The Evolution of Lactation in Mammalian Species.

Lactation is a defining characteristic of all mammals, and, indeed, mammals draw their name from mammae, or mammary glands. The evolution of mammary glands has been the subject of debate since Charles Darwin. The purpose of this brief review is not to examine all past theories of mammary evolution but to consider the evolution of the mammary gland in relation to (1) modern paleobiology, giving special attention to the mammaliaforms which had many mammalian features, including delayed tooth development suggestive of milk intake. (2) Comparative aspects of mammary development in monotremes, marsupials, and eutherians, which reveal the close developmental relation of mammary glands to other skin glands and hair follicles. (3) The evolution of caseins, which are now known to derive from secretory calcium-binding phosphoproteins, which have a long history in regulating biomineralization. (4) The evolution of lipid secretion, and especially the evolutionary incorporation of immune system components (such as xanthine oxidoreductase) into the fat globule membrane. (5) The evolution of lactose synthesis, and especially the synthesis of the wide array of oligosaccharides found in some milks, including monotremes, marsupials, caniform carnivores, and humans.