While ubiquitous in nature, melanin, which provides the coloring found in hair, skin, eyes, feathers, scales, etc., is an especially important substance as far as the human condition is concerned. After all, melanin's role in determining skin color makes it the primary physiological basis for racial differentiation among humans. Entire civilizations, no doubt, have risen and fallen due to their conceptions (and misconceptions) about this pigment's effects on human behavior, to the point that the very notion of humanness itself has been called into question depending on how little or how much melanin a body possessed.
It is for this reason that melanin's lesser known, functional properties should be considered more closely. In fact, being more pigmented, i.e. darker skinned, or put oppositely, being less de-pigmented, may confer a unique set of health benefits which over the course of human history have been repressed or intentionally misrepresented in order to fuel the sociopolitical construct of race.
In biological science melanin is known to possess a diverse set of roles and functions in a wide range of organisms. These include:
- Protection against biochemical attack: e.g. the smokeshield-like ink of the octopus, and the melanin-based protective colorings of bacteria and fungi which are capable of encapsulating and oxidizing invading organisms in a process known as melanization.
- Mitigating chemical stresses associated with exposure to heavy metals and oxidizing agents.
- Acting as a natural sunscreen: shielding light-sensitive tissue from the potentially damaging effects of ultraviolet light.
Melanin is capable of transforming ultraviolet light energy into heat in a process known as "ultrafast internal conversion"; more than 99.9% of the absorbed UV radiation is transformed from potentially genotoxic (DNA-damaging) ultraviolet light into harmless heat.
If melanin can convert light into heat, could it not also transform UV radiation into other biologically/metabolically useful forms of energy? This may no seem so far fetched when one considers that even gamma radiation, which is highly toxic to most forms of life, is a source of sustenance for certain types of fungi and bacteria.
Single-celled fungi, for instance, have been observed  thriving within the collapsed nuclear reactor at Chernobyl, Ukraine, using gamma radiation as a source of energy. Albino fungi, without melanin, were studied to be incapable of using gamma radiation in this way, proving that gamma rays initiate a yet-unknown process of energy production within melanin. There is also the curious discovery of bacteria living within vats of radioactive waste.
Given these examples, it is no surprise that vertebrate animals may be capable of converting light directly into metabolic energy through the help of melanin . In a review on the topic published in 2008 in the Journal of Alternative and Complementary Medicine, titled "Melanin directly converts light for vertebrate metabolic use: heuristic thoughts on birds, Icarus and dark human skin," Geoffrey Goodman and Dani Bercovich offer a thought-provoking reflection on the topic. Their abstract is well worth reading:
Pigments serve many visually obvious animal functions (e.g. hair, skin, eyes, feathers, scales). One is 'melanin', unusual in an absorption across the UV-visual spectrum which is controversial. Any polymer or macro-structure of melanin monomers is 'melanin'. Its roles derive from complex structural and physical-chemical properties e.g. semiconductor, stable radical, conductor, free radical scavenger, charge-transfer. Clinicians and researchers are well acquainted with melanin in skin and ocular pathologies and now increasingly are with internal, melanized, pathology-associated sites not obviously subject to light radiation (e.g. brain, cochlea). At both types of sites some findings puzzle: positive and negative neuromelanin effects in Parkinsons; unexpected melanocyte action in the cochlea, in deafness; melanin reduces DNA damage, but can promote melanoma; in melanotic cells, mitochondrial number was 83% less, respiration down 30%, but development similar to normal amelanotic cells. A little known, avian anatomical conundrum may help resolve melanin paradoxes. One of many unique adaptations to flight, the pecten, strange intra-ocular organ with unresolved function(s), is much enlarged and heavily melanized in birds fighting gravity, hypoxia, thirst and hunger during long-distance, frequently sub-zero, non-stop migration. The pecten may help cope with energy and nutrient needs under extreme conditions, by a marginal but critical, melanin-initiated conversion of light to metabolic energy, coupled to local metabolite recycling. Similarly in Central Africa, reduction in body hair and melanin increase may also have lead to 'photomelanometabolism' which, though small scale/ unit body area, in total may have enabled a sharply increased development of the energy-hungry cortex and enhanced human survival generally. Animal inability to utilize light energy directly has been traditionally assumed. Melanin and the pecten may have unexpected lessons also for human physiology and medicine.
If the authors are correct, a longstanding assumption that animals are incapable of utilizing light energy directly is thrown out the window. In other words, melanized tissue within our body may be capable of "ingesting" sunlight, and not unlike plants, using the "harvested" light in biologically useful ways.
Should it be any surprise, really, that our skin was designed to benefit from being bathed in sunlight? We already know that sunlight exposure  can reduce the risk of over 30 diseases, and that its primary metabolite in our skin, vitamin D , may reduce the risk of over 150 additional conditions. Our biological connection to, and dependence on, the sun, is so profound that the very variation in human skin color from African, melanin-saturated dark skin, to the relatively melanin de-pigmented, Caucasian lighter-skin, is a byproduct of the offspring of our last common ancestor from Africa (as determined by mitochondrial DNA ) migrating towards sunlight-impoverished higher latitudes, which began approximately 60,000 years ago.
In order to compensate for the lower availability of sunlight, the body rapidly adjusted, essentially requiring the removal of the natural "sunscreen" melanin from the skin, which interferes with vitamin D production; vitamin D, of course, is involved in the regulation of over 2,000 genes, and therefore is more like a hormone, without which our entire genetic infrastructure becomes destabilized.
While a life-saving adaptation, the loss of melanin likely has adverse health effects, which include losing the ability to convert sunlight into metabolic energy, increased prevalence of Parkinson's disease (which involves de-melanization of the substantia nigra and disproportionately affects those of Caucasian descent), and others effects which have yet been investigated in any detail.
For now, it is important to point out that within the span of only 60,000 years (a nanosecond in biological time), many of the skin "color" differences among the world's human inhabitants reflect how heavily genetically-conserved was the ability of the human body to produce vitamin D. Furthermore, the trade-off involved in maintaining the ability create enough vitamin D within a sunlight-deprived clime by sacrificing melanin may have had adverse health effects that are only now being investigated.
For those who are not naturally gifted with large quantities of melanin, tanning is an attractive prospect. However, it is important to differentiate between UVA light-induced tanning and UVB light-induced tanning. Although visually there is little, if any discernable difference, UVA light results from the photoxidation of existing melanin and its precursors, whereas UVB stimulates melanocytes to up-regulate melanin synthesis and increases pigmentation coverage.1
Because UVA light does not provide any additional photoprotection and is far more toxic to cellular DNA, it is important to maximize exposure to the UVB wavelengths which predominate around solar noon (approximately 12 o' clock), tapering off in intensity several hours before and after. It is within this window of time that vitamin D production also happens to be at its greatest, as UVB radiation is responsible for stimulating its synthesis as well.
1 The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin. Pigment Cell Melanoma Res. 2011 Feb ;24(1):136-47. Epub 2010 Oct 6. PMID: 20979596