Recent evolution in human brain size

 

Human brain size remained stable from 300,000 to 60,000 years ago. It then diversified, becoming larger in some populations and smaller in others. This was when modern humans were spreading out of Africa and into new environments in Eurasia.

 

 

With the end of the last ice age, some 10,000 years ago, northern hunting peoples found themselves in a new environment. Men could no longer pursue herds of wandering reindeer over the vast steppe-tundra. They now had to hunt over shorter distances, and the game would be smaller and more varied. Meanwhile, women now had opportunities for gathering fruits, berries, roots, and other small food items. They thus turned toward food gathering, while men moved into the formerly female domain of crafts, kiln operation, and shelter construction. Cognitive demands were thus changing. Men no longer had to store huge amounts of spatiotemporal data when tracking prey, and women were losing their dominance of artisanal work (Frost 2019).

 

The post-glacial period also brought an apparent decrease in brain size. Henneberg (1988) found that male brains shrank by 9.9% and female brains by 17.4% between the ice age and modern times. He attributed the decrease to a reduction in body size. In a reanalysis of Henneberg's data, Hawks (2011) showed that the reduction in body size explains only one-fifth to one-seventh of the decrease in brain size. He also showed that the declining ratio of brain size to body size did not affect all populations equally. In fact, it can be securely demonstrated only for Europeans and Chinese. No decline is discernable for Nubians, the only non-Eurasian population for which we have a large cranial sample.

 

In a recent analysis of cranial data, DeSilva et al. (2021) argue that brain size began to decrease with farming and the rise of larger, more complex societies. They argue more specifically that the decrease was due to an increasing ability to store knowledge externally either in written form (on tablets, paper, or parchment) or in the brains of scribes, skilled tradesmen, and other knowledge workers. People no longer had to rely solely on their own brains to store the knowledge they needed:

 

 […] the recent decrease in brain size may instead result from the externalization of knowledge and advantages of group-level decision-making due in part to the advent of social systems of distributed cognition and the storage and sharing of information. (DeSilva et al. 2021, p. 1)

 

That hypothesis has been challenged by Villmoare and Grabowski (2022). Because farming was adopted at different times in different populations, they argue that DeSilva et al. (2021) should have analyzed the cranial data on a regional basis. But this was not done:

 

Since this transition [to farming] occurred at different times across the globe, rather than over a single 3–5 ka year period, under the hypothesis of DeSilva et al. (2021) we should detect the change in different modern human populations at different times. However, the dataset of DeSilva et al. (2021) is not organized to test the hypothesis in this fashion. Populations from around the globe are lumped together, with only 23 crania sampled over what we would argue to be a critical window with regards to their hypothesis, 5–1 ka, and coming from Algeria, England, Mali, China, and Kenya, among other locations. Later modern human samples are focused on Zimbabwe (at 1.06 ka), the Pecos Pueblo sample from the United States (1 ka), and finally, 165 crania (28% of the total sample) are from Australian pre-Neolithic hunter-gatherer populations and dated in DeSilva et al. (2021) to 100 years ago. (Villmoare and Grabowski 2022, p. 2)

 

The cranial dataset suffers from other problems:

 

In that same dating category [100 years ago], 307 (53% of the total sample) are from unspecified Morton Collection crania, where we have no way of knowing how many may be from pre-Neolithic and post-Neolithic populations. We also observe that the sample of DeSilva et al. (2021) generates a modern human mean of 1,297 cc in the final 100-year category, which is well below other published estimates of contemporary world-wide modern mean human cranial capacity that range from ?1,340 cc up to ?1,460 cc. (Villmoare and Grabowski 2022, p. 2)

 

When Villmoare and Grabowski (2022) reanalyzed the cranial data for the last 300,000 years, they found a very different picture:

 

[…] our analyses showed no changes in brain size associated with the transition to agriculture during the Holocene. Overall, our conclusion is that, given a dataset more appropriate to the research question, human brain size has been remarkably stable over the last 300 ka. (Villmoare and Grabowski 2022, p. 4).

 

Actually, their reanalysis shows that brain size remained stable from 300,000 to 60,000 years ago. It then diversified, becoming larger in some populations and smaller in others. This was when modern humans were spreading out of Africa and into new environments in Eurasia (see chart at top of post).

When the authors looked more narrowly at the last 30,000 years, they found no discernable change in mean brain size or in variation around the mean. They did not attempt a regional analysis. That’s a pity because DeSilva et al. (2021) may have been right within a more limited context, specifically that of complex Eurasian societies. We still have John Hawks’ finding that brain size decreased in Eurasians after the last ice age. But when exactly? Immediately after the ice age? Or during the much later increase in social complexity?

 

Today, more than a decade later, John Hawks has still not published that paper in a journal. When I asked him why, he replied: "I did not feel it was necessary to pursue formal journal publication for this, because I did not think it fit well into the journals at the time." Yet, at that time, the paper was exciting a lot of interest. This is what he wrote on his blog:

 

I've had a dozen requests from colleagues to cite the paper (which anyone is welcome to do by using the arXiv number). I also had two great interactions with colleagues who had comments and suggestions on the preprint, which I am now incorporating into a revision. (Hawks 2012)

 

He might have had trouble publishing the paper in a top-tier journal. But the main problem lay elsewhere. Once it got published, some academics might have viewed him the wrong way. Perhaps not, but why take the risk? Why risk opportunities for getting funding and invitations to work on big projects with big names?

