Jablonski asserts that it was evolutionarily advantageous for pre-humans to retain the hair on their heads in order to protect 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 given that 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 (and which also is very vulnerable at birth), was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). During the gradual process by which Homo erectus made a transition from furry to naked skin, their hair texture putatively changed gradually from straight[citation needed] (the condition of most mammals, including humanity's closest cousins—chimpanzees), to Afro-textured hair or 'kinky' (i.e. tightly coiled). This argument is based on the principle that curly hair impedes the passage of UV light into the body relative to straight hair (thus curly and/or coiled hair would be particularly advantageous for pale-skinned hominids living at the equator). It is substantiated by Iyengar's (1998) findings that UV light can enter into straight human hair roots (and thus into the body/skin) via the hair shaft. Specifically, the results of this study suggest that this phenomenon occurs in a manner similar to the way that light passes through fiber optic tubes (which do not function as effectively when kinked/sharply-curved/coiled). In this sense, during the period in which hominids (i.e. Homo Erectus) 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. Hence, tightly coiled or 'kinky' Afro-hair may have (initially) evolved to prevent the entry of UV light into the body during the transition toward dark, UV-protected skin.