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

Recent evolution in Estonia

 

Estonian women at a song festival (Wikicommons – Anastasia Lakhtikova)

 

Estonian women had more reproductive success during the late 20th century if they possessed a more masculine body build, narrower hips, and shorter legs. Such women married earlier and were less likely to stay on the mate market as long as possible.

 

Human evolution didn’t end in the Pleistocene. In fact, there has been more genetic change within our species over the past 10,000 years than over the previous 100,000, and perhaps more than over the previous million. The growing importance of culture did not slow down the pace of genetic change. In fact, culture became the main driving force of genetic evolution by replacing adaptation to a limited number of natural environments with adaptation to an ever-widening range of cultural environments (Cochran and Harpending 2009; Hawks et al. 2007; Rinaldi 2017).

 

Two years ago, I reviewed a study on recent evolution in the Estonian population (Frost 2020; Hõrak and Valge 2015). Among Estonians born between 1937 and 1962, women with only primary education had 0.5 to 0.75 more children than did women with tertiary education. This difference in reproductive success correlated with difference in cranial volume: children with larger crania were more likely to go on to secondary or tertiary education, independently of sex, socioeconomic position, and rural vs urban origin (Valge et al. 2019). Thus, for Estonian women in the late 20th century, higher education decreased fertility, probably by postponing the age of marriage.

 

That finding was found only for women. Perhaps Estonian men with higher education enjoyed greater reproductive success, in which case selection for less intelligent women may have been cancelled out by selection for more intelligent men.

 

The same research team has now published a new study of the same dataset, this time on both sexes. They confirm the original finding that female fertility correlated negatively with education and cranial volume. As for male fertility, although it correlated positively with education, the most fertile males had only average cranial volume. The authors had no explanation for that finding:

 

Stabilizing selection on the cranial volume of boys was an unexpected result, given that cranial volume in our study population predicts educational attainment independently of sex, socioeconomic background, and height. Since educational attainment was a strong predictor of fatherhood in our study, we would have expected positive directional selection on cranial volume. However, we found only evidence for stabilizing selection (Valge et al. 2022)

 

Perhaps women prefer men who are well-educated but not excessively intelligent. As one goes farther and farther away from the mean IQ of a population, higher intelligence becomes more and more often due to genetic “accidents”—unusual genetic variants or combinations of variants that may adversely affect other aspects of mind and behavior. A very intelligent person may seem autistic or have poor social skills.

 

The new study also shows that women had greater reproductive success if they possessed a more masculine body build, narrower hips, and shorter legs. That finding may seem counterintuitive. Don’t men prefer feminine-looking women? They do. However, as the authors show by citing earlier findings, shorter women are also less selective and likelier to marry earlier:

 

Similar reasoning might also explain why selection favored girls with masculine body build, narrow hips, and absolutely and relatively shorter legs in our study. If choosiness in women increases with desirability, this could lead to women with more feminine phenotypes engaging in a more time-consuming mate selection process, delaying their age of first birth, and thereby negatively affecting reproduction. (Valge et al. 2022)

 

Finally, the new Estonian study shows that heavier and stronger boys had more reproductive success.

 

The results relating to height and strength are consistent with studies of sexual selection showing that men who are taller, stronger, and more physically fit are generally perceived as more physically attractive by women, and therefore, have better opportunities for partnering and becoming a father. For instance, in a sample of Polish men born in the 1930s, childless men appeared significantly shorter than those with at least one child. In West Point graduates, the number of children increased linearly with height because taller men had higher probabilities of marrying more than once. Barclay and Kolk showed in a sample of 405,427 Swedish conscripts born between 1965 and 1972 that men in the lowest deciles of height, and in particular, physical fitness in early adulthood, had the lowest probabilities of transition to parenthood. (Valge et al. 2022)

 

Final thoughts

 

This is a study of Estonians who were born more than a half-century ago, long before the breakup of the Soviet Union. Things may be different now. Estonians have rapidly converged on Western social, behavioral, and ideological norms over the past three decades. Although their country is nominally independent, they are now strongly influenced by the inflow of Western culture via the media, and this new media environment is having a decisive impact on how they think and act (Karlin 2018).

