I’ve been chronicling the VDH recently since it has great explanatory—and predictive—power. Light skin is a clear adaptation to low UVR, while dark skin is a clear adaptation to high UVR. Dark, highly melanized skin confers advantages in high UVR environments, such as protection against DNA damage, and also absorbs sufficient UV for vitamin D production while also protecting against folate depletion. However, when our ancestors migrated out of Africa, dark skin would not cut it in temperate environments with highly variable UV rays. This is where our highly adaptive physiology came into play, ensuring that we survived in highly variable environments. Light skin was important in low UVR environments in order to synthesize ample vitamin D, however, that synthesized vitamin D then conferred numerous other physiological advantages to the cold.
Eighty to ninety percent of the vitamin D required for humans comes from the sun, whereas ten to twenty percent comes from the diet, such as fatty fish, eggs, and dairy products (fortified with vitamin D, of course) (Ajabshir, Asif, and Nayer, 2014). Humans need to rely on high amounts of UV rays for vitamin D synthesis (Carlberg, 2014) other than Arctic peoples. Since dark skin does not synthesize vitamin D as well as light skin, skin gradually lightened as our ancestors migrated out of Africa (Juzeniene et al, 2009). This was then imperative to the physiologic adaptations that then occurred as our physiology had to adapt to novel, colder environments with fewer UV rays.
Sufficient amounts of vitamin D are highly important for the human musculoskeletal system (Wintermeyer et al, 2016), which is extremely important for birthing mothers. Along with the increased vitamin D synthesis in low UV environments, the heightened production of vitamin D conferred numerous other physiologic benefits which then helped humans adapt to colder environments with more varying UVR.
Vasoconstriction occurs when the blood vessels constrict which leads to heightened blood pressure, whereas vasodilation is the dilation of blood vessels which decreases blood pressure. So evolutionarily speaking, we had to have adaptive physiology in order to be able to “switch” back and forth between vasoconstriction and vasodilation, depending on what the current environment needed. Vasodilation, though, most likely had no advantage in high UV environments, and thus must have been an advantage in low UV environments, where it was more likely to be colder and so, when the blood vessels constrict, blood pressure increases and thus, heat loss could be considerably slowed in these environments due to these physiologic adaptations.
The races also differ, along with many other physiologic abilities, in nitric oxide-mediated vasodilation. Vasodilation is the dilation of blood vessels, which increases blood pressure. Mata-Greenwood and Chen (2008) reviewed the relevant literature regarding black/white differences in nitric oxide-dependent vasorelaxation and concluded that nitric oxide vasodilation is reduced in darker-skinned populations. Thus, we can infer that in lighter-skinned populations nitric oxide vasodilation is increased in lighter-skinned populations, which would have conferred a great physiological advantage when it came to colonizing environments with lower UV rays.
VDR and vitamin D metabolizing enzymes are present in adipose tissue. Tetrahydrobiopterin; which acts as a cofactor in the synthesis of nitric oxide and its primary function is as a vasodilator in the blood vessels (meaning that blood pressure is increased, to keep more heat in the cold) (Chalupsky and Cai, 2005). Since vasodilation is the body’s primary response to heat stress, blood flow increases which allows heat to leave the body. Therefore, the human body’s ability regarding vasodilation and vasoconstriction mechanisms were important in surviving areas with varying UVR.
One function of our adipose tissue is the storage of vitamin D, while vitamin D metabolizing enzymes and VDR are also expressed in the adipocyte (Abbas, 2017). With these known actions of vitamin D on adipose tissue, we can speculate that since vitamin D and the VDR are expressed in adipose tissue, it may have exerted a role in the adipose tissue which may have been important for surviving in cold, low UV environments (see below).
Furthermore, since these mechanisms are brought on by short-term changes, we can infer that it would hardly be of any use in high UVR environments and would be critical in temperate environments. So, vasodilation and vasoconstriction have little to no benefit in high UVR environments but seem to be imperative in temperate environments where UVR varies. It’s also likely that vitamin D influences vasodilation by influential nitric oxide synthesis (see Andrukhova et al, 2014) and vasoconstriction by influencing the renin-angiotensin system (Ajabshir, Asig, and Nayer, 2014).
This would have conferred great benefit to our ancestors as they migrated into more temperate and colder climates. You can read this for information on how adaptive our physiology is and why it’s like that. Because we went into numerous new environments and natural selection couldn’t act quickly enough, therefore the human body’s physiology is extremely adaptive.
What this suggests is that as skin lightened and adapted to low UV, the increased synthesis in vitamin D influenced vasodilation by a strong influence on nitric oxide synthase, along with vasoconstriction, implies that it would have been easier to survive in novel environments due to adaptive physiology and skin color, along with body fat reserves and the physiologic effects of vitamin D on adipose tissue. These physiologic adaptations would have been of no to little use in Africa. Thus, they must have been useful after we migrated out of Africa and experienced wildly varying environments—the whole reason why our physiology evolved (Richardson, 2017: chapter 5).
When the human body is exposed to cold, a few things occur: cutaneous vasoconstriction, shivering (Castellani and Young, 2016), “behavioral thermoregulation” (Young, Sawka, and Pandolf, 1996), while the human body can adapt physiologically to the cold (Young, 1994). The physiologic functions that vitamin D and folate in regard to vasodilation and vasoconstriction, there is a great chance that these effects were important in maintaining energy homeostasis in colder climates.
In sum, the evolution of light skin conferred a great survival advantage to our ancestors. This then upped the production of vitamin D synthesis in the body, which where then of utmost importance in regard to the adaptation of the human physiology to colder, lower-UV environments. Without our adaptive physiological systems, we would not have been able to leave Africa into novel environments. We need both behavioral thermoregulation as well as adaptive physiology to be able to survive in novel environments. Thus, the importance of skin lightening in our evolution becomes clearer:
As humans migrated out of Africa, lighter skin was needed to synthesize vitamin D. This was especially important to women, who needed higher amounts of vitamin D, in order to produce enough calcium for lactation and pregnancy—so the babe had enough calcium to grow its skeleton in the womb. With the uptake in vitamin D synthesis, this then allowed more adaptive physiologic changes that occurred due to the cold, and along with vasodilation and vasoconstriction, along with shivering and adapting behaviorally to the new environments, were our ancestors able to survive. Dark skin cannot synthesize vitamin D as well as light skin in low UV environments; this also can be seen with the lowered production of nitric oxide-dependent vasodilation in dark-skinned populations. Thus, vasoconstriction conferred no physiologic benefit in high UV environments, but almost certainly conferred a physiologic benefit in low UV environments.