What skin color was primitive for the hominid lineage?
Before questions about changes in integumentary pigmentation in modern human evolution can be addressed, consideration must be given to the probable primitive condition of the integument in the earliest members of the human lineage. It is likely that the integument of the earliest protohominids was similar to that of our closest living relative, the chimpanzee, being white or lightly pigmented and covered with dark hair (Post et al., 1975b). In the chimpanzee, exposed areas of skin vary considerably in their coloration depending on the species and subspecies under consideration, but in all groups facial pigmentation increases with age and exposure to UV radiation (Post et al., 1975b). Except for the face, eyelids, lips, pinnae, friction surfaces, and anogenital areas, the epidermis of most nonhuman primates is unpigmented due to an absence of active melanocytes (Montagna & Machida, 1966; Montagna et al., 1966a,b), suggesting that this is the primitive condition for primates in general. The hairless areas listed above are pigmented to greater or lesser extents in all primate species (Montagna & Machida, 1966; Montagna et al., 1966a,b), suggesting that the potential for induction of melanogenesis (Erickson & Montagna, 1975) in exposed skin is also primitive for the group.
Physiological models have demonstrated that the evolution of hairlessness and an essentially modern sweating mechanism were coordinated with the higher activity levels associated with the modern limb proportions and striding bipedalism (Montagna, 1981; Schwartz & Rosenblum, 1981; Wheeler, 1984, 1996; Chaplin et al., 1994). Throughout this transitional period, the critical function of the integument in thermoregulation was maintained through evolution of an increased number of sweat glands, particularly on the face (Cabanac & Caputa, 1979; Falk, 1990), that increased the maximum rate of evaporative cooling available at any one time (Wheeler, 1996; see also Mahoney, 1980). The brain is extremely heat sensitive, and its temperature closely follows arterial temperature (Nelson & Nunneley, 1998). Evolution of a whole-body cooling mechanism capable of finely regulating arterial temperature was, therefore, a prerequisite for brain expansion and increased activity levels. Naked skin itself affords a thermoregulatory advantage because it makes for a reduced total thermal load requiring evaporative dissipation (Wheeler, 1996). As the density of body hair decreased and the density of sweat glands increased, the need for protection of subepidermal tissues against the destructive effects of UV radiation, particularly UVB, also increased. This protection was accomplished by an increase in melanization of the skin.
Jablonski and Chaplin. "The Evolution of Human Skin Coloration". J Hum Evol, 2000.