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
