Blonde Girl With Long Hair Posing On The Tree

According to the recent single origin hypothesis, anatomically modern humans arose in East Africa approximately 200,000 years ago (Tishkoff, 1996). Then, ~150,000 years later, modern humans began to expand their range to regions outside of (and within) this continent (Tishkoff, 1996). The skin of those in the group that left the African continent had developed the ability to manufacture vitamin D (which was essential for bone development) upon exposure to UV light. However, the UV light of northern regions was too weak to penetrate the highly pigmented skin of the initial migrants in order to provide enough vitamin D for healthy bone development. Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies; leading to the death of both the mother and the infant during labor. Hence, those with lighter skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.
In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions that the need for vitamin D triggered the transition back from dark to light skin. Specifically, the distribution of this trait suggests that this need may have grown so intense at certain points that Northerners with mutations for straighter hair survived and had children at higher rates. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by Iyengar's (1998) findings that UV light can pass through straight human hair roots in a manner similar to the way that light passes through fiber optic tubes (Iyengar, 1998).
Nonetheless, some argue against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to militate against the adaptive usefulness of straight hair at northern latitudes. They further argue that UV light also is poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable, cannot be fully discounted in this regard. In addition, as mentioned in the previous section, straight hair also may have contributed to enhanced comfort levels in the north. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.
The latter hypothesis seems the more plausible evolution determinant as the surface area of the head is minute compared to the remainder of the body, thus the energy required in producing long hair for the express purpose of "optical" amplification of UV light reflected from the snow seems counterproductive. Scientists point to the fact that straight hair found in many ethnic groups is denser as well and has a greater ability to "show" as it does not coil, hence providing more warmth as the likely deterministic factor for the evolution of straight long hair. Some scientists argue that since the head and appendages are the greatest areas for heat loss from the body, the ability to grow long hair on the crown of the head as well as the face provides a distinct advantage in a cold climate. Since the main sensory organs are anatomically located on the head, long hair provides the necessary warmth and protection in a cold climate that allows the use of these organs by exposing them to the elements to "sense", in for example a hunt, yet still providing necessary warmth and protection to sustain prolonged exposure. It may be argued, therefore, that the ability to grow long, straight, densely packed hair provides a distinct evolutionary advantage in cold climate; however, it would be a distinct disadvantage in a hot climate, when compared to loosely packed, spongy, closely cropped hair.