Human hair, a protein filament growing from the skin, displays a wide spectrum of textures, from straight to tightly coiled. Curliness is a common biological trait determined by several interacting factors. Understanding why hair grows in a spiral shape requires examining the microscopic mechanics of the hair fiber and the genetic instructions that govern its formation. The existence of curly hair is a function of molecular architecture and a product of evolutionary pressures.
The Physical Structure of Curl
The shape of a hair strand is determined by the hair follicle, the tunnel-like structure in the skin. Straight hair emerges from a nearly circular follicle that tunnels vertically into the scalp. For curly hair, the follicle is asymmetrical, possessing an oval or elliptical shape and emerging at an angle to the scalp surface. The flatter the oval cross-section, the tighter the resulting curl will be.
The curved path causes an uneven distribution of cells and proteins, particularly keratin, as the hair fiber forms. Keratin contains cysteine, which forms strong chemical cross-links called disulfide bonds. In curly hair, the asymmetrical follicle forces an uneven alignment of keratin, leading to an asymmetrical distribution of these bonds on the inner curve of the shaft. This uneven internal tension causes the hair to bend and twist into its characteristic spiral shape.
The Genetic Influence
The specific shape and angle of the hair follicle, which dictates the curl, is primarily determined by an individual’s genetic makeup. Hair texture is a complex trait, meaning multiple genes interact to produce the final outcome.
A significant genetic marker is a single-nucleotide polymorphism (SNP) within the TCHH gene, which codes for the protein trichohyalin. Trichohyalin is a structural protein in the inner root sheath that helps stabilize and harden the growing hair shaft. Variations in the TCHH gene can influence the precise shape of the follicle and the way keratin filaments are cross-linked, affecting the degree of curliness.
Other genes, such as KRT74 and PADI3, also affect the structural proteins and enzymes that form the hair fiber. Inheriting certain combinations of these gene variants makes a person more likely to develop the asymmetrical follicles that produce curly hair.
Evolutionary Adaptation
The prominence of curly hair in early human populations, particularly those originating in equatorial Africa, suggests an adaptive purpose related to climate. The most widely supported hypothesis centers on thermoregulation and the protection of the brain from heat stress. As early hominins adopted bipedalism, their heads were increasingly exposed to intense, overhead solar radiation near the equator.
Tightly curled hair provides a unique solution to this heat challenge, acting as a “peculiar parasol” for the scalp. The dense, helical structure creates a layer of air between the hair and the scalp, which provides shade and maximizes protection from direct solar heat gain.
This structure is superior to straight hair because it achieves high solar reflectance without lying flat against the scalp, thereby minimizing the insulation effect that would trap heat. By effectively reducing the heat absorbed by the head, tightly curled hair also minimizes the body’s need to cool the brain through evaporative sweating.
Conserving water was a major advantage in the hot, arid environments where humans evolved. Studies using thermal manikins have shown that tightly curled hair provides the most efficient shield against solar radiation, reducing the amount of sweat required to maintain a stable brain temperature. This passive cooling mechanism may have been a factor that allowed the human brain, which is highly sensitive to heat and metabolically demanding, to grow to its modern size without being constrained by the risk of heatstroke.