|MadSci Network: Anatomy|
This is an excellent question Steve, and I am sorry to say that no one is sure for certain, but that there are some exciting theories that I will summarize for you. Most of what I have drawn upon here is the work of Dr. Daniel Blackburn at Trinity College in Connecticut. You might want to try contacting him for more details.
The answer to your first question, "what fed young before Mammary glands evolved", is easier to address. If you look at most fish, reptiles and amphibians, you will see that, after they hatch from their eggs, young animals of these species are left to fend for themselves. Some species are born with a bit of yolk left in a residual yolk sac, and they can live off of that for a while, but eventually, they have to find whatever it is that they like to eat on their own, with no assistance from their parents. Unlike fish, reptiles and amphibians, most birds take care of their own young and bring them partially digested food to eat. In fact, pigeons mix a type of ?milk? of their own with the partially digested food, as a supplement for their young.
Unlike birds, mammals don?t have to regurgitate their food for their offspring because we have mammary glands. It seems unlikely that the ancestors of mammals (or mammal-like reptiles known as Cynodont Therapsids) would have regurgitated food for their offspring either. Since baby birds cannot fend for themselves the way that baby reptiles can, their parents need to take care of them until they can develop the ability to fly. This seems a likely motivation for the development of regurgitation as a means of feeding offspring in birds. So, before mammary glands began to evolve, it seems likely that Cynodont Therapsids and other mammalian ancestors had to fend for themselves after birth, the way that reptiles and amphibians do today.
Now, your second question , "what series of steps was involved in the evolution of the mammary gland," is harder to answer. One of the many things that distinguish mammals from reptiles is the presence of various skin glands, evolved for things like thermoregulation and the maintenance of hair and fur. Glands are secretory organs, and several modes of secretion are possible. These range from apocrine (secretory products are synthesized by glandular cells and secreted without disintegration of the cell), eccrine (secretes a salty fluid but cells are not lost as part of the secetion), and holocrine (secretion consists of disintegrated cells of the gland itself), to merocrine (the gland is repeatedly functional and cells are not destroyed while secreting).
Mammary glands have been theorized to have evolved from either holocrine sebaceous glands, eccrine sweat glands, or apocrine scent glands. As you suggested in your question, ancestral mammary glands are usually assumed to have produced a watery secretion like that of human sweat glands. However, Daniel Blackburn suggests that if you consider the anatomy, physiology, development, and topographic distribution of mammary glands, they appear more similar to apocrine and sebaceous glands than to typical eccrine glands, and are in general best described as using a merocrine and aprocrine mode of secretion. The mammary gland then, is a highly derived compound gland; it represents several different types of glands which have evolved together into a new organ.
Now, as to how mammary glands might have come about, one scenario begins with the development of a vascularized patch of skin intended to facilitate the incubation of eggs of warm-blooded pre-mammlian reptiles. Since hair insulates the body, a patch of bare skin in contact with the egg could have been of benefit as far as warming eggs would have gone. If these pre-mammals were warm blooded, it seems safe to suggest that they had some sort of skin glands to help radiate heat and keep hair soft and pliable. At the same time, a higher body temperature would promote the growth of micro-organisms, which would threaten the development of eggs. So, the second step in this scenario supposes the production of anti-bacterialand anti-viral secretions from cutaneous glands of the incubation patch. At some point, these secretions could have supplemented the nutrients contained in the developing embryo?s yolk sack during the period following hatching, perhaps in concert with the evolution of