 

Those are questions that many anthropologists end up asking themselves. I have no easy answer, other than to say that you can never control what other people think of you. You only get to own your own thoughts, not those of others.

 

References

 

DeSilva, J. M., Traniello, J. F. A., Claxton, A. G., and Fannin, L. D. (2021). When and why did human brains decrease in size? A new change-point analysis and insights from brain evolution in ants. Frontiers in Ecology and Evolution 9: 742639. https://doi.org/10.3389/fevo.2021.742639

 

Frost, P. (2019). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181. https://doi.org/10.3390/psych1010012

 

Hawks, J. (2011). Selection for smaller brains in Holocene human evolution. arXiv:1102.5604 [q-bio.PE] https://arxiv.org/abs/1102.5604  

 

Hawks, J. (2012). Spreading preprints in population biology. John Hawks Weblog, August 1. https://johnhawks.net/weblog/topics/meta/population-biology-arxiv-callaway-2012.html

 

Henneberg, M. (1988). Decrease of human skull size in the Holocene. Human Biology 60: 395-405. https://www.jstor.org/stable/41464021

 

Villmoare, B. and M. Grabowski. (2022). Did the transition to complex societies in the Holocene drive a reduction in brain size? A reassessment of the DeSilva et al. (2021) hypothesis. Frontiers in Ecology and Evolution 10: 963568. https://doi.org/10.3389/fevo.2022.963568

Ancestral East Asians and adaptation to coronaviruses

 

Early farming village in China (Wikicommons – Xinyang City Museum, Gary Todd)

 

Respiratory viruses began to propagate more easily when hunting and gathering gave way to farming and as settlements grew larger. Humans may have then evolved to use coronaviruses as a natural vaccine against deadlier respiratory diseases, like tuberculosis and pneumonia.

 

 

A new genomic study has found that East Asians had to adapt to epidemics of coronaviruses some 25,000 years ago. The authors looked at gene variants for proteins that interact with coronaviruses in five East Asian populations: Han Chinese (Beijing); Han Chinese (South China); Dai (Yunnan, China); Japanese; and Vietnamese. There were three main findings:

 

·         Ancestral East Asians had to adapt to coronavirus epidemics around 25,000 years ago

·         They adapted by acquiring mutations that are close to genes that regulate the development of lung tissue and other tissues affected by COVID-19

·         Those mutations either promote or block infection by coronaviruses (Souilmi et al. 2021, p. 3505).

 

The last finding is puzzling. Did those ancestral East Asians become more vulnerable or less vulnerable to coronaviruses? The authors simply say that half of those mutations from 25,000 years ago have “anti- or proviral effects” versus 29% of all proteins that interact with coronaviruses (Souilmi et al. 2021, p. 3509). Fine. But how many of those mutations were antiviral and how many proviral?

 

It might seem strange that natural selection would actually make people more susceptible to coronavirus infections. Yet such susceptibility could be beneficial. A viral infection can boost immunity to other pathogens, including deadly ones that cause tuberculosis, pneumonia, or pneumonic plague. Until recently, coronaviruses were typically mild in their effects, producing what we call the “common cold.” They may thus act as a natural vaccine against deadlier respiratory diseases (Frost 2020).

 

Respiratory diseases are believed to have become serious for humans when hunting and gathering gave way to farming. People became sedentary and their settlements grew larger with time, thus providing respiratory viruses with better conditions for propagation (Comas et al. 2013). This theoretical model is in conflict, however, with the above finding that ancestral East Asians began adapting to coronaviruses some 25,000 years ago, long before they adopted farming and became sedentary. We’re thus left with the unlikely conclusion that coronavirus epidemics began among scattered bands of hunter-gatherers.

 

The estimate of 25,000 years ago is probably wrong. The authors arrived at that figure by calculating the latest date when the ancestors of the four East Asian groups were still a single population. But East Asians are not descended from a single population. Their origins are best described by the "Two-Layer" (TL) hypothesis:

 

·         Modern humans spread into East Asia through a northern route and a southern route.

·         The southerners were then replaced to varying degrees by northerners who spread out of northeast Asia and successively occupied northern China, southern China, and Southeast Asia (Oxenham and Buckley 2016; Xu et al. 2006).

·         Thus, as you go farther south in East Asia, the population has a greater admixture from the earlier southern “layer”—from hunter-gatherers who closely resemble the relic groups that still exist in parts of Southeast Asia, i.e., the Andaman Islanders, the Aeta of the Philippines and the Maniq and Semang of the Malayan Peninsula.

 

Admixture from that older southern substrate pushes back in time the latest common ancestors, who never existed. Adaptation to coronaviruses therefore happened at a later date, probably when the “northerners” pushed into what is now northern China and adopted farming. They then grew in population, pushed farther south, and intermixed with the hunter-gatherers who lived there. 

 

 

References

 

Comas, I., M. Coscolla, T. Luo, et al. (2013). Out-of-Africa migration and Neolithic coexpansion of Mycobacterium tuberculosis with modern humans. Nature Genetics 45: 1176–1182. https://doi.org/10.1038/ng.2744

 

Frost, P. (2020). Does a commensal relationship exist between coronaviruses and some human populations? Journal of Molecular Genetics 3(2): 1-2. https://researchopenworld.com/does-a-commensal-relationship-exist-between-coronaviruses-and-some-human-populations/

 

Frost, P. (2022). A natural vaccine. Evo and Proud, February 21 http://evoandproud.blogspot.com/2022/02/a-natural-vaccine.html

 

Oxenham, M., and H.R. Buckley. (2016). The population history of mainland and island Southeast Asia, in M. Oxenham and H.R. Buckley (eds) The Routledge Handbook of Bioarchaeology in Southeast Asia and the Pacific Islands. Routledge.