 

Estonia is generally following the lead of the West. With respect to education and fertility, the negative correlation has become stronger throughout the West: “In all countries [Australia, United States, Norway, Sweden], however, education is negatively associated with childbearing across partnerships, and the differentials increased from the 1970s to the 2000s” (Thomson et al. 2014).

 

This differential is increasing not only between families but also within “families.” Second and third children are born increasingly to women who have divorced and are in relationships with low-quality fathers who often seem to be little more than sperm donors. In Norway, multi-partner fatherhood has become most common among men with the lowest level of education (10 years of schooling, "i.e., compulsory education"):

 

At age 45, about 15 percent of all men in the 1960-62 cohort with a compulsory education had had children with more than one woman, compared to about 5 percent among men with a tertiary degree. If looking at fathers only (Figure 6), the pattern becomes even more pronounced. At the lowest educational level, 19.3 percent of those who had become fathers, had children with more than one woman, compared to 6.1 percent of those at the highest educational level. (Lappegård et al. 2011)

 

This trend may partly explain the slowing down and reversal of the Flynn effect, i.e., the steady rise in mean IQ over the 20th century. There is some debate over whether the Flynn effect was a real increase in intelligence or simply an increase in familiarity with doing tests. In any case, its reversal seems real enough.

 

With respect to Norway, Bratsberg and Rogeberg (2018) have shown that the decline in mean IQ can be explained by “within-family variation.” In other words, mean IQ is declining among people who supposedly share the same genetic background, i.e., siblings. In Norway, however, siblings are increasingly half-siblings. Among Norwegian women with only two children, 13.4% have had them by more than one man. The figure rises to 24.9% among those with three children, 36.2% among those with four children, and 41.2% among those with five children (Thomson et al. 2014). 

 

The family unit is decomposing throughout the West. It is becoming little more than an administrative entity that can be repeatedly dissolved and reconstituted (Frost 2018a; Frost 2018b).

 

 

References

 

Bratsberg, B., and O. Rogeberg. (2018). Flynn effect and its reversal are both environmentally caused. Proceedings of the National Academy of Sciences 115 (26) 6674-6678

https://doi.org/10.1073/pnas.1718793115

 

Cochran, G. and H. Harpending. (2009). The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books: New York.

 

Frost, P. (2018a). Why is IQ declining in Norway? Evo and Proud, June 19. https://evoandproud.blogspot.com/2018/06/why-is-iq-declining-in-norway.html

 

Frost, P. (2018b). Yes, the decline is genetic. Evo and Proud, June 26. https://evoandproud.blogspot.com/2018/06/yes-decline-is-genetic.html

 

Frost, P. (2020). Declining intelligence in the 20th century: the case of Estonia. Evo and Proud, August 3. https://evoandproud.blogspot.com/2020/08/declining-intelligence-in-20th-century.html

 

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA) 104: 20753-20758. https://doi.org/10.1073/pnas.0707650104

 

Hõrak, P., and M. Valge. (2015). Why did children grow so well at hard times? The ultimate importance of pathogen control during puberty. Evolution, Medicine, and Public Health (1): 167–178, https://doi.org/10.1093/emph/eov017

 

Karlin, A. (2018). Gay marriage in Estonia. The Unz Review, October 30. https://unz.com/akarlin/estonian-freezer/

 

Lappegård, T., Rønsen, M., and Skrede, K. (2011). Fatherhood and fertility. Fathering 9: 103-120. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.839.2752&rep=rep1&type=pdf

 

Rinaldi, A. (2017). We're on a road to nowhere. Culture and adaptation to the environment are driving human evolution, but the destination of this journey is unpredictable. EMBO reports 18: 2094-2100. https://doi.org/10.15252/embr.201745399

 

Thomson, E., T. Lappegård, M. Carlson, A. Evans, and E. Gray (2014). Childbearing across partnerships in Australia, the United States, Norway, and Sweden. Demography 51(2): 485-508. https://doi.org/10.1007/s13524-013-0273-6 

 

Valge, M., R. Meitern, and P. Hõrak.  (2022). Sexually antagonistic selection on educational attainment and body size in Estonian children. Annals of the New York Academy of Sciences Early view https://doi.org/10.1111/nyas.14859

Brain size and family structure in Estonia

 

Estonian schoolchildren (Wikicommons). Estonian children have smaller brains if raised by a biological parent and a step-parent. Therefore, two committed parents are better than one, right? Well, not in this case. Brains aren't smaller in Estonian children raised by a single parent (and no step-parent).

