Jablonski asserts head hair was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense African equatorial UV light. While some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet was arguably a more urgent issue. Sometime during the gradual process by which Homo erectus began a transition from furry skin to the naked skin expressed by Homo sapiens, hair texture putatively gradually changed from straight hair the condition of most mammals, including humanity's closest cousins—chimpanzees to Afro-textured hair or 'kinky'. This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair. It is substantiated by Iyengar's findings that UV light can enter into straight human hair roots via the hair shaft. Specifically, the results of that study suggest that this phenomenon resembles the passage of light through fiber optic tubes. In this sense, when hominids were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. By inverse logic, later, as humans traveled farther from Africa and/or the equator, straight hair may have evolved to aid the entry of UV light into the body during the transition from dark, UV-protected skin to paler skin.
Some conversely believe that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool because although African peoples is much less dense than its European counterpart, in the intense sun the effective 'woolly hat' that such hair produced would have been a disadvantage. However, such anthropologists as Nina Jablonski oppositely argue about this hair texture. Specifically, Jablonski's assertions[ suggest that the adjective "woolly" in reference to Afro-hair is a misnomer in connoting the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, Jablonski argues, more likely facilitates an increase in the circulation of cool air onto the scalp. Further, wet Afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. In this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair.