People have dark skin because their skin cells produce high levels of a pigment called eumelanin, which absorbs ultraviolet (UV) radiation from the sun. This trait evolved over more than a million years in human populations living near the equator, where UV exposure is intense year-round. It is one of the clearest examples of natural selection shaping the human body to match its environment.
What Makes Skin Dark
All human skin contains specialized cells called melanocytes, and every person has roughly the same number of them: about 1,200 per square millimeter, regardless of ethnicity. The difference between light and dark skin is not the number of these cells but how active they are and what type of pigment they produce.
Melanocytes make two kinds of pigment. Eumelanin is dark brown or black and absorbs UV radiation effectively. Pheomelanin is yellowish-red and is actually photo-unstable, meaning it can generate harmful reactions when exposed to sunlight. In people with very dark skin, eumelanin makes up roughly 70 to 78% of total pigment, with pheomelanin accounting for the rest. In people with blond hair and light skin, that ratio flips: pheomelanin can represent 87% or more of total pigment. The amount of pheomelanin stays roughly constant across skin tones. What changes is the eumelanin.
In dark skin, the pigment-containing packages inside melanocytes are larger, more numerous, and distributed individually throughout the outer skin layer. In lighter skin, these packages tend to be smaller and clumped together, which reduces their UV-blocking effect. Eumelanin-rich skin scatters and absorbs 50 to 75% of incoming UV radiation and neutralizes the free radicals that UV generates. It functions, essentially, as a built-in sunscreen.
Why Evolution Favored Dark Skin Near the Equator
The earliest members of the human lineage, living more than 2.8 million years ago, likely had light skin hidden beneath dark body hair, similar to chimpanzees today. As early humans lost most of their body hair (beginning around 2 million years ago, probably to stay cool while walking and running on the open savanna), their bare skin was suddenly exposed to the full force of equatorial UV radiation.
This created intense evolutionary pressure. Genetic evidence points to a selective sweep roughly 1.2 million years ago that locked in gene variants favoring heavy eumelanin production. The gene MC1R, which is one of the key regulators of eumelanin, shows almost no functional variation in African populations. That uniformity is the signature of purifying selection: mutations that would have lightened the skin were so disadvantageous near the equator that they were eliminated generation after generation.
Selection for dark pigmentation continued through the origin of our species, Homo sapiens, about 300,000 years ago. Additional genes involved in dark pigmentation, including MFSD12 and DDB1, show evidence of ongoing selection across that entire timespan.
The Folate Connection
The leading explanation for why dark skin was so strongly favored centers on a B vitamin called folate. UV radiation penetrating the skin breaks down folate circulating in the blood. Folate is essential for DNA synthesis and repair, and its role in reproduction is especially critical. Low folate levels during pregnancy dramatically increase the risk of neural tube defects in developing embryos, and folate deficiency in men reduces sperm count and motility.
Dark skin blocks most UV before it can reach the blood vessels beneath, keeping folate levels intact. In high-UV environments, people with less melanin would have had their folate stores depleted more quickly, leading to lower reproductive success over time. This is known as the vitamin D-folate hypothesis: skin pigmentation evolved as a balancing act between protecting folate (which UV destroys) and producing vitamin D (which UV helps create). Near the equator, where UV is relentless, the balance tipped heavily toward protection, and dark skin became the norm.
It Involves Many Genes, Not One
Skin color is not controlled by a single gene. A landmark study of ethnically diverse African populations identified variants in at least six gene regions significantly associated with pigmentation, including SLC24A5, MFSD12, DDB1, TMEM138, OCA2, and HERC2. Even so, the top eight identified variants together explain only about 29% of the variation in skin color among Africans. The remaining variation is influenced by many other alleles that scientists have not yet fully mapped.
This complexity matters because it shows that skin pigmentation is a trait shaped by many small genetic contributions rather than a simple on-off switch. It also means that the genetic architecture of skin color in African populations differs substantially from that in European or Asian populations. Variants that explain pigmentation differences in Europe account for very little of the variation seen within Africa.
Skin Color Varies Widely Within Africa
The idea that all people of African descent have the same skin tone is incorrect. Africa contains the widest range of skin pigmentation of any continent. Nilo-Saharan-speaking populations in East Africa have some of the darkest skin measured anywhere on Earth, while KhoeSan hunter-gatherers in Botswana have relatively light skin. This range reflects both the deep genetic diversity within Africa (greater than on any other continent) and the fact that different African environments present different levels of UV exposure, from equatorial forests to southern savannas.
Variants associated with dark pigmentation in African populations are also found, identical by descent, in southern Asian and Australo-Melanesian populations. This pattern suggests that some of the genetic machinery for dark skin is ancient, predating the migration of modern humans out of Africa, and was maintained in other high-UV regions of the world like coastal South Asia, Papua New Guinea, and Aboriginal Australia.
What Happens When Dark Skin Meets Low UV
The same melanin that protects folate also reduces the skin’s ability to produce vitamin D from sunlight. Near the equator, this is not a problem because UV is strong enough year-round. But when people with dark skin live at higher latitudes, where UV is weak for much of the year, vitamin D production drops significantly.
Data from the United Kingdom illustrates the scale of this challenge. To maintain adequate vitamin D levels year-round, a person with very dark skin living in the UK needs about 25 minutes of midday sun exposure daily from March through September, with forearms and lower legs uncovered during summer months. Exposing only the hands and face is not enough. In winter at UK latitudes, UV levels are so low that no appreciable vitamin D is produced in skin of any color.
The consequences show up in population-level data. In UK national surveys, roughly 60% of participants with darker skin had vitamin D levels below the deficiency threshold, compared to about 20% of white participants. Because few foods naturally contain meaningful amounts of vitamin D (fatty fish being the main exception), and typical Western diets fall short of the recommended daily intake, supplementation is often the most practical solution for people with dark skin living far from the equator.
This trade-off is itself a product of evolution. As human populations migrated out of Africa into Europe and northern Asia over the past 50,000 to 100,000 years, lighter skin evolved independently in several lineages, allowing more efficient vitamin D production under weaker sunlight. Skin color across the globe maps remarkably well onto UV intensity: darker near the equator, progressively lighter toward the poles. It is one of the most visible examples of how the human body adapts to its environment over deep time.