 

 

In Estonia, cranial volume was one of several anthropometric traits that were routinely measured in schoolchildren during the Soviet era. The data didn't suffer from volunteer bias because the measurements were mandatory. Mortality bias was minimal because the subjects were young. This data source is thus better in many respects than data from Western biobanks. It is now being mined by Peeter Hõrak, a University of Tartu professor, to learn more about nature and nurture in human brain development.

 

I discussed this data source in a previous post (Frost 2020). One problem is that the study population is not as homogeneous as it may seem. In fact, 16% of the fathers and 7% of the fathers were not Estonian (Hõrak 2020). This factor might explain some differences in the data, especially changes over time.

 

 

The latest study

 

This data source has now been used to see whether the brain size of children is influenced by family structure, specifically whether the child was raised by biological parents or by step-parents. The data came from 822 children born between 1980 and 1987 in Tartu, Estonia and were measured at around 14 years of age.

 

The children had significantly larger brains when the household had both biological parents:

Cranial volume was related to family structure and paternal education. Children living with both birth-parents had larger heads than those living in families containing a step-parent. [...] our findings suggest that families including both genetic parents provide non-material benefits that stimulate predominantly cranial growth. (Lauringson et al. 2020)

 

That's what we read in the Abstract. The brain was bigger on average in children who had been raised by both biological parents, rather than by a biological parent and a step-parent, presumably because a step-parent contributes less to the child's upbringing.

 

That finding is rejected, however, in the Results section. It turns out that there was no difference in brain size between children raised by both biological parents and children raised by a single parent (in almost all cases the biological mother). The brain was smaller only in children raised by a biological parent and a step-parent:

 

At the same time, cranial volumes of children living with a single parent were similar to those living with two providers, even though the former reported on average lower resource availability and more frequent meat shortage. Associations between family type and cranial volume thus cannot be explained on the basis of dilution of material resources. (Lauringson et al. 2020)

Differences in family structure also failed to correlate with differences in the child's height. If life in a stepfamily had somehow harmed the child's development, that harm was much less observable in overall body growth than in cranial volume.

 

So what's going on here? Keep in mind two things about Estonian society of the late 20th century:

 

- A single parent was almost always a woman, often a widow who refused to remarry, either because she still felt attached to her deceased spouse or because she considered the potential husbands available to be more trouble than they were worth.

 

- A step-parent could be of either sex. A stepfather often took over from a man who had sired the child out of wedlock or during a short-lived marriage.

 

Thus, on average, the biological father was a different kind of man in the two situations. In the first situation, he was usually the sort of man who would remain with the mother of his child until his death. In the second, he was often the sort of man who would leave the mother of his child once a more interesting woman came into view. One may presume there are differences in genetic quality between the two kinds of men. This hypothesis is actually advanced in the study:

 

An alternative (yet not mutually exclusive) explanation to the observed associations between family type and cranial volume of children would be that parents prone to remarrying possess on average (genetically) smaller heads than those prone to avoiding divorce or remaining single after divorcing. Such a scenario would assume robust genetic correlations between cranial volume and personality traits related to marriage stability. Twin studies have shown that genetic factors account for 13-53% of the variation in divorce [...], and if personality traits associated with a propensity to divorce are genetically correlated with cranial volume or its growth rate, one would detect smaller heads of children growing up in divorced/separated families. Such an explanation would be consistent with the predictions of life history theory, assuming that qualities characteristic of slow pace of life-including high somatic investment into body and brain growth and propensity for relatively low mating effort (in relation to parenting effort)-have coevolved (and cluster) with higher mental abilities and conscientious and risk-averse personality traits [...]. Consistent with this view are also the findings in our sample where fathers with only primary education were shorter and more prone to divorce/separate than others. (Lauringson et al. 2020)

 

We see a similar problem of interpretation with the relationship between father absence and early sexual maturity in daughters. Using a large sample of 1,247 daughters, Surbey (1990) found that daughters with an absent father matured four to five months earlier than those who lived with both parents continuously and seven months earlier than those with an absent mother. Surbey argued that the presence of a strange male accelerates the speed of sexual maturation. In other words, at a subconscious level, the girl does not recognize the man as a father. She recognizes him as a potential mate, and her body gears up for procreation.

