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Anatomy and physiology



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A.G. Messenger , pp. 66.1–66.16 Hair has no vital function in humans, yet its psychological functions are extremely important, as any clinical dermatologist or cosmetician can readily attest from routine daily practice. If the inevitability of scalp baldness makes it reluctantly tolerable to genetically disposed men, in women, loss of hair from the scalp is distressing as is the growth of body or facial hair in excess of the culturally accepted norm. The evolutionary history of hair is no less enigmatic. Mammals probably evolved from Therapsid reptiles during the Late Triassic period over 200 million years ago (MyA). The earliest direct evidence of hair in mammals comes from fossilized casts and impressions in coprolites and pellets from the Late Paleocene beds of Inner Mongolia [1] . Hairs from at least four extinct mammalian taxa were identified, notably the multituberculate Lambdopsalis bulla , all showing striking preservation of the cuticular scale pattern. The three extant mammalian groups—monotremes, marsupials and placental mammals—all possess hair, indicating its presence prior to the divergence of therian mammals from monotremes, which probably took place between 170 and 230 MyA [2,3] . The multituberculate lineage extends back into the Triassic, suggesting that hair is a very ancient and possibly defining feature of mammals. Whatever its origin, it is clear that ... log in or subscribe to read full text

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