Curly hair is almost certainly the ancestral state of human head hair, and it appears to have evolved primarily as a solution to a thermoregulatory problem: keeping the brain cool under the tropical sun. Research using thermal manikins has shown that tightly curled hair reduces solar heat gain to the scalp more effectively than straight hair, offering early humans a passive cooling advantage as they moved across open landscapes in equatorial Africa. But the story does not end with heat. The genetics of hair shape turn out to be surprisingly complex, with different populations arriving at straight hair through entirely unrelated genetic pathways, and the structural mechanics of a single curly strand involve an asymmetry built into each hair fiber at the cellular level.
Keeping Cool on the Savanna
The most robust evolutionary explanation for curly hair centers on solar radiation. When early human ancestors lost most of their body hair, they kept a dense covering on the scalp, the body part most directly exposed to overhead sunlight. A 2023 study using a heated thermal manikin fitted with wigs of different textures measured the actual heat transfer at the scalp surface under simulated solar conditions. The researchers found that all hair reduced solar heat reaching the scalp compared to bare skin, but tightly curled hair provided the greatest protection against heat gain from solar radiation. Crucially, while hair did reduce how much sweat could evaporate from the scalp, the net effect was still beneficial: curly-haired scalps needed less sweat to maintain thermal balance than bare or straight-haired scalps under the same sun exposure.1PubMed Central. Human scalp hair as a thermoregulatory adaptation
The mechanism is intuitive once you visualize it. Tightly coiled hair creates a porous, insulating layer above the scalp. Air circulates within the coils, but the dense canopy of curl absorbs and scatters incoming radiation before it reaches the skin. Straight hair lies flat against the head, conducting more heat directly to the scalp surface. Think of it like the difference between wearing a loosely woven straw hat versus pressing a dark towel against your head. Both cover you, but one allows ventilation while blocking light, and the other traps heat against you.
This thermal advantage would have been significant for early hominins who were bipedal, walking upright in open grasslands. An upright posture reduces the total body area exposed to overhead sun, but the head remains a target. And because the brain is exquisitely sensitive to overheating, any trait that protected it even modestly could have been strongly favored by natural selection. The thermoregulatory hypothesis does not explain everything about human hair diversity, but it provides a compelling reason why tightly curled hair was the starting point from which other textures later diverged.
What Makes a Strand Curl
A curly hair fiber is not simply a straight fiber that has been bent. The curl is built into the strand’s internal architecture from the moment it forms inside the follicle. The hair follicle itself is asymmetric in curly-haired individuals: rather than being a perfectly round tube, it has an oval or irregularly shaped cross-section, and the cells on one side of the growing fiber develop differently from those on the other side.
Research examining the cortical cell types inside curved hair fibers has shown that two dominant cell types distribute themselves in a bilateral pattern, roughly perpendicular to the direction of curvature. The cell type on the convex (outer curve) side contains discrete bundles of filaments arranged in a helical pattern, while the cell type on the concave (inner curve) side contains larger, fused bundles with filaments running more parallel to the fiber’s length.2PubMed. Cortical cell types and intermediate filament arrangements correlate with fiber curvature in Japanese human hair This asymmetry means the two sides of the hair strand respond differently to moisture and mechanical stress, which is what causes the fiber to bend. If the arrangement shifts along the length of the hair, the curl changes direction, producing the complex spirals and coils you see in natural curly hair.
A complementary analysis of hair mechanics concluded that while an irregular, flatter cross-section plays a role, it is not the determining factor of curl formation by itself. Instead, the degree of cell-type segregation along the hair shaft, combined with shifts in where the plane of that segregation sits, can account for the full range of curl patterns from loose waves to tight coils.3PubMed. Why is hair curly? – Deductions from the structure and the biomechanics of the mature hair shaft In other words, curl is a three-dimensional problem produced by the internal organization of the fiber, not just its shape when sliced in half.
The Genetics Are More Complicated Than You Would Expect
Hair shape is one of the most visible differences among human populations, and you might assume it is controlled by a handful of genes. The reality is messier. A meta-analysis of genome-wide association studies identified eight distinct genetic locations associated with variation in hair shape, and the frequency patterns of these variants showed substantial differences across major continental groups, including Africans, Asians, Europeans, and Native Americans.4Human Molecular Genetics. Meta-analysis of genome-wide association studies identifies 8 novel loci involved in shape variation of human head hair That is eight loci from one study alone, and each contributes a small piece to the overall phenotype.