 

This hypothesis was challenged by Mendle et al. (2006) who examined the daughters of twin mothers.

 

In a pair of twin mothers of which only one raises her children with a stepfather, the offspring of both twins are equally likely to display early age of menarche. It therefore appears that some genetic or shared environmental confound accounts for the earlier association found in female children living with stepfathers.

 

It seems, then, that people who end up as step-parents are, on average, genetically different from other parents. They tend to have the mental and behavioral characteristics of a "fast" life history.

 

 

References

 

Frost, P. (2020). Declining intelligence in the 20th century: the case of Estonia. Evo and Proud, August 3 http://evoandproud.blogspot.com/2020/08/declining-intelligence-in-20th-century.html

 

Hõrak, P. (2020). Personal communication.

 

Lauringson, V., G. Veldre, and P. Hõrak. (2020). Adolescent Cranial Volume as a Sensitive Marker of Parental Investment: The Role of Non-material Resources? Frontiers in Psychology 15 December https://doi.org/10.3389/fpsyg.2020.602401

 

Surbey, M.K. (1990). Family composition, stress, and the timing of human menarche. In T.E. Ziegler & F.B. Bercovitch (eds.) Socioendocrinology of Primate Reproduction, pp. 11-32, New York: Wiley-Liss Inc.

Declining intelligence in the 20th century: the case of Estonia

Soviet-era stamp. In Estonia, cranial volume shrank between the cohort of girls born in 1937 and those born in 1962, apparently because the intellectually gifted were more likely to pursue higher education and postpone childbearing.

 

Is the genetic basis for intelligence declining from one generation to the next? That’s the conclusion of several recent studies on alleles associated with high educational attainment. By adding up such alleles over the genome, we can get a person's "polygenic score." By calculating the mean polygenic score for each generation, we can then find out whether this genetic basis is declining.

The polygenic score has declined among Icelanders since the cohort born in 1910 and among Euro Americans between the 1931 and 1953 cohorts (Beauchamp 2016; Kong et al. 2017). The Icelandic study is especially interesting because that country took in very few immigrants during the period under study. The decline was thus driven by internal factors. One reason seems to be the tendency of university-educated people to delay reproduction and have fewer children. But that's not the whole story. Even among Icelanders who didn’t pursue higher education, fertility was lower among the intellectually gifted, apparently because their intelligence was associated with a desire to plan for the future and delay gratification.

Before the twentieth century, such forward-looking people were reproductively successful. They were the ones who had enough resources to survive disasters of one sort or another: famine, disease, the Little Ice Age, etc. Today, such disasters are a lot less deadly, and it no longer matters so much whether one is a grasshopper or an ant.

Moreover, because of demographic and cultural changes during the twentieth century, it’s no longer possible to count on the same degree of assistance for child-raising from relatives and grandparents. With childbearing at later ages, grandparents are either dead or too frail to help. With people moving around more, not all relatives live nearby. If you’re the sort of person who plans for the future and delays gratification, you may be a lot more intimidated than your forbears by the costs of raising a family.

Shrinking cranial volume in Estonians

Cranial volume correlates with IQ and with educational attainment, albeit imperfectly (see Frost 2020). Has it been declining in tandem with the decline in alleles for educational attainment?

