Evolution of Hair Follicles, Mammary Glands, and Sweat Glands in Humans and Other Mammals
Skin appendages such as hair follicles, mammary glands and sweat glands develop during embryonic life through a series of interactions between surface epithelial cells and underlying mesenchymal cells. In each case, these events are regulated by similar sets of conserved cell-cell signaling pathways. These observations suggest that diverse appendages may have evolved from a common precursor. Most evidence from evolutionary studies suggests that this precursor was a simple glandular structure whose initial function was to prevent organismal dehydration and permit greater independence from an aquatic environment. Secretions from such glands, containing water, salts and anti-microbial peptides, may have first been used to help hydrate and protect thin-walled eggs, and later developed a nutrient function to support the growth of hatchlings. Consistent with this hypothesis, the gene encoding the anti-microbial enzyme lysozyme became duplicated during evolution, and the duplicated copy evolved into a-lactalbumin, a whey protein unique to the mammary gland. Specialization of simple glands resulted in the formation of eccrine sweat glands, and hair follicles with associated apocrine sweat glands and sebaceous glands. In some mammals, such as the echidna (spiny anteaters), mammary secretions are conveyed to the young via specialized hairs, suggesting that hair may first have evolved for this purpose, and that mammary glands may have originally developed from apocrine glands associated with hair follicles. In line with this, mammary glands and hair follicles are both unique to mammalian species, and, while marsupials and eutherian mammals have specialized nipples developed to deliver milk, in koala bears each nipple retains a tuft of hairs. Most mammals, including humans, have hairless nipples, which may have been a subsequent evolutionary adaptation to facilitate feeding. I will summarize our current state of knowledge of the molecular events that control hair follicle, mammary gland and sweat gland development, and will highlight major questions still remaining. Using the mammary gland as an example, I will discuss how evolutionary pressures may have driven specific changes in molecular pathways to permit organ diversification, and further refinements in glandular number and location that permitted efficient feeding of newborn mammals including humans.