Gene expression work has pushed the number even higher. A comparison of the gene activity in follicles producing very curly hair versus straight hair identified 85 genes with statistically significant differences in expression between the two hair types.5PubMed Central. Changing human hair fibre colour and shape from the follicle Many of these had not been previously linked to hair curvature at all. This suggests that the molecular machinery controlling curl is spread across many genes, each nudging the follicle’s shape, the asymmetry of cell development, or the composition of structural proteins in one direction or another.
One of the best-studied individual genes is trichohyalin, known as TCHH, which is expressed in the developing inner root sheath of the hair follicle. Variants in this gene are associated with straight hair in Europeans, and a large study found that these variants explain roughly six percent of the variance in hair shape in European populations.6PubMed Central. Common variants in the trichohyalin gene are associated with straight hair in Europeans Six percent from a single gene sounds small, but it is actually a large effect for a polygenic trait. The straight-hair-associated variants of TCHH are most common in Northern Europeans, which is consistent with the observation that very straight hair is more prevalent in those populations.
Two Roads to Straight Hair
One of the more surprising findings in this field is that European and East Asian populations arrived at straight hair through completely different genetic mechanisms. While Europeans owe some of their hair straightness to TCHH variants, East Asians rely on a different gene entirely: EDAR, specifically a variant called EDARV370A.
A study of more than 1,700 individuals across four East Asian populations found a strong association between the 370A variant and straight hair in Han, Tibetan, and Mongolian groups. The effect was additive, meaning that people with two copies of the variant had straighter hair than those with one, with each copy roughly doubling the odds of having straight hair.7PubMed. The adaptive variant EDARV370A is associated with straight hair in East Asians Experiments in transgenic mice confirmed the mechanism: boosting EDAR signaling converted the animals’ coat to a phenotype resembling typical East Asian hair, with thicker, straighter, rounder individual fibers.8PubMed. Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form
The researchers characterized this as convergent evolution: two populations independently evolving a similar outward appearance through unrelated biological pathways. The TCHH variants that contribute to straight hair in Europeans are not the same gene, not even on the same chromosome, as the EDAR variant in East Asians. This is a striking example of how the same physical trait can have entirely different molecular stories behind it, depending on ancestry. It also implies that the ancestral condition, tightly curled hair, was modified separately in multiple lineages as humans migrated out of Africa into cooler climates where the thermoregulatory advantage of curly hair may have mattered less.
Why straight hair might have been favored outside equatorial regions is less clear than why curly hair was favored within them. Some researchers have suggested that straight hair could retain heat in cold environments by lying closer to the scalp, but the evidence for this is thin. The EDAR variant in East Asians also affects sweat gland density and tooth shape, which suggests it may have been selected for reasons unrelated to hair at all, with straighter hair being a byproduct. Whatever the selective pressure was, the genetic architecture makes clear that curly hair was the starting point, not an innovation.
Hair as a Social Signal
Thermoregulation may explain why humans kept hair on their heads, but it does not fully explain why human scalp hair grows so long compared to every other primate. Most mammals have hair that reaches a genetically set length and stops. Human head hair has an unusually long growth phase that, in some individuals, allows hair to reach waist length or beyond. A 2024 review of the evolutionary evidence argues that long scalp hair was also selected because it conveyed social information about age, health, sexual maturity, and status.9Oxford Academic. Evolution of long scalp hair in humans
Hair quality is genuinely hard to fake. Healthy, well-maintained hair signals nutritional status and general health, because hair growth is one of the first things the body sacrifices during illness or malnutrition. In prehistoric societies, the ability to style hair into elaborate forms would have also demonstrated manual skill and social investment. The ornamental potential of long hair, the review argues, likely made hairstyling an important part of social communication, which in turn reinforced the selective pressure to grow longer hair. This sexual and social selection would have operated on top of the thermoregulatory pressures, layering additional evolutionary forces onto an already complex trait.
This framing helps explain why hair texture carries so much cultural weight. If hair has been a social signal for hundreds of thousands of years, humans are essentially wired to notice it, interpret it, and draw inferences from it. The enormous global industry around hair styling, straightening, curling, and treatment is, in a sense, an industrial-scale version of a behavior that goes back to the earliest modern humans.