In Soviet-era Estonia, cranial volume was one of several anthropometric traits that were measured in girls born between 1937 and 1962. Because the measurements were mandatory, there was no volunteer basis; mortality bias was minimal because all the participants were younger than 20. In this respect, the study is better than Western biobank studies. On the other hand, the results may be less applicable to Western populations, given the differences in demographic history. Estonia had no postwar baby boom. Fertility then rose from the late 1960s until the breakup of the Soviet Union. By the late 1980s, fertility was actually higher in Estonia than in any other major region of Europe.

Nonetheless, there were demographic similarities between Soviet Estonia and the West, particularly the rising prevalence of single mothers and the influence of education on fertility:

- Divorce rates began to rise during the interwar years, equalling or exceeding those of Scandinavia from the 1970s onward.

- Throughout the twentieth century, Estonian women with only primary education bore 0.5 to 0.75 more children on average than women with tertiary education. In the population under study, taller children and those with larger crania were more likely to go on to secondary and/or tertiary education, independently of sex, socioeconomic position, and rural vs urban origin (Valge et al. 2019).

The second factor seems to explain why cranial volume declined from the older cohorts to the younger ones:

[...] the majority of selection for smaller cranial volume acted indirectly via educational attainment, whereas the direct path of selection in the SEM model was non-significant (Figs. 2 and 4). In other words, consistent with our prior expectations, girls with larger heads were selected against because they were more likely to obtain higher education than girls with smaller heads. Lower education (Tiit, 2013) and rural origin (Kulu, 2005) have been independently and additively associated with higher fertility in Estonia throughout the past century. The reason for the link between education and fertility is that early reproduction, a major determinant of LRS, is not compatible with schooling for both cultural and genetic reasons. (Valge 2020)

 It is doubtful that this decline is due to ethnic change. All of the girls were from Estonian schools (Russian-speakers had their own schools). Nonetheless some of them were of mixed background. According to a personal communication from the corresponding author, 84% of the fathers and 93% of the mothers were Estonian. Ideally, the study should be redone without individuals of mixed parentage. The problem here is not only that one of the parents was non-Estonian but also that such individuals were disproportionately economic migrants who had trouble finding suitable work elsewhere in the Soviet Union.

Other anthropomorphic changes

Height also declined. Unlike cranial volume, this decline was not wholly explained by educational/socioeconomic differences:

Notably, higher reproductive success of shorter girls in Estonia could not be entirely ascribed to indirect selection via educational attainment, nor via other measured socioeconomic variables such as rural/urban origin, although indirect selection via education did account for a large portion of total selection (Fig. 4). The finding that selection against height remains after controlling for education or income (that favours less-educated individuals who are generally shorter than highly-educated ones) is consistent with findings of studies reviewed by Stulp and Barrett (2016).

 

Female hips and female jaws became narrower even after controlling for educational/socioeconomic differences. There seems to have been selection for rounder female faces, but this selection is significant only if one allows for nonlinear effects. Finally, there was no direct selection on two markers of overall health and nutritional status: handgrip strength and lung capacity.

In general, "direct selection favoured shorter, slimmer and lighter girls with smaller heads, more masculine facial and body shapes and slower rates of sexual maturation."

Conclusion

The genetic basis for intelligence has declined in European populations, apparently since the early twentieth century. This decline is attested by two "hard" measures: 1) alleles associated with educational attainment; and 2) cranial volume. Furthermore, it is attested in two relatively homogenous societies, i.e., Iceland and Estonia.

In Estonia, the decline seems entirely due to the intellectually gifted going to university and postponing family formation. In Iceland, this factor explains only part of the decline: the intellectually gifted chose to postpone family formation even when they didn't go to university. Perhaps the Soviet system was better at steering gifted individuals into higher education.

On a final note, this problem will not go away on its own. If we wish to have large numbers of intellectually gifted people who plan for the future and delay gratification, we will need to reverse certain social and cultural changes of the twentieth century.

Comments by Peeter Horak

In an email, Peeter pointed out that the decrease in height due to natural selection might be offset by an increase in height due to lower pathogen load (as a result of vaccination and antibiotics, see Hõrak and Valge 2015). In addition, we currently don't know the direction of selection on boys. It may entirely cancel out natural selection on girls if men's income and education correlate positively with their reproductive success. In the sample under study, taller boys and those with larger heads went on to obtain more education; if they were reproductively successful, there would be sexually antagonistic selection: selection would favor larger boys and smaller girls at the same time.