 

Souilmi, Y., M.E. Lauterbur, R. Tobler, C.D. Huber, A.S. Johar, S.V. Moradi, W.A. Johnston, N.J. Krogan, K. Alexandrov, and D. Enard. (2021). An ancient viral epidemic involving host coronavirus interacting genes more than 20,000 years ago in East Asia. Current Biology 31(16), 3504–3514.e9. https://doi.org/10.1016/j.cub.2021.05.067

 

Xue, Y., T. Zerjal, W. Bao, S. Zhu, Q. Shu, J. Xu, R. Du, S. Fu., P. Li, M.E. Hurles, H. Yang, C. Tyler-Smith. (2006). Male demography in East Asia: A north-south contrast in human population expansion times. Genetics 172: 2431-2439, https://doi.org/10.1534/genetics.105.054270

Humans and the olfactory environment

 

Perfume burner, Egypt, c. 700-900 (Wikicommons, Musée du Louvre, Marie-Lan Nguyen)

 

We have sought to remake our environment in ever more appealing ways, including its smell. But the change hasn’t been one-way. By remaking our olfactory environment, we’ve ended up remaking ourselves.

 

 

I’ve published a new paper in Psych. When I wrote it, I had three aims:

 

·         Explain the concept of gene-culture coevolution

·         Provide a concrete example, i.e., how we have coevolved with the odors around us, not only in our ability to emit and sense them but also in our ability to represent them mentally

·         Develop the theoretical basis of gene-culture coevolution

 

Please feel free to comment. The following is the abstract:

 

 

As hunter-gatherers, humans used their sense of smell to identify plants and animals, to find their way within a foraging area, or to distinguish each other by gender, age, kinship, or social dominance. Because women gathered while men hunted, the sexes evolved different sensitivities to plant and animal odors. They also ended up emitting different odors. Male odors served to intimidate rival males or assert dominance. With the rise of farming and sedentism, humans no longer needed their sense of smell to find elusive food sources or to orient themselves within a large area. Odors now came from a narrower range of plants and animals. Meanwhile, body odor was removed through bathing to facilitate interactions in enclosed spaces. This new phenotype became the template for the evolution of a new genotype: less sensitivity to odors of wild plants and animals, lower emissions of male odors, and a more negative response to them. Further change came with the development of fragrances to reodorize the body and the home. This new olfactory environment coevolved with the ability to represent odors in the mind, notably for storage in memory, for vicarious re-experiencing, or for sharing with other people through speech and writing.

 

 

References

 

Frost, P. (2022). Humans and the olfactory environment: a case of gene-culture coevolution? Psych 4(2): 301-317. https://doi.org/10.3390/psych4020027  

 

The myth of selective neutrality

Paleolithic tent (Wikicommons - Michal Mañas). Did Europeans lose haplogroup U because they were replaced by farmers from the south? Or because they needed less energy for body heat?



Blood group systems have long been used to reconstruct prehistory. A good example is the Diego antigen. One of its alleles, DI*A, has helped us chart the prehistory of indigenous peoples in the Americas. Among other things, we have learned that most of them originated in Siberia some 12,000 years ago. This is not the case with the Eskimo-Aleut and Na-Dene peoples, who seem to have entered North America later. 

It’s assumed here that the Diego antigen has mutated at a steady rate and that the mutations have displaced earlier ones at a steady rate. So this antigen can act as a clock. If two populations have separated from each other, we can estimate their time of separation by measuring the mean genetic difference between them at the Diego antigen.

The "clock" assumption has its limitations. Diego mutations are neither kept nor lost at a constant rate. Both processes can be slowed down or speeded up by natural selection: 

Our study also revealed a significant correlation between DI*A allele frequency and warm tropical conditions, domesticated crop type, and presence of disease-carrying vector species. The circumscribed areas defined by these factors compose a mosaic of specific biocenoses and pathocenoses. It is thus reasonable to consider natural selection in the distribution of human genetic polymorphisms. (Bégat et al. 2015)

It's widely believed that all blood groups have the same survival value, so differences between them should be "selectively neutral." That belief is mistaken. In fact, nothing in the genome is truly of neutral value, not even noncoding genes that supposedly do nothing. Even if a gene doesn't code for anything, it still affects the spatial configuration of genes on the chromosome, thus altering how one gene may regulate another. 

According to a recent study, 80% of our genome has some kind of function, even noncoding genes (The ENCODE Project Consortium 2012). Indeed, such genes may have disproportionately contributed to human evolution. Comparison of our genome with other primate genomes has shown that almost all human-specific deletions are in noncoding regions (Bae et al. 2015). Furthermore, DNA is mostly noncoding in human accelerated regions (HARs)—genomic regions that have been well conserved throughout vertebrate evolution but are strikingly different in humans, perhaps in ways that alter how coding genes regulate each other (Bae et al. 2015). This would be consistent with the belief that our ancestors evolved largely through new ways of regulating existing systems, particularly the pace and timing of development (King and Wilson 1975).


Loss of haplogroup U: population replacement or change in natural selection?

