Explaining The Genetic Fitness Advantage Of LP and EDAR and Skin Color

In West Eurasia, the strongest target of genetic selective fitness adaptation is lactase persistence (LP) (see also here) which allows adults to drink milk, which surged around the time that Indo-Europeans migrated en masse across Europe in the late Neolithic era and early Bronze Age (often were more herding based food production than the first wave Neolithic populations of Europe derived from Anatolian farmers), and with sometimes regionally distinct genes that have the same effect, upon the arrival of pastoralism in the Near East and Africa.

In East Eurasia and the Americas, the EDAR gene which is responsible for much of the distinctively "East Asian" phenotype is a similarly powerful selective fitness adaptation in the Upper Paleolithic era (roughly speaking, after humans expanded out of Africa and before agriculture was developed). What specifically does it do?

A derived G-allele point mutation (SNP) with pleiotropic effects in EDAR, 370A or rs3827760, found in most modern East Asians and Native Americans but not common in African or European populations, is thought to be one of the key genes responsible for a number of differences between these populations, including the thicker hair, more numerous sweat glands, smaller breasts, and the Sinodont dentition (so-called shovel incisors) characteristic of East Asians. This mutation is also implicated in ear morphology differences and reduced chin protrusion. 

The mutation arose in humans approximately 30,000 years ago, and now is found in 93% of Han Chinese and in the majority of people in nearby Asian populations. 

It has been hypothesized that natural selection favored this allele during the last ice age in a population of people living in isolation in Beringia, as it may play a role in the synthesis of breast milk under Vitamin D-poor conditions. 

Previous research has shown that both LP and EDAR genes may make it easier for mothers to get Vitamin D and/or to pass Vitamin D to their children during pregnancy and in breast feeding. 

But, what is so great about mothers having high levels of Vitamin D and passing it on to their children?

Well, it turns out that the presence of Vitamin D in a mother during pregnancy enhances the IQ of her children, which has great potential to be fitness enhancing, on top of benefits like preventing miscarriage, enhancing fertility, and having immune system enhancing properties.

The importance of Vitamin D could also explain apparent selective fitness effects of matching skin color appropriately to latitude, i.e. favoring pale skin at high latitudes to enhance Vitamin D levels, while favoring darker skin at lower latitudes where solar exposure is more intense to prevent sun burns and skin cancer where it is possible to get sufficient Vitamin D even with darker skin.

The paper and its abstract are as follows:

Background

Vitamin D is critical to embryonic neuronal differentiation and other developmental processes that may affect future neurocognitive function. However, observational studies have found inconsistent associations between gestational vitamin D and neurocognitive outcomes. 

Objectives

We examined the association of gestational 25-hydroxyvitamin D [25(OH)D] with children's IQ at 4–6 y, and explored whether associations differed by race. 

Methods

This study used data from the CANDLE (Conditions Affecting Neurocognitive Development and Learning in Early Childhood) cohort. Between 2006 and 2011, CANDLE recruited 1503 women in their second trimester of healthy singleton pregnancies. Inclusion criteria for this analysis were gestation of ≥34 wk and availability of 25(OH)D and IQ data. Associations between second-trimester 25(OH)D plasma concentration and Stanford-Binet IQ scores in offspring at 4–6 y were examined using multivariable linear regression; interaction terms were used to explore possible effect modification by race. 

Results

Mean ± SD 25(OH)D concentration among 1019 eligible dyads was 21.6 ± 8.4 ng/mL, measured at a mean ± SD gestational age of 23.0 ± 3.0 wk. Vitamin D deficiency [25(OH)D < 20 ng/mL] was observed in 45.6%. Maternal 25(OH)D differed by race with a mean ± SD of 19.8 ± 7.2 ng/mL in Blacks sand 25.9 ± 9.3 ng/mL in Whites ( P < 0.001). In adjusted models a 10-ng/mL increase in 25(OH)D was associated with a 1.17-point higher Full Scale IQ (95% CI: 0.27, 2.06 points), a 1.17-point higher Verbal IQ (95% CI: 0.19, 2.15 points), and a 1.03-point higher Nonverbal IQ (95% CI: 0.10, 1.95 points). We observed no evidence of effect modification by race. 

Conclusions

Second-trimester maternal 25(OH)D was positively associated with IQ at 4–6 y, suggesting that gestational vitamin D status may be an important predictor of neurocognitive development. These findings may help inform prenatal nutrition recommendations and may be especially relevant for Black and other dark-skinned women at high risk of vitamin D deficiency.

Melissa M Melough, et al., "Maternal Plasma 25-Hydroxyvitamin D during Gestation Is Positively Associated with Neurocognitive Development in Offspring at Age 4–6 Years." The Journal of Nutrition (2020). DOI: 10.1093/jn/nxaa309