References

Beauchamp, J.P. (2016). Genetic evidence for natural selection in humans in the contemporary United States. Proceedings of the National Academy of Sciences 113(28): 7774-7779 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4948342/

Frost, P. (2020). Did women jumpstart recent cognitive evolution? Evo and Proud, July 1 https://evoandproud.blogspot.com/2020/07/did-women-jumpstart-recent-cognitive.html

Hõrak, P., and M. Valge. (2015). Why did children grow so well at hard times? The ultimate importance of pathogen control during puberty, Evolution, Medicine, and Public Health, 2015 (1): 167–178, https://doi.org/10.1093/emph/eov017

Kong, A., M.L. Frigge, G. Thorleifsson, H. Stefansson, A.I. Young, F. Zink, G.A. Jonsdottir, A. Okbay, P. Sulem, G. Masson, D.F. Gudbjartsson, A. Helgason, G. Bjornsdottir, U. Thorsteinsdottir, and K. Stefansson. (2017). Selection against variants in the genome associated with educational attainment. Proceedings of the National Academy of Sciences 114(5): E727-E732 https://core.ac.uk/download/pdf/154416179.pdf

Valge, M., P. Horak, and J.M. Henshaw. (2020). Natural selection on anthropometric traits of Estonian girls. Evolution and Human Behavior in press. https://doi.org/10.1016/j.evolhumbehav.2020.07.013

Valge, M, R. Meitern, and P. Horak. (2019). Morphometric traits predict educational attainment independently of socioeconomic background. BMC Public Health 19: 1696. https://link.springer.com/article/10.1186/s12889-019-8072-7

Did women jumpstart recent cognitive evolution?

Scatter plots of frequencies of CASC5 variants by sex (Shi et al. 2017). During the last ice age, natural selection favored an increase in the gray matter of ancestral East Asians ... primarily in women.

Back in 2005 there was much interest in genes that regulate brain size, particularly in the ways they varied geographically within our species. It was found that two of these genes, Microcephalin and ASPM, continued to evolve as modern humans spread out of Africa. The latest variant of Microcephalin arose some 37,000 years ago in Eurasia and is still largely confined to the indigenous peoples of Eurasia and the Americas (Mekel-Bobrov et al. 2005). The latest variant of ASPM appeared even later, some 5,800 years in the Middle East (Evans et al. 2005).

Interest fell off when no association could be shown between the new variants and IQ or brain size (Mekel-Bobrov et al. 2007; Rushton et al. 2007; see also Frost 2020). Since then, we have learned that the new ASPM variant is associated with a larger cerebral cortex, and not a larger brain as a whole. Overall brain volume seems to be constrained in modern humans, perhaps by the breadth of a woman's pelvis during childbirth or simply by the high metabolic costs of brain tissue (Ali and Meier 2008; Frost 2020). As for the lack of an association with IQ, we now know that IQ correlates poorly or not at all with some cognitive abilities, like executive function and face recognition.

But what do the new variants actually do? Perhaps a specialized mental task.  It has been suggested that the new ASPM variant assists the brain in processing non-tonal language or alphabetical script (Dediu and Ladd 2007; Frost 2007).

CASC5, another gene for brain growth

Interest has since grown in another gene that regulates brain growth, CASC5. Like Microcephalin and ASPM, it has undergone recent evolution in the modern human lineage:

[...] the CASC5 gene contains mutations in modern humans, but not in Denisovans (Meyer et al. 2012) and this gene also shows distinct sequence divergence between modern humans and Neanderthals (Prufer et al. 2014). These data suggest that CASC5 is an important gene for human neurogenesis, and may harbor modern human specific mutations contributing to the recent evolutionary change of the human brain. (Shi et al. 2017)

Shi et al. (2017) found evidence of recent evolutionary change. Specifically, two nucleotides of CASC5 have been replaced with a new variant in all modern humans. Six other nucleotides have become polymorphic, with some people having the new variants and others not. These polymorphisms show regional differences:

- In four of the polymorphisms, the new variant has a much higher frequency in East Asians than in Europeans or Africans.