Let's now look at haplogroup U. This, too, is assumed to be "selectively neutral" and is used to reconstruct prehistory, specifically the replacement of hunter-gatherers by farmers in Europe. Haplogroup U is a group of mitochondrial genes that was widespread among Mesolithic hunter-gatherers throughout Europe and is now common only among the Sami of Finland and the Mansi of northwestern Siberia, both of whom were hunter-gatherers until recently (Derbeneva et al. 2002). Indeed, according to ancient mtDNA from central and western Europe, the population frequency of haplogroup U shows a sharp break at the time boundary between late hunter-gatherers and early farmers (Bramanti et al. 2009). That break strongly suggests that European hunter-gatherers were largely replaced by farmers spreading into Europe from the Middle East.

Yet things are not always as they seem. In Denmark, haplogroup U persisted at high frequencies long after the transition to farming, in fact as late as the Early Iron Age (Melchior et al. 2010). In Latvia and Ukraine it persisted into Neolithic times (Jones et al. 2017).

Perhaps haplogroup U disappeared because it ceased to be adaptive and was removed by natural selection. This haplogroup shifts the energy balance away from ATP synthesis and toward production of body heat—a useful cold adaptation for hunter-gatherers, who had to sleep in makeshift shelters and pursue game animals in all kinds of weather (Balloux et al. 2009; Montiel-Sosa et al. 2006). Farmers slept in a warmer environment and could more easily plan their outdoor activities.

This being said, the loss of haplogroup U was not the only genetic change across the Mesolithic-Neolithic divide. Were those other changes due to natives being replaced by farmers from the Middle East? Or was natural selection again responsible? Researchers have tried to exclude the second cause by examining how noncoding genes changed across the divide, on the assumption that such genes are generally non-functional and make no difference to one’s chances of survival and reproduction. As we've seen, that assumption is unfounded.

Clearly, some of this genetic change was due to natural selection. I mentioned the shift in energy balance, but there were others. Farmers had less need for odor recognition, monotony avoidance, and sensation seeking (Majid and Kruspe 2018; Zuckerman 2008). They also had to process reciprocal obligations with a larger number of people while interacting less, on average, with each person. All in all, farming did not impose the same demands on mind and body. Going from one way of life to the other required many physiological adjustments.

To explain the genetic divide between hunter-gatherers and farmers, we should also allow for founder effects. When bands of hunter-gatherers are given the opportunity, only a few will choose to become farmers. Because this minority is a small sample of the hunter-gatherer gene pool, the new farming population will differ genetically from the previous one in many random ways.


Conclusion

When reconstructing the past, particularly the transition from hunting and gathering to farming, we shouldn't interpret genetic change solely in terms of one population replacing another. Some of the change may also be due to a new regime of natural selection, as well as founder effects.

I once made this point to Greg Cochran, and his reply was that changes in natural selection couldn't possibly account for all of the genetic change we see in ancient DNA between late hunter-gatherers and early farmers. True, but that's not my point. Some population replacement did happen, but its magnitude is exaggerated by a methodology that attributes all genetic change to that one factor alone. 


Interview with Grégoire Canlorbe

I was recently interviewed by Grégoire Canlorbe, a young French author and scholar. The interview covers a variety of topics and can be read in its entirety (in two parts) at American Renaissance:

https://www.amren.com/features/2020/03/how-did-whites-get-their-appearance/ 
https://www.amren.com/features/2020/03/why-are-human-groups-so-different/

Une traduction française est disponible sur le site Evopsy de Philippe Gouillou :

http://www.evopsy.com/concepts/coevolution-frost.html
http://www.evopsy.com/concepts/hbd-frost.html


References

Bae, B-I., D. Jayaraman, and C.A. Walsh. (2015). Genetic changes shaping the human brain. Developmental Cell 32: 423-434. 
https://www.sciencedirect.com/science/article/pii/S1534580715000787

Balloux F., L.J. Handley, T. Jombart, H. Liu, and A. Manica. (2009). Climate shaped the worldwide distribution of human mitochondrial DNA sequence variation. Proceedings of the Royal Society B. Biological Sciences 276: 3447-3455. 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817182/

Bégat, C., Bailly, P., Chiaroni, J., & Mazières, S. (2015). Revisiting the Diego Blood Group System in Amerindians: Evidence for Gene-Culture Comigration. PloS one 10(7), e0132211.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4493026/

Bramanti, B., M.G. Thomas, W. Haak, M. Unterlaender, P. Jores, K. Tambets, I. Antanaitis-Jacobs, M.N. Haidle, R. Jankauskas, C.J. Kind, et al. (2009). Genetic discontinuity between local hunter-gatherers and Central Europe's first farmers. Science 326: 137-140.
http://roceeh.mediatis.de/fileadmin/download/Publications/Bramanti_Sci09_Meso_Neo.pdf

Derbeneva, O.A., E.B. Starikovskaya, D.C. Wallace, and R.I. Sukernik, (2002). Traces of early Eurasians in the Mansi of Northwest Siberia revealed by mitochondrial DNA analysis. American Journal of Human Genetics 70: 1009-1014. 
https://www.sciencedirect.com/science/article/pii/S0002929707603085

Jones, E.R., G. Zarina, V. Moiseyev, E. Lightfoot, P.R. Nigst, A. Manica, et al. (2017). The Neolithic transition in the Baltic was not driven by admixture with early European farmers. Current Biology 27(4): 576-582.
https://www.sciencedirect.com/science/article/pii/S0960982216315421

King, M-C, and A.C. Wilson. (1975). Evolution at two levels in humans and chimpanzees. Science 188: 107-116.
http://hydrodictyon.eeb.uconn.edu/people/schwenk/KingWilsonHumansChimps75.pdf