Why Curly Hair Is More Prone to Damage
The same structural asymmetry that produces curl also creates mechanical vulnerability. Every twist in a curly strand is a point where stress concentrates, and research into the mechanics of breakage in curly hair has mapped the sequence of failure: first, the outer cuticle layers begin to slide; then the junction between the cuticle and the cortex fails; small cracks form between cells and grow along the cuticle-cortex boundary; and finally these cracks propagate toward the surface and merge, causing the strand to snap.10British Journal of Dermatology. Understanding breakage in curly hair This process is accelerated by grooming. Each time a comb or brush passes through tightly curled hair, it straightens the coils against their natural shape, applying force at the weakest points.
Tightly coiled hair also has a harder time distributing the natural oils produced at the scalp down the length of the shaft. In straight hair, sebum travels down the strand by gravity and capillary action. In coiled hair, every twist interrupts that flow, leaving the ends relatively dry while the scalp may remain oily. This creates a paradox where the scalp can be prone to conditions like seborrheic dermatitis, while the mid-lengths and ends of the same hair are brittle from lack of moisture.11Wiley Online Library. High Curl Pattern Hair and Scalp Care Considerations to Mitigate Seborrheic Dermatitis This has real implications for hair care routines, particularly for people with type 3 and type 4 curl patterns who may benefit from less frequent shampooing, more emollient-based cleansing, and careful detangling while wet rather than dry.
What Happens to Curl Over a Lifetime
Anyone who had pin-straight hair as a child and developed waves or curls after puberty, or vice versa, knows that hair texture is not fixed. Hormonal changes are the primary driver of these shifts. Puberty, pregnancy, menopause, and thyroid changes can all alter follicle shape, and a follicle that becomes more asymmetric over time will begin producing curlier hair. The reverse also happens: some people with childhood curls find their hair gradually straightens with age as hormonal balances shift.
This lifetime variability reinforces the point that hair shape is not a simple on-off trait controlled by one switch. The same person can produce different curl patterns at different life stages, from different follicles on the same head, and even from the same follicle over time. The genetic architecture creates a predisposition, but hormones, nutrition, and even mechanical stress from years of grooming can modify the final result. This is partly why identical twins can have noticeably different hair texture as they age, even though their DNA is the same.
Can Hair Shape Tell You About Your Ancestry
Because the genetics of hair shape differ so dramatically between continental populations, hair texture might seem like a reliable marker of ancestry. In practice, it is a poor one. The trait is polygenic, meaning dozens of genes contribute, and many of those genes are common across populations even if their frequencies differ. The genome-wide association data shows that the vast majority of hair-shape-associated variants occur at some frequency in African, Asian, European, and Native American populations, just in different proportions.4Human Molecular Genetics. Meta-analysis of genome-wide association studies identifies 8 novel loci involved in shape variation of human head hair
People of mixed ancestry may inherit curl-promoting variants from one parent and straightening variants from another, with the result depending on which specific combination they received. Two siblings with the same parents can have visibly different hair textures. And within any given population, there is wide variation: not all East Asians have poker-straight hair, and not all sub-Saharan Africans have tightly coiled hair. The statistical trends are real at the population level, but they are unreliable for any individual. Forensic science once attempted to use hair morphology to infer ancestry, but that practice has been largely abandoned in favor of DNA analysis, which is both more accurate and less prone to observer bias.
Lessons From Other Species
Humans are not the only mammals with genetic variation in hair or fiber shape. Sheep breeding has provided some of the clearest evidence for how specific genes regulate fiber curliness. A study on IRF2BP2, a gene expressed in sheep skin, found that specific genetic variants dramatically altered the ratio of fine wool fibers to coarser hair fibers and changed the diameter of those fibers.12ScienceDirect. IRF2BP2 genes provide new insights into coat type and fiber composition variation in sheep The gene acts as a repressor of wool development, so animals with lower expression produce more abundant, finer wool. In two-locus combinations, specific genotypes yielded fiber compositions that were over 80 percent fine wool.
These findings matter because they illustrate a general principle: fiber shape in mammals is governed by regulatory genes that control follicle behavior, not by single structural mutations. The same architecture appears in human hair genetics, where the relevant genes tend to affect follicle development, inner root sheath composition, or the signaling pathways that determine how cells differentiate inside the follicle. The specifics differ between sheep and humans, but the general logic, in which small genetic changes in regulatory machinery produce large visible effects on fiber shape, is conserved across species. Researchers studying wool genetics and human hair genetics increasingly reference each other’s work, since the underlying biology of keratin fiber production is shared across mammals.