Our brains are the most metabolically demanding organ we have, sapping 5-600 kcal per day (25 percent of our daily energy needs). Due to how cost-efficient the brain is, it only would have evolved if it gave us a bigger fitness advantage (which it obviously did). In the news a couple of days ago, it broke that C-sections may be affecting our evolution. But in all of the articles I read about it I didn’t see any one of them talking about how C-sections may affect human evolution in America between race. Clearly, if C-sections are having this effect on the country as a whole, there must be racial differences as well. Could this have an effect on brain size between race in America?
C-sections have increased in frequency since 1996. Clearly, if there is any selection it’s for more narrow-hipped women and bigger-brained babies. The regular use of C-sections has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20%. (Mitteroecker et al, 2016) Fetopelvic disproportion is the inability of a babe’s head to pass through the mother’s birth canal. This is because the head—and along with it the brain—is too big, leading to emergency C-sections. Mitteroecker et al (2016) also say (which slightly amused me):
Mitteroecker et al (2016) also say (which slightly amused me):
Neonatal size and maternal pelvic dimensions influence fitness (i.e., reproductive success) of the newborn and the mother in multiple ways. Undoubtedly, relative brain size had increased during human evolution in response to directional selection. Recently, it has also been suggested that the large human brain may be the result of runaway selection for the childcare of infants that are born prematurely because of their large brain (12). It is unclear whether any of this selection still persists after the slight decrease of brain size in the late Pleistocene. However, birth weight, which correlates with brain size at birth, is strongly positively associated with infant survival rate (13) and has also been reported to correlate negatively with the risk of multiple diseases (14). Reducing neonatal brain size by shortening gestation length seems to be equally disadvantageous: Delivery before term clearly increases the likelihood of impaired cognitive function in later life (15, 16).
Brain size is decreasing. Associate professor of anthropology at the University of Wisconsin John Hawks also states in his blog post, Selection for smaller brains in Holocene human evolution, where he says (contrary to Pumpkin Person’s assertion) that human brain size has gotten smaller in the past 10,000 years:
The available skeletal samples show a reduction in endocranial volume or vault dimensions in Europe, southern Africa, China, and Australia during the Holocene. This reduction cannot be explained as an allometric consequence of reductions of body mass or stature in these populations. The large population numbers in these Holocene populations, particularly in post-agricultural Europe and China, rule out genetic drift as an explanation for smaller endocranial volume. This is likely to be true of African and Australian populations also, although the demographic information is less secure. Therefore, smaller endocranial volume was correlated with higher fitness during the recent evolution of these populations. Several hypotheses may explain the reduction of brain size in Holocene populations, and further work will be necessary to uncover the developmental and functional consequences of smaller brains.
The reduction in brain size began around 28 kya and accelerated around 10 kya after the dawn of agriculture. The planet getting warmer also played a part in the decrease in brain size, which also allowed for the beginning of agriculture. Anyway, I’m sidetracking, I will return to this point in the future.
Large brains were also selected for since we needed to care for helpless babies. Natural selection for large brains led to more premature births which itself selected for even larger brains.
One-hundred years ago, a narrow-hipped mother who was pregnant with a big-headed baby would have died. Narrow-hipped women with big-headed babies can now survive, transmitting genes for both big brains and narrower pelvises. This is natural selection currently at work as we speak.
One thing that I obviously didn’t see in any article I’ve read on this matter is how will this affect racial differences in brain size? Which race has the most C-sections and will that select for bigger heads and smaller pelvises in that population?
Black women are substantially more likely to deliver by C-section than are white women (pg. 4). Though, one reason that C-sections occur is due to obesity. Black women are the most likely to be obese, which is part of the reason why they have more C-sections. If this trend continues, I could see a slight uptick in black brain size, as even smaller hips get selected for in black women, along with an increase in brain size. That’s one reason why Africans have smaller heads and brains than East Asians and Europeans: they have narrower hips which allows for better athleticism. Conversely, Europeans and East Asians have wider hips which allows for bigger-brained children but hampers athletic ability.
While on the topic of race and C-sections, Asian female-European Male couples have higher rates of C-sections. The obvious explanation is that the Asian woman’s pelvis is too narrow to birth bigger babies. In the study, Asian female-white male couples had babies that had a median weight of 8 pounds, while Asian-Asian couples had babies that had a median weight of 7.1 pounds and finally Asian male-white female couples’ babies had a median weight of 7.3 pounds. However, Asian female-white male couples had an increased rate of C-section deliveries, proving that a significant differences exist between sex of the parent (whether the father or mother is Asian or white influences birth weight) which leads to increased C-section rates due to the white father passing clearly influencing the birth weight more, thusly making it difficult for his Asian partner to birth the baby. There are 100 deaths per 100,000 live births per year in the U.S., a rate of .1 percent. Clearly, though the death rate is low, C-sections lead to maternal mortality and since Asian females are more likely to have a C-section when the father is white due to the baby being bigger, the mortality rate is slightly increased when this interracial pairing occurs.
C-sections are causing natural selection, favoring for bigger heads and narrower hips. This helped us, evolutionarily speaking, as human bipedalism is promoted by a narrow pelvis. C-sections could possibly select for bigger-brained African Americans. Though brain size has decreased in the past 10,000 years, our brain size will slightly increase over time due to this selection pressure. Asian women and white male couples have C-sections more often. Pretty good case against race-mixing, if I don’t say so myself.