Majid, A., and N. Kruspe. (2018). Hunter-gatherer olfaction is special. Current Biology 28: R108-R110. 
https://www.sciencedirect.com/science/article/pii/S0960982217316160

Melchior, L., N. Lynnerup, H.R. Siegismund, T. Kivisild, and J. Dissing. (2010). Genetic diversity among ancient Nordic populations. PLoS One 5(7): e11898
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912848/

Montiel-Sosa, F., E. Ruiz-Pesini, J.A. Enriquez, A. Marcuello, C. Diez-Sanchez, J. Montoya, D.J. Wallace, and M.J. López-Pérez, (2006). Differences of sperm motility in mitochondrial DNA haplogroup U sublineages. Gene 368: 21-27.
http://cnc.cj.uc.pt/BEB/private/pdfs/2007-2008/RepBiology/ExtraBibliog/MontielSosa2006.pdf

The ENCODE Project Consortium. (2012). An integrated encyclopedia of DNA elements in the human genome. Nature 489: 57-74 
https://www.nature.com/articles/nature11247

Zuckerman, M. (2008). Genetics of Sensation Seeking. In J. Benjamin, R.P. Ebstein, and R.H. Belmaker (Eds) Molecular Genetics and the Human Personality, (pp. 193-210). Washington D.C.: American Psychiatric Publishing Inc.
https://books.google.ca/books?id=mfANqS-SnwgC&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

The ghosts of West Africa

Bushmen in the Kalahari (Wikicommons, Andy Maano). When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 

Most Americans think of native Africans as black and of white Africans as recent intruders; and when they think of Africa's racial history they think of European colonialism and slave trading. But very different types of peoples occupied much of Africa until as recently as a few thousand years ago.

When Jared Diamond penned those words, analysis of ancient DNA was years away. Even when it began, there was a feeling that such analysis would always be impractical in Africa or anywhere else in the tropics. The climate is too warm for that stuff to last thousands upon thousands of years.

Apparently not. DNA has been retrieved from the remains of four individuals at a site in Cameroon, two of them going back 8,000 years and the other two 3,000 years. The main finding? The individuals were most similar to Pygmies, who still exist as isolated groups of hunter-gatherers in the Congo basin. There was no genetic similarity to the Bantu peoples who now predominate throughout central, eastern, and southern Africa (Lipson et al. 2020).

This finding is no surprise. Linguistic evidence has shown that the Bantu are all descended from a group of farming peoples who, some two to three thousand years ago, began to expand eastward and southward from what is now the Cameroon-Nigeria border. 

More intriguing is the discovery of admixture from an extinct West African people. These were hunter-gatherers who shared common ancestry with the Pygmies of central Africa and the Khoisans of southern Africa; however, they had intermixed much more with an archaic hominin that had diverged from ancestral modern humans at about the same time as the Neanderthals:

The West African clade is distinguished by admixture from a deep source that can be modelled as a combination of modern human and archaic ancestry. The modern human component diverges at almost the same point as Central and southern African hunter-gatherers and is tentatively related to the deep source that contributes ancestry to the Mota individual, and the archaic component diverges close to the split between Neanderthals and modern humans (Lipson et al. 2020)

This suggests that the Bantu expansion was the second leg of an earlier expansion of farming peoples who had first replaced the hunter-gatherers of West Africa. This is in line with the thinking of George Murdock, an American anthropologist who argued that black Africans originated with the spread of agriculture from the Niger's headwaters, near the Mali-Guinea border. This region was the cradle of the Sudanic food complex: sorghum, pearl millet, cow pea, and other crops.

Murdock’s scenario is supported by linguistic evidence. Speakers of proto-Niger-Congo broke up around 10,000 years ago, and the oldest group appears to be proto-Mande speakers, whose descendants inhabit the Niger's headwaters (Blench 1984, pp. 128-129; Ehret 1984; Murdock 1959, pp. 44, 64-68). Farming itself seems to have begun later. According to Harris (1976, p. 352), “the problem of dating must be left in abeyance, but it is clear that some form of seed-crop cultivation was underway in the interior at least by the second millennium B.C.”

It looks like a stable population of hunter-gatherers took shape on the Niger’s headwaters around 10,000 years ago. They gradually became proto-agricultural, i.e., more sedentary and better able to manage their food sources. By 4,000 years ago, they had become true farmers and were entering a phase of sustained demographic expansion that would see them colonize the banks of the Niger farther and farther downstream until they reached the rain forest in southern Nigeria. As they adapted to this new environment, they reached a modus vivendi with the Pygmy inhabitants, at first as tenants and then as de facto landowners who took over more and more of the land. Meanwhile, the Pygmies were pushed back farther and farther into the forest until they were no more.

In sum, farming can support a much larger population, and it was this demographic advantage that enabled farming peoples to replace hunter-gatherers, first in West Africa and eventually throughout almost all of sub-Saharan Africa.


Memories of the first West Africans

Those hunter-gatherers are remembered in the traditions of West Africa: 

Pygmies may have been the first inhabitants of Côte d'Ivoire. In their oral tradition, most of the present-day peoples, in particular the Dan-Yacouba, recount that their ancestors, on arriving in the country, found "little red men" whom they pushed back into the forest. Others speak of "little brown men", who had supernatural powers and to whom presents were given to win them over. (Mantongouine 2012)

According to some authors like Allou and Gonnin, the presence of these mysterious beings appears in the oral traditions. They are presented as short beings about 1m 44 to 1m 55 according to J.N. Loucou, with reddish skin, abundant hair, and feet pointing backward. They appear in almost all of the regions of prehistoric Côte d’Ivoire in the sense that almost all of the oral traditions of Côte d’Ivoire’s ethnic groups affirm that they found pygmies in the area before they became established. (Afri 2013; see also Gonnin and Allou 2006; Loucou 1984, p. 18)

Everywhere, but mainly in the countries from which the Pygmies have long disappeared, the Blacks who are considered to be the oldest occupants of the land say that it does not really belong to them and that, when their distant ancestors, coming from the East, established themselves, they found it in the possession of little men with reddish complexions and large heads who were the real natives and who, in exchange for fulfilment of certain agreements, permitted the Negroes who first arrived on a piece of land to enjoy its use and cultivate it. Eventually, those little men disappeared, but the memory of them has persisted. (Delafosse 1922, p. 14)

The Mano of Liberia say that the forested area used to contain only “talking chimpanzees.” These small creatures, called Lam, inhabited the area when the Mano first came. A Lam and his family would live in a hole in the ground (Riddell 1970, p. 27).


Year-round farming, polygyny. and increased stature and robustness

In addition to their means of subsistence, this expanding population of farmers differed from the hunter-gatherers in another way: a much higher rate of polygyny. Farming, especially year-round farming, makes women more self-reliant in feeding themselves and their children, thus cutting the costs, for a man, of having a second wife (van den Berghe 1979, p. 65). The result is a high polygyny rate: 20-50% of all marriages in sub-Saharan farming societies (Bourguignon and Greenbaum 1973, p. 51; Goody 1973; Pebley and Mbugua 1989; Welch and Glick 1981; White 1988).

If some men have more wives, others have to do without. In general, men must compete more keenly with each other for access to women. When such rivalry intensifies in nonhuman species, there is selection for larger, stronger, and more muscular males. This may explain the physical robustness of polygynous farming peoples in sub-Saharan Africa.

This point was studied by Butovskaya et al. (2015) in their study of two East African peoples: the polygynous Datoga and the monogamous Hadza. Datoga men were larger and more robust than Hadza men. They also scored higher on measures of physical aggression, verbal aggression, anger, and hostility. In fact, the two groups differed fundamentally in their attitudes toward aggression:

There is a negative attitude toward aggression among the Hadza but not among the Datoga. In situations of potential aggression, the Hadza prefer to leave. In contrast, aggression is an instrument of social control — both within the family and in outgroup relations — in Datoga society. Datoga men are trained to compete with each other and to act aggressively in particular circumstances.  (Butovskaya et al. 2015).

The two groups also differed at the androgen receptor gene, with the polygynous Datoga more often having an allele that correlated in men with aggressiveness and number of children fathered. Thus, through a process of gene-culture coevolution, a highly polygynous culture has produced a different sort of man, both mentally and physically.

There are other explanations for the diminutive and less robust appearance of African hunter-gatherers. O'Dea (1994) has argued that Pygmies are smaller and less robust because they are less exposed to sunlight in the rain forest and thus less able to synthesize vitamin D and maintain a large and strong skeleton. But how would this theory explain the small, gracile appearance of the Khoisan hunter-gatherers of the Kalahari, who live in an open environment with high solar radiation?


Darker skin

The polygyny rate correlates with darkness of skin, even after you control for latitude (Manning et al. 2004). This is particularly so in sub-Saharan Africa, where highly polygynous farming peoples are noticeable darker than the largely monogamous Pygmy and Khoisan hunter-gatherers. The reason may be a widespread mental association between gender and skin color. Because women are naturally lighter-skinned than men, traditional cultures tend to associate light skin with femininity and dark skin with masculinity (van den Berghe and Frost 1986). There is thus a selective compromise between natural selection for darker skin as a protection against solar radiation and sexual selection for lighter skin as a criterion of femininity (or darker skin as a criterion of masculinity). 

Because unmated women of any kind are scarce in a polygynous society, there is weaker sexual selection for women with lighter skin. This may be why farming peoples are noticeably darker-skinned in sub-Saharan Africa (Frost 2008).


Archaic admixture in West Africa

The ancient DNA study is also consistent with evidence that a partially archaic population used to live in West Africa. One piece of evidence is a skull from a Nigerian site (Iwo Eleru), which is only about 16,300 years old and yet is intermediate in shape between the skulls of modern humans on the one hand and the skulls of Neanderthals and Homo erectus on the other (Harvati et al. 2011; Stojanowski 2014). Furthermore, genomic analysis shows an apparently higher level of Neanderthal ancestry in the Yoruba of Nigeria than in the Luhya of Kenya. This admixture seems to come from a Neanderthal-like population that once lived in West Africa (Hawks 2012).


Conclusion

In the fifteenth century, Europeans discovered a continent whose inhabitants would have looked quite different a millennium earlier. Two tenth-century Arab geographers reported that "in the outer reaches of the land of the Zanj there are cool highlands in which live white Zanj" (Lewis 1990, p. 121, n. 3). The Zanj are the dark-skinned peoples of east Africa and the “white Zanj” were probably the Khoisan hunter-gatherers who once inhabited the inland plateau of southern Africa.

