I'll stick to breast feeding for this example as thought, feeling and emotions are all clearly evident in primates (you did see the recent videos shot of Gorillas making and using various tools?). (the following is taken from multiple sources and edited together to make it coherent)

While Darwin devised a plausible scenario for the evolution of breast feeding, new information later proved it false. Subsequently, biologists have proposed at least ten hypotheses, but none of them were entirely satisfactory, according to a recent review by National Zoo scientist Olav Oftedal. In two papers published in 2002 in the Journal of Mammary Gland Biology and Neoplasia, Oftedal offers a new scenario to explain, in his words, “How could such an intricate process, involving radical innovation in both mother and suckling young, come into being?”

It is no easy matter to trace the evolution of a structure like the mammary gland and a process like lactation because it does not fossilize. Through painstaking examination of fossils, paleontologists have been able to “see” progressively more mammal-like forms evolve by following gradual changes in teeth and bones.

No such luck with mammary glands and lactation. Instead, to develop his scenario, Oftedal had to review mammalian evolutionary history as revealed by changes in bones and teeth, as well as amass evidence on the nature and evolution of skin glands, the difference between the eggs of birds and reptiles and those of the few mammals that lay eggs, ecological changes over evolutionary time, embryonic development of mammary glands, the functions of scales, skin, and hair, and more. In addition, he looked for clues in parental care exhibited by various vertebrates.

While lactation occurs only in mammals today, lactation appears to be an ancient reproductive trait that predates the origin of mammals. The synapsid branch of the amniote tree that separated from other taxa in the Pennsylvanian (>310 million years ago) evolved a glandular rather than scaled integument. The ancestors of birds and some other groups solved this problem with the evolution of a hard, calcified egg shell, eventually the typical chicken egg. The synapsids appeared to have solved it by keeping the eggs in contact with skin kept moist by secretions from skin glands. Glandular skin was present in the ancestors of the amniotes and proved handy to meet this new need. And, it turns out that mammary glands are specialized skin glands, most similar to apocrine glands and both likely evolved from the same ancient gland type. Some apocrine glands make and secrete lipids and other complex organic molecules, and mammary glands also secrete lipids and the other complex organic components of milk.

How, though, did animals get from secreting a glandular substance to keep eggs moist to feeding their young the secretions of mammary glands?

The behavior of modern amphibians provides a clue to how this might have occurred. Some amphibians that nest in dry habitats carefully tend their eggs and may keep them moist through glandular secretions; glandular secretions may also provide protection from fungus and bacteria. Further, there are hints that the hatchlings of some caecilian species (legless amphibians) feed on their mother’s skin or skin gland secretions. As Oftedal says, “If verified, this remarkable discovery might provide a direct analogy for a transition from skin secretions as egg supplements to skin secretions as hatchling food.”

Closer to home than amphibians, however, are the monotremes—one species of platypus and two species of echidnas that are the only egg-laying mammals. And, of course, by definition, female monotremes lactate. The ancestors of the living monotremes split off very early on from those of the rest of the mammals, indicating that lactation had evolved before that split, which occurred about 150 million years ago. The modern monotremes are certainly not identical to the first ones, having evolved their own unique specializations. But they may reveal a link between the waterers and feeders.

Female platypus and echidnas lay eggs, which they incubate for about 12 days. When the young hatch, they feed on milk produced by the mother’s mammary glands. In a significant departure from all other mammals, however, the mammary glands do not open into a nipple from which young suck milk. Instead, the glands open in a hairy patch and the young suck the milk from the hair and skin.

During incubation, monotreme eggs are covered with a moist, sticky substance of unknown origin. Moreover, before they are laid, the eggs have three eggshell layers; afterwards, a fourth layer appears that is quite different from the earlier layers, and it seems to be applied to the egg surface in a fluid form. Thus, it is entirely possible that both the sticky substance and the fourth eggshell layer are secretions of the mammary glands.

This dual mammary gland function of coating the eggs and feeding young may also help explain why the monotremes lack nipples: secretions from a largish, flat patch might more easily coat an egg than secretions from the single point of a smallish nipple.

Repeated radiations of synapsids produced a gradual accrual of mammalian features. The mammary gland apparently derives from an ancestral apocrine-like gland that was associated with hair follicles. This association is retained by monotreme mammary glands and is evident as vestigial mammary hair during early ontogenetic development of marsupials. The dense cluster of mammo-pilo-sebaceous units that open onto a nipple-less mammary patch in monotremes may reflect a structure that evolved to provide moisture and other constituents to permeable eggs. Mammary patch secretions were coopted to provide nutrients to hatchlings, but some constituents including lactose may have been secreted by ancestral apocrine-like glands in early synapsids. Advanced Triassic therapsids, such as cynodonts, almost certainly secreted complex, nutrient-rich milk, allowing a progressive decline in egg size and an increasingly altricial state of the young at hatching. This is indicated by the very small body size, presence of epipubic bones, and limited tooth replacement in advanced cynodonts and early mammaliaforms. Nipples that arose from the mammary patch rendered mammary hairs obsolete, while placental structures have allowed lactation to be truncated in living eutherians.

Now aren't you glad you asked?


Arthur