The Great LGM Bottleneck; Or, Has It Been Downhill Since the Days of the Gravettian Mammoth Hunters?

Because Europeans have been obsessed with ‘the races of Europe’ — the title of Ripley (1899); later echoed by Coon (1939) — and craniologists prevailed against philologists in identifying them after the professionalization of science in the late-nineteenth century, all human taxa (specific and subspecific) are still identified through craniology. This is as it should be since there is a stronger population history signal in craniometrics relative to linguistics (although the strongest population history signal is in dental polymorphisms). However, what physical anthropology (molecular or otherwise) of the living can tell us is the phylogenetic relationship of living populations. It cannot tell us about population history, or about the histories of vanished paleo-demes. For that we need molecular or cranial paleontology. That is, we need fossils.

Contemporary populations of European ancestry are admixtures of Yamnaya pastoralists who arrived from the steppe 5ka and Neolithic farmers who arrived from the Near East around 9ka. Reich et al. assumed that under these Neolithic populations was a uniform substratum of Upper Paleolithic hunter gatherers presumably descended from the Cro-Magnons, although they did distinguish between western and eastern paleo-demes.

That presumption turns out to have been badly wrong. The reason is that the great ice age that lasted from 30-20ka — the last glacial maximum (LGM) is dated to 26ka — generated a very severe population bottleneck. The impact was particularly drastic in Europe where faunal populations, including human demes, became isolated in refugia (largely in southern Europe). The drastic reduction in population size meant a major loss of genetic diversity. For instance, the diagnostic haplogroup for Asians that present-day Europeans do not have, has been found in pre-LGM populations. It was lost during the LGM-bottleneck.

Replicating the work of Brewster et al. (2014), I will show that pre-LGM paleo-demes were physically different from post-LGM paleo-demes. These paleo-demes were the authors of remarkable Upper Paleolithic cultures, particularly the Gravettian. Indeed, the LGM discontinuity is so drastic that we may speak of the vanished civilization of the Gravettian peoples. We’ll look at fossil skeleton metrics for pre-Neolithic paleo-demes in Europe. Unlike Brewster et al. (2014), we’ll distinguish between three chrono-groups: pre-LGM/Early Upper Paleolithic 35-20ka, late glacial/Late Upper Paleolithic 18-10ka, and Holocene/Mesolithic 10-5ka. We’ll label them with the predominant cultures in these sample: Gravettian, Magdalenian, and Mesolithic. All cranial data is from Brewster et al. (2014), and all postcranial data is from Ruff et al. (2018), Skeletal Variation and Adaptations in Europeans: Upper Paleolithic to the Twentieth Century.

We begin with postcranial size variables. There was a steady decline in stature over time. The Gravettians were considerably taller than later paleo-demes. Stature.png

They also weighed a bit more. Although the t-test is not significant (p=0.152).

body_mass.pngRecall that these are allometric estimates. We have much more reliable measures of body size. Femur head diameter and pelvic bone width are not only the main weight carrying parameters of the human skeleton, they are also under tighter genetic control.

femur_head_diameter.png

Gravettian and Magdalenian femur head diameter and pelvic bone width are not significantly different. However, Gravettian and Mesolithic populations are significantly different by both measures (p=0.078 for femur head diameter and p=0.016 for pelvic bone width).

Pelvic_bone_width.png

Gravettian and Magdalenian femur length differences are highly significant (p<0.001). Recall that femur lengths and stature are controlled by net nutritional status. This suggests that the Gravettians had much higher nutritional standards than later paleo-demes.

femur_max_length.pngMoving onto craniometrics, we look first at skull size calculated as the geometric mean of skull length, height, and width. The difference between Gravettian and Magdalenian skull size is not statistically significant (p=0.706). In sum, the Gravettians were taller than Magdalenians but not otherwise bigger. Both were however absolutely bigger than later Mesolithic Europeans.

Skull_size.png

We now look at cranial variables after adjusting them for skull size. So these are shape variables that contain information largely orthogonal to the size variables we have so far examined. Of the 10 cranial measurements in the Brewster et al. dataset, only in the following four is there a statistically significant difference between the pre-LGM Gravettian and the post-LGM Magdalenian skulls. Gravettian skulls were longer.

Skull_length.png

Gravettian foreheads were broader.

Forehead_width.png

They had taller noses …

Nasal_height.png

… that we narrower.

Nasal_breadth.png

Table 1 reports the results of our t-tests for size-adjusted Gravettian-Magdalenian craniometric differences.