If we could rewind history, we would see true black Africans retreating progressively to West Africa and then to the area of the Niger’s headwaters. This leads us to a strange conclusion. When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. Perhaps they didn’t yet exist anywhere. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 


References

Afri, A. (2013). Existait-il des peuples en Côte d’Ivoire avant le XVIIIème siècle ?
http://anicetafri.over-blog.com/existait-il-des-peuples-en-cote-d’ivoire-avant-le-xviiième-siÈcle

Blench, R. (1995). Recent developments in African language classification and their implications for prehistory. In T. Shaw, P. Sinclair, B. Andah, and A. Okpoko (Eds.) The Archaeology of Africa (pp. 126-138). London: Routledge.

Butovskaya M.L., O.E. Lazebny, V.A. Vasilyev, D.A. Dronova, D.V. Karelin, A.Z.P. Mabulla, et al. (2015). Androgen receptor gene polymorphism, aggression, and reproduction in Tanzanian foragers and pastoralists. PLoS ONE 10(8): e0136208.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4546275/ 

Delafosse, M. (1922). Les Noirs de l’Afrique. Paris: Collection Payot. 
https://www.herodote.net/Textes/delafosse_noirs_afrique.pdf

Diamond, J. (1994). How Africa Became Black. Discover, February 1
https://www.discovermagazine.com/planet-earth/how-africa-became-black

Ehret, C. (1984). Historical/linguistic evidence for early African food production. In J.D. Clark and S.A. Brandt (Eds.) From Hunters to Farmers: The Causes and Consequences of Food Production in Africa (pp. 26-35). Berkeley: University of California Press.

Frost, P. (2008). Origins of black Africans, Evo and Proud, February 10
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Gonnin, G. and R.K. Allou. (2006). Côte-d’Ivoire : les premiers habitants. Abidjan: Les éditions du CERAP.

Goody, J. (1973). Polygyny, Economy and the Role of Women, in J. Goody (Ed.) The Character of Kinship, Cambridge: Cambridge University Press, pp. 175-190.

Harris, D.R. (1976). Traditional systems of plant food production and the origins of agriculture in West Africa. In J.R. Harlan, J.M.J. De Wet, and A.B.L. Stemler. (ed.) Origins of African Plant Domestication, (pp. 311-356), The Hague: Moulton.
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Harvati, K., C. Stringer, R. Grün, M. Aubert, P. Allsworth-Jones, C.A. Folorunso. (2011). The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology. PLoS ONE 6(9): e24024. doi:10.1371/journal.pone.0024024
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Hawks, J. (2012). Which population in the 1000 Genomes Project samples has the most Neandertal similarity? John Hawks Weblog, February 8
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Lewis, B. (1990). Race and Slavery in the Middle East. An Historical Enquiry. New York: Oxford University Press.

Lipson, M., I. Ribot, S. Mallick, et al. (2020). Ancient West African foragers in the context of African population history. Nature 577: 665-670.
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Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25(1): 38-50.
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Have we been selected for long-term thinking?

GDP per capita as a function of future orientation (Preis et al. 2012)



To what degree do we value the short term over the long term? The answer varies not only from individual to individual but also from society to society. Hunter-gatherers, for instance, value the short term. Perishable food cannot be stored for future use and, in any case, is not normally obtained in large enough amounts to make storage worthwhile. If a hunter gets more meat than his family can consume, he'll give it away to others in the local band.

There are exceptions, especially at northern latitudes. Meat can be stored in caches during winter and in cold lake waters during summer. With limited opportunities for food gathering, women specialize in technologies that need more cognitive input and longer-term thinking, like garment making, needlework, weaving, leatherworking, pottery, and use of kilns. Finally, men hunt over longer distances and therefore plan over the longer term. Northern hunting peoples thus broke free of the short-term mental straitjacket imposed by hunting and gathering. In time, their descendants would spread south and rise to the challenges of social complexity (Frost 2019).

Those northern hunting peoples were better able to exploit the opportunities created by farming, but the transition from one lifestyle to the other was still far from easy. Farming requires not only longer-term thinking but also less monotony avoidance and higher thresholds for expression of personal violence. In recent times, hunter-gatherers usually refused offers to be settled on farms. They saw farming as akin to slavery.

The change in mindset didn't end with the transition to farming. There were different types of farming, and some required longer-term investment than others. Those types generated stronger selection for future orientation.


Language as a mirror of cultural evolution

Galor et al. (2018) argue that language is a mirror of cultural evolution. It can show a society’s degree of commitment to a long-term mindset, as well as other psychological traits.


The periphrastic future tense

The authors studied the relationship between future orientation and forms of the future tense that express intention and obligation, rather than simply prediction:

Languages differ in the structure of their future tense. In particular, linguists distinguish between languages that are characterized by an inflectional versus periphrastic future tense [...]. Inflectional future tense is associated with verbs that display morphological variation (i.e., a change in the verb form that is associated with the future tense). In contrast, periphrastic future tense is characterized by roundabout or discursive phrases, such as `will', `shall', `want to', `going to' in the English language [...] (Galor et al. 2018, p. 6)

[U]nlike the inflectional future tense, the periphrastic future tense is formed by terms that express a desire, an intention, an obligation, a commitment as well as a movement towards a goal. In particular, in the English language, "shall has developed from a main verb meaning 'to owe', will from a main verb meaning 'to want', and the source of be going to is still transparent" [...]. Moreover, "intention and prediction are most commonly expressed by the periphrastic future, while the synthetic one is more common in generic statements, concessives, and suppositions" [...]. Inflectional futures "also appear systematically (often obligatorily) in sentences which express clear predictions about the future (which are independent of human intentions and planning), whereas less grammaticalized constructions [i.e., periphrastic] often tend to be predominantly used in talk of plans and intentions - a fact which is explainable from the diachronic sources of future tenses" [...] (Galor et al. 2018, p. 6)

Galor et al. (2018, p. 16) used pre-1500 AD data to estimate the return on agricultural investment ("crop return") in the homeland of a language’s speakers. They found a positive correlation between this return on investment and the existence of a periphrastic future tense. They concluded that "a one standard deviation increase in crop return in the language's contemporary homeland is associated with a 6 percentage points increase in the probability that the language is characterized by a periphrastic future tense."

Using the World Values Survey, the authors also found a positive correlation between the existence of a periphrastic future tense and future orientation. The correlation held true both for the people of the world as a whole and for Old World peoples who speak languages originating in the Old World (Galor et al. 2018, p. 23).

Interestingly, the return on agricultural investment did not correlate with other linguistic characteristics, like the existence of the past tense or the perfect tense, the existence of possessive classifications, the existence of coding for evidentiality, the number of consonants, and the number of colors (Galor et al. 2018, pp. 18-19).


Grammatical gender

The authors also looked into the relationship between grammatical gender and the sexual division of labor in a language's homeland:

Further, consider ancient civilizations that had been characterized by a sexual division of labor and consequently by the existence of gender bias. Linguistic traits that had fortified the existing gender biases have plausibly emerged and persisted in these societies over time. In particular, geographical characteristics that had been associated with the adoption of agricultural technology that had contributed to a gender gap in productivity, and thus to the emergence of distinct gender roles in society (e.g., the suitability of land for the usage of the plow […]), may have fostered the emergence and the prevalence of sex-based grammatical gender in the course of human history. (Galor et al. 2018, p. 2)

Galor et al (2018, p. 24) found a negative correlation between grammatical gender and “plow negative” crops (i.e., crops not requiring use of the plow and, hence, requiring less male participation). A one standard deviation increase in the potential caloric yield of plow negative crops was associated with a 13 percentage point decrease in the probability that the language has grammatical gender.  The correlation was reversed in the case of all crops, the caloric yield now being associated with a 17 percentage point increase in the probability that the language has grammatical gender.


Politeness distinctions in pronouns

Finally, Galor et al. (2018) looked into the relationship between politeness distinctions in pronouns and ecological diversity, which they related to the emergence of hierarchical societies.

Linguistic traits that had reinforced existing hierarchical structures and cultural norms had conceivably emerged and persisted in these stratified societies in the course of human history. In particular, politeness distinctions in pronouns (e.g., the differential use of "tu" and "usted" in the Spanish language, "Du" and "Sie" in German, and "tu" and "vous" in French) had conceivably appeared and endured in hierarchical societies. Thus, geographical characteristics, such as ecological diversity that had been conducive to the emergence of hierarchical societies (Fenske, 2014), may have contributed to the emergence of politeness distinctions. (Galor et al. 2018, p. 2)

Galor et al. (2018, p. 32) found a significant relationship between politeness distinctions and ecological diversity in a language's homeland. A one standard deviation increase in ecological diversity corresponded to a 15 percentage point increase in the probability that the language has politeness distinctions.

I'm skeptical about the last finding. Is ecological diversity conducive to hierarchical societies? The authors refer to a study that mostly uses African data. More to the point, the study seeks to link ecological diversity to centralized states. Centralization of state power and social hierarchization are not the same thing. Japan, for instance, had a weak central state for much of its history and yet was very hierarchical, as seen in the politeness distinctions of the Japanese language.


Conclusion

Although the authors refer to work by L.L. Cavalli-Sforza, Peter Richerson, and Robert Boyd on gene-culture coevolution, they avoid discussing the possibility that selection for future orientation, gender specialization, and hierarchical politeness has influenced not only culture and language but also human biology. The coevolution they propose is simply between culture and language. It can be summed up as follows:

- Certain patterns of mind and behavior have been favored to varying degrees in different societies.

- These cultural patterns are transposed into language.

- Language then reinforces those cultural patterns: "In light of the apparent coevolution of cultural and linguistic characteristics in the course of human history, emerging linguistic traits have conceivably reinforced the persistent effect of cultural factors on the process of development" (Galor et al. 2018, p. 1).

Language is not a passive mirror of culture. It can also act upon culture. For instance, the way we perceive the future, and its relative importance to us, may be shaped by the way we speak. This is of course the Sapir-Whorf hypothesis.  In a farming society, the periphrastic future tense might make it easier to envision farming methods and technologies that pay off over the longer term. Similar arguments have been made for grammatical gender and politeness distinctions. The way we speak influences our thoughts and behavior.

Again, the authors leave it to the reader to go one step farther: patterns of mind and behavior may influence the frequencies of alleles in the gene pool.


References

Fenske, J. (2014). Ecology, trade, and states in pre-colonial Africa. Journal of the European Economic Association 12(3): 612-640.
https://www.eeassoc.org/doc/paper/20130704_190013_FENSKE.PDF 

Frost, P. (2019). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181
https://doi.org/10.3390/psych1010012 

Galor, O., O. Özak, and A. Sarid. (2018). Geographical Roots of the Coevolution of Cultural and Linguistic Traits (November 7, 2018). Available at SSRN: https://ssrn.com/abstract=3284239 or http://dx.doi.org/10.2139/ssrn.3284239  

Preis, T., H.S. Moat, H.E. Stanley, and S.R. Bishop. (2012). Quantifying the advantage of looking forward. Scientific Reports 2: 350
https://www.nature.com/articles/srep00350