- In one polymorphism, it has a much higher frequency in Europeans than in the other two regional groups.

- The remaining polymorphism shows no differences in frequency between the three regional groups.

By and large, the new variants have been under strong positive selection, particularly among East Asians. When the authors examined the six polymorphisms, they found signals of selection for five of them in East Asians and for one in Europeans.

The new variants and brain characteristics

The authors then looked for correlations between the new variants and certain characteristics of the brain, specifically total brain volume, gray matter volume, and white matter volume. To this end, 267 healthy participants were recruited for brain imaging (Han Chinese, 178 females and 89 males, mean age 35.4 ± 12.5 years). All of them were free from mental disorders, drug abuse, alcohol dependence, and brain injury.

Gray matter was significantly larger in participants with the new variant than in those with the ancestral variant at five of the nucleotide sites, including the four polymorphic ones—the same ones that showed differences in variant frequency between East Asians and Europeans. When the authors examined the one polymorphism whose variants were equally common in East Asians, Europeans, and Africans, they found no brain differences between participants with the new variant and those with the ancestral one.

When the authors broke their data down by sex, they found that the new variants were significantly associated with a higher volume of gray matter only in women, not in men, although men seemed to trend in the same direction. The authors suggest that this effect would be significant in men if the number of male participants were larger. Probably. But it seems to me there would still be a sex difference, the number of participants being already large enough.

Ice age origin of the new variants

The authors say the new variants became prevalent "after modern humans migrated out of Africa less than 100,000 years ago." We can narrow down the time range further. The new variants are also present at high frequencies among the indigenous peoples of North and South America; therefore, they must have become prevalent before ancestral Amerindians crossed into North America some 12,000 years ago, apparently in a population that was ancestral both to Amerindians and to East Asians. That would be long before the time of recorded history and even before the Holocene, at a time when northern Eurasia was experiencing glacial conditions.

Did those conditions select for cognitive ability? Cold, seasonal environments did impose new cognitive demands on early modern humans, first by increasing their need to plan ahead over a yearly cycle and second by providing them with new tasks: garment making, needlework, weaving, leatherworking, and kiln operation. Women performed those tasks because the environment offered them few opportunities for food gathering—the usual female activity before the advent of farming. They thus moved into artisanal tasks that not only required greater cognitive ability but also offered much potential for further development. This was the "original industrial revolution" and it was led by women (Frost 2019a).

We can better understand this sexual division of labor by studying northern hunter-gatherers of recent times. According to a cross-cultural study, if women are less involved in food gathering, they specialize in activities unrelated to food procurement, i.e., house building, leatherworking, and burden carrying (Waguespack 2005). A study of two Inuit groups found the highest degree of technological complexity in garment making and shelter building, both of which are wholly or largely women's work (Oswalt 1976). Cold environments thus change the sexual division of labor among hunter-gatherers in a crucial way: while men continue to be food providers, women develop new technologies.

These findings may explain the recent evolution of CASC5: women were the focus of selection for cognitive ability during Ice Age times. But why was the selection stronger among ancestral East Asians than among ancestral Europeans? It looks like the climate at that time was more severe in northern Asia than in northern Europe. Europe benefited from the moderating influence of the Atlantic, which made for a milder and moister climate. Conditions were much colder and drier in northern Asia.

The evolution of human intelligence cannot be reduced to a single unified theory. Cold environments emancipated women from the mental straitjacket of food gathering, thus putting humans on the path to social complexity. That path, however, would take them to latitudes farther south in temperate and even tropical environments where they would be exposed to new cognitive demands. With the end of hunting, men moved not only into farming but also into the artisanal activities that women had developed. The same period saw a decline in brain volume that was greater in women than in men—an indication that cognitive demands were particularly high before the Holocene, and even more so for women (Frost 2019b).

The Holocene thus saw northern populations expand southward and eventually cover almost all of Eurasia, North Africa, Oceania, and the Americas. Those populations had a cognitive advantage that made them better able to exploit the social complexity emerging farther south. This point was made by Darwin's colleague Alfred Russel Wallace:

So when a glacial epoch comes on, some animals must acquire warmer fur, or a covering of fat, or else die of cold. Those best clothed by nature are, therefore, preserved by natural selection. Man, under the same circumstances, will make himself warmer clothing, and build better houses; and the necessity of doing this will react upon his mental organisation and social condition [...] a hardier, a more provident, and a more social race would be developed, than in those regions where the earth produces a perennial supply of vegetable food, and where neither foresight nor ingenuity are required to prepare for the rigours of winter. And is it not the fact that in all ages, and in every quarter of the globe, the inhabitants of temperate have been superior to those of tropical countries? All the great invasions and displacements of races have been from North to South, rather than the reverse.

References

Ali, F. and R. Meier. (2008). Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size. Molecular Biology and Evolution 25(11): 2247-2250.

https://www.researchgate.net/profile/Rudolf_Meier/publication/23185977_Positive_Selection_in_ASPM_Is_Correlated_with_Cerebral_Cortex_Evolution_across_Primates_but_Not_with_Whole-Brain_Size/links/00b7d53ad237958edd000000.pdf

Dediu, D., and R. Ladd. (2007). Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin. Proceedings of the National Academy of Sciences 104(26): 10944-10949

https://langev.com/pdf/dediu07linguisticTonePNAS.pdf

Evans, P. D., Gilbert, S. L., Mekel-Bobrov, N., Vallender, E. J., Anderson, J. R., Vaez-Azizi, L. M., et al. (2005). Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans. Science 309: 1717-1720.

http://web.stanford.edu/class/cs273a/papers.spr07/lecture15/Microencephalin.pdf

Frost, P. (2007). The spread of alphabetical writing may have favored the latest variant of the ASPM gene. Medical Hypotheses 70: 17-20.

http://www.sciencedirect.com/science/article/pii/S0306987707003234

Frost, P. (2019a). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181

https://doi.org/10.3390/psych1010012

Frost, P. (2019b). Why did brain size decrease after the ice age? Evo and Proud, July 6

https://evoandproud.blogspot.com/2019/07/why-did-brain-size-decrease-after-ice.html

Frost, P. (2020). A second look at ASPM. Evo and Proud, April 14

https://evoandproud.blogspot.com/2020/04/a-second-look-at-aspm.html

Mekel-Bobrov, N., S.L. Gilbert, P.D. Evans, E.J. Vallender, J.R. Anderson, R.R. Hudson, S.A. Tishkoff and B.T. Lahn. (2005). Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens. Science 309: 1720-1722

https://www.researchgate.net/publication/7611130_Ongoing_Adaptive_Evolution_of_ASPM_a_Brain_Size_Determinant_in_Homo_Sapiens

Mekel-Bobrov, N., D. Posthuma, S.L. Gilbert, P. Lind, M.F. Gosso, et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Human Molecular Genetics 16(6): 600-608.

https://academic.oup.com/hmg/article/16/6/600/610971

Oswalt, W.H. (1976). An Anthropological Analysis of Food-Getting Technology, 1st ed.; John Wiley and Sons: New York, NY, USA.

Rushton, J.P., P.A. Vernon, and T.A. Bons. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK 3(2): 157-60.

https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2006.0586

Shi, L., Hu, E., Wang, Z. et al. (2017). Regional selection of the brain size regulating gene CASC5 provides new insight into human brain evolution. Human Genetics 136: 193-204. https://doi.org/10.1007/s00439-016-1748-5

http://ir.ioz.ac.cn/bitstream/000000/12812/1/Regional%20selection%20of%20the%20brain%20size%20regulati1.pdf

Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist 107: 666-676.

https://pdfs.semanticscholar.org/8e50/02b23a25d1a97054f8bf9b8b852d2151a093.pdf

Wallace, A.R. (1864). The Origin of Human Races and the Antiquity of Man deduced from the Theory of "Natural Selection." Journal of the Anthropological Society of London, 2, clviii-clxxxvii, Alfred Russel Wallace Classic Writings. Paper 6. http://digitalcommons.wku.edu/dlps_fac_arw/6