The appearance of hair, whether straight, wavy, or tightly coiled, is determined by a complex interplay of physical structure, molecular chemistry, and genetic programming. Curly hair is fundamentally a non-linear growth pattern, an intrinsic twist and curvature that develops before the hair emerges from the scalp. The variation in hair texture across the human population results from distinct biological mechanisms that shape the hair shaft at its root.
The Follicle Factor
The initial shape of a hair strand is determined by the hair follicle, the small, tunnel-like structure within the scalp that anchors the hair root. For straight hair, the follicle is typically circular and symmetrical in cross-section, allowing the hair fiber to grow in a uniform, straight line. In contrast, people with curly hair possess asymmetrical follicles, often shaped like an ellipse or a flattened oval. This asymmetrical shape molds the developing hair fiber into a curved structure as it grows.
The degree of flatness or ellipticity in the follicle directly correlates with the tightness of the curl pattern. A slightly oval follicle produces wavy hair, while a more flattened, ribbon-like oval results in tighter curls or coils. The angle at which the follicle is inserted into the scalp also influences curl development. A vertically inserted follicle tends to produce straight hair, whereas a follicle angled or curved beneath the skin surface forces the hair to curl as it emerges.
The Role of Keratin and Disulfide Bonds
The physical curvature established by the follicle is maintained by the internal, molecular structure of the hair shaft, which is primarily composed of the protein keratin. The hair fiber’s core, known as the cortex, contains spindle-shaped cells. Straight hair typically has a symmetrical distribution of two types of cortical cells, known as the orthocortex and the paracortex. In curly hair, however, these two cell types are distributed non-symmetrically in a bilateral pattern.
This asymmetric arrangement creates a differential tension within the hair strand as the cells mature and harden. The paracortex cells are generally found on the concave, or inner, side of the curve, while the orthocortex cells are located on the convex, or outer, side. Because these two regions have different physical properties and contract at slightly different rates, the imbalance forces the entire hair fiber to twist and spiral. This built-in torque is the underlying mechanical reason why the hair strand maintains its curled shape outside of the follicle.
The twisted structure is then locked into place by strong chemical links called disulfide bonds, which form between the amino acid cysteine molecules within the keratin protein chains. These sulfur bridges act as permanent molecular anchors, stabilizing the hair’s coiled configuration. The greater the density and uneven distribution of these disulfide bonds along the hair shaft, the more tightly the curl is set. Chemical treatments like perms and relaxers work by temporarily breaking and then reforming these specific bonds to either straighten or further curl the hair.
The Genetic Blueprint
The instructions for the physical and molecular characteristics of curly hair are encoded in a person’s genes, making hair texture a highly heritable trait. Hair curl is a polygenic trait, meaning multiple genes interact to determine the final curl pattern. These genes essentially provide the blueprint that dictates the shape of the follicle and the asymmetric production of keratin proteins within the hair shaft.
A prominent gene associated with hair texture is TCHH (Trichohyalin), which plays a direct role in forming the inner structure that stabilizes the hair fiber. Variations in this gene have been linked to the variance in hair curliness across different populations. Other genes, such as EDAR and WNT10A, also influence hair follicle shape and development. The cumulative effect of these genetic markers determines whether a person will have straight, wavy, or tightly coiled hair.
Curly hair is often characterized as a genetically dominant trait, meaning that a child is likely to inherit a curly or wavy texture even if only one parent passes on the associated gene variants. However, because the trait is polygenic, the inheritance pattern is more complex than a simple dominant/recessive model, allowing for a wide spectrum of hair types within families.
Variation and Distribution Across Populations
The global prevalence of different hair types suggests that hair texture provided evolutionary advantages in specific geographic locations. Tightly curled or coiled hair is more common in populations with ancestry in equatorial regions, particularly Africa. One leading hypothesis is that this hair texture evolved to protect the scalp from intense solar radiation while simultaneously promoting thermoregulation.
The springy, coiled structure creates a dense barrier that effectively shields the scalp from direct UV exposure, reducing the risk of overheating. At the same time, the curls lift the hair mass away from the scalp, allowing for better air circulation and sweat evaporation to cool the head. This combination of protection and cooling was beneficial for early humans living in hot climates. Conversely, straighter hair is more common in populations that migrated to colder climates with less intense sun exposure.