Table 1. Gravettian vs. Magdalenian craniometric characters. 
Martin number Description pVal
M1 Maximum cranial length 0.026
M8 Maximum cranial breadth 0.970
M9 Least frontal breadth 0.000
M17 Basibregmatic height 0.194
M45 Bizygomatic breadth 0.454
M48 Nasoalveolar height 0.846
M51 Orbital breadth 0.968
M52 Orbital height 0.171
M54 Nasal breadth 0.003
M55 Nasal height 0.016
Source: Brewster et al. (2014), adjusted for skull size.

Who were these people with long heads, narrow and tall noses, and broad foreheads? Why were they so tall? People have tried to trace the tall stature of contemporary Bosnians to inheritance from this paleo-deme. That patent nonsense for stature is controlled by health insults. Whatever the correlations with haplotype, this variable is just too plastic. Moreover, the genotype of the Gravettians largely vanished in the LGM bottleneck. All later demes, including and especially contemporary populations, inherited little of their genotype. As far as present-day populations of European ancestry are concerned, they might as well be Neanderthals.

But I would go much further. Ancient paleo-demes, particularly pre-LGM ones like the authors of the Gravettian, cannot be projected onto contemporary classifications. Not only do they have different genotypes and different cranial morphologies, they also had different skins and eye colors than contemporary Europeans. For instance, we know that the genetic basis for depigmentation came under selection in Europe only 7ka; while that for blue eyes came under (probably sexual) selection 14ka. We don’t know yet if the Gravettians looked fair or if some of them had blue eyes. Possibly. But if so, it had a different genetic basis from contemporary morphotypes.

What is clear is that they had much higher standards of living than later paleo-demes. Why? The answer is the great mammoth steppe.

mammoth_steppe.png
Source: Clive Gamble, Settling the Earth.

During the Upper Pleistocene, vast herds of woolly mammoth and other megafauna roamed across the northern Eurasian steppe, extending from the great European plain all the way to north America. How was it possible to sustain such high secondary productivity during the ice ages? As Gamble reports,

The mammoth steppe comes down to a simple observation by zoologist Dale Guthrie – that today’s boreal and tundra vegetation of Beringia could not have supported the large herds of bison, horse, reindeer and above all mammoth whose bones, and sometimes carcasses, are preserved by their millions in its frozen silts. Tundra plants and boreal trees produce a toxic litter that affects the soil, leaving very little for animals to feed on. Furthermore, they act as a blanket so that the level of annual freeze–thaw is small. The factor which changed this balance between plants and soil was the aridity of the Pleistocene. Creating the mammoth steppe depended on high evaporation and a deeper thaw in summer that released nutrients from lower down in the soil. This, according to Guthrie, broke the cycle of low soil nutrients and toxic plants. It produced a richer soil and the conditions for the abundant growth of grass. These grasses were resistant to grazing pressure, grew quickly and formed a rich mosaic of vegetation conditions, likened to the weave in a plaid, and contrasted with the stripes, or bands, of vegetation found in the warmer Holocene. Isotope studies suggest that mammoths consumed higher quantities of dry grass than the other grazers such as reindeer, horse and woolly rhino. The mammoth steppe developed across Western Europe to Beringia during a major glaciation, MIS12, 500ka. It supported a diverse, high-biomass animal community, compared to the present tundra and boreal forests. Besides the major herd animals — mammoths, woolly rhinos, bison, horse, red deer, reindeer, musk ox and saiga antelope — there were major carnivore guilds of lions, hyenas, wolves, leopards and foxes, and omnivores such as bears.

The Gravettian civilization was based on the western extremity of the mammoth steppe. As the mammoth steppe contracted east, the Gravettians moved with it. The west-east movement is very marked in Gravettian sites. While some Gravettian populations specialized in mammoth hunting, the notion that they were all specialized mammoth hunters has been discredited. The Gravettians certainly hunted mammoths — they were probably the first humans to do so — but they also hunted other game. Their kitchen middens display significant variability. They did however collect mammoth bones at scale. They build shelters with them, used them to make all sorts of tools. Mammoth bones very likely also had a religious function in Gravettian civilization. For as Soffer (1993) reported and was recently confirmed for a Russian site, the spatial patterns of Gravettian camp sites suggest that mammoth bones were segregated from those of other animals. 

So the Gravettians not only had a highly distinct morphology (and genotype), they also had a very distinctive way of life that was highly advanced. They were certainly better fed than later populations. We are just beginning to appreciate the ways of life, and indeed, histories, of this ancient civilization.

This foray into the Gravettian mammoth hunters will help us think about combined and uneven development in the Upper Pleistocene. More on that later.

Mammoth_home.jpg

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