and other male bonding rituals. They conquer the first-farmers of Europe and eventually the isolated pockets of hunter-gatherers (in particular, in Scandinavia). These migrations are extremely sex-biased. Yamnaya warrior-pastoralists likely took the women and slaughtered the men in raids and skirmishes as the horizon moved inexorably westward.

Pop_history.png

Source: David Reich (2018).

The end-result of this population history is that contemporary European populations are a sex-biased admixture of Pleistocene hunter-gatherers, Neolithic farmers, and Yamyaya pastoralists; in the reverse order in terms of weight in the population structure. These three populations were morphologically-adapted to very different macroclimates during the Late Pleistocene. Specifically, the first can be expected to be adapted to local conditions in Europe that were highly polarized by latitude (southern Europe was never glacial whereas northern Europe witnessed the massive glacial-interglacial whipsaw), the second to the considerably warmer conditions of the Late Pleistocene in the Near East, and the last, the Yamyaya, to the more-permanently glacial conditions of the Russian steppe. We thus expect systematic time-variation in European morphology consistent with this population history. More precisely, we expect the slower-moving morphological parameters (eg, pelvic bone width, femur head diameter) to fall after the invasion of the farmers from the central Eurasian region and rise after the invasion of the pastoralists from the Eurasian steppe.

Figure 1 and 2 display the pelvic bone width of the skeletons in the Ruff et al. (2018) dataset. We have resized the points by the number of skeletons in the dataset for given region and period. We have also merged some periods in the original dataset for simplicity. [Early Upper Paleolithic (33-26 Ka) and Late Upper Paleolithic (22-11 Ka) have been folded into Upper Paleolithic (33-11 Ka); Mesolithic (11-6 Ka) and Neolithic (7-4 Ka) into Neolithic (11-4 Ka); Bronze (4-3 Ka) and Iron/Roman (2.3-1.7 Ka) into Yamnaya (4-1.7 Ka); Early Medieval (c. 600-950) and Late Medieval (c. 1000-1450) into Medieval; and Early modern (c. 1500-1850) and Very recent (c. 1900-2000) into Modern (c. 1500-2000).]

m_pelvic.png

Figure 1. Pelvic bone width in Europe, Men. Source: Ruff et al. (2018).

The evidence that emerges is pretty unambiguous. For both men and women there is a significant fall in pelvic bone width during the Neolithic transition, and a substantial rise contemporaneous with the Yamnaya transition. Since there was no major population replacement in the Medieval-Modern passage, the decline in body size cannot be traced to population history. Note the French outlier that attenuates the modern decline for women. Without the outlier, the modern decline in women’s pelvic bone width would be as significant as men’s.

f_pelvic.png

Figure 1. Pelvic bone width in Europe, Women. Source: Ruff et al. (2018).

Similar results hold if we look at femur head diameter which is also strongly canalized (very slow-moving). Femur head diameter is the main weight-carrying parameter of the human body and as slow to change as pelvic bone width.

m_femur_head_si.png

f_femur_head_si.png

The length of the thigh bone (femur) is much more developmentally-plastic than either pelvic bone width of femur head diameter. Yet, we know that even femur length (and hence stature) exhibits morphological adaptation to the macroclimate. The evidence that emerges from this dataset is consistent with our previous findings.

m_femur.png

f_femur.png

Finally, for the sake of completeness, we include graphs for stature. These ought to be congruent with the results for femur length since the former is a linear function of the latter.

m_stature.png

f_stature.png

The evidence that emerges is consistent with the idea that population history confounds the interpretation of the time-variation (as opposed to the just the cross-section as we have argued until now) of morphological parameters of the human body. In order to make valid inferences, population history must be kept in mind.


I constructed an index of body size by adding up the z-scores of femur head diameter and pelvic bone width. It is a less noisy measure of body-size that those considered above. The overall pattern revealed by the Body-Size Index is very compelling. We observe that size falls in the Upper Paleolithic-Neolithic passage and rises with the arrival of the Yamnaya precisely as predicted by population history. The modern decline does not correspond to any major population movement and therefore cannot be explained by population history.

body_size.png

The decrease in body-size is consistent with other evidence of gracialization during the transition to modernity. Men have not only become smaller, even their faces have become less aggressive (not to speak of manners and behavior). Could it be that changing social norms against the warrior code rewarded variants with traits less associated with aggression with greater reproductive success? Or did the rewards for body-size decline with better technology for grunt work? We don’t know. There is certainly a case to be made for a general process of gracialization related to modernity.


P.S. On second thoughts, it may not be wise to combine sexes after all. There is good reason to think that women’s pelvic bone width is more plastic than men’s because maternal mortality can adjust the latter quite rapidly. The best measure we have is men’s pelvic bone width. Here I graph the mean pelvic bone width of European men without combining periods. The evidence is consistent with the population history noted above. The overall pattern suggests strong gracialization after the Last Glacial Maximum, a further fall in body-size with the arrival of Neolithic farmers, a dramatic rise with the arrival of Yamnaya pastoralists; followed by slow upward drift until the end of the Middle Ages, strong gracialization in the early modern period, and very partial restoration after c. 1900. The big question thrown up by the present investigation is of course the dramatic decline in European body-size after the Black Death. But the overall elephant shaped pattern is very interesting as well.

men_biiliac.png

 

 

 

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