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