Abstract
In many humans, head hair can grow to a much greater length than hair elsewhere on the body. This is a “derived” form that evolved outside Africa and probably in northern Eurasia. The ancestral form, which is frizzier and much shorter, survives in sub-Saharan Africans and in other groups whose ancestors never left the tropics. This original hair form is nonetheless relatively straight and silky during infancy. Head hair thus seems to have lengthened in two stages: 1) retention of the infant hair form at older ages; and 2) further lengthening to mid-back and even waist length. These changes seem to have gone farther in women, whose head hair is thicker and somewhat longer. The most popular evolutionary explanations are: 1) relaxation of selection for short hair; and 2) sexual selection for women with long hair. Neither hypothesis is satisfactory. The first one cannot explain why head hair lengthened so dramatically over so little time. The second hypothesis suffers from the assumption that some populations have remained naturally short-haired because they consider long-haired women undesirable. Almost the opposite is true in traditional African cultures, which have a long history of lengthening and straightening women’s hair. It is argued here that sexual selection produced different outcomes in different populations not because standards of beauty differed but because the intensity of sexual selection differed. In the tropical zone, sexual selection acted more on men than on women and was thus too weak to enhance desirable female characteristics. This situation reversed as ancestral humans spread northward into environments that tended to limit polygyny while increasing male mortality. Because fewer men were available for mating, women faced a more competitive mate market and were selected more severely.
Highlights
In many humans, head hair can grow to a much greater length than hair elsewhere on the body
Darwin noted “the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground” (Darwin, 1936 [1888]: p. 906)
It is best known to the public through the writings of Elaine Morgan, who argued that head hair lengthened during an aquatic phase of human evolution that forced infants to hang on to their mother’s hair while in the water “and if the hair floated around her for a yard or so on the surface [the infant] wouldn’t have to make so accurate a beeline in swimming towards her” (Morgan, 1972: p. 36)
Summary
Head hair can grow to a much greater length than hair elsewhere on the body. As ancestral humans spread out of Africa and northward, the infant hair form must have persisted into adulthood while becoming ever longer and straighter, eventually reaching down to the waist if left uncut This evolutionary trajectory has gone farther in women, their scalp hairs having a greater mean diameter and more volume, even in the shorter-haired New Guineans (Walsh & Chapman, 1966). Other visible infant traits (i.e., smooth, hairless, and fair skin and a more childlike face shape) have pursued this kind of evolutionary trajectory, i.e., the infant phenotype has become the adult phenotype, and more so in women than in men In some cases, this trajectory has gone farther in those human groups that entered northern Eurasia. 7600 years ago (Frost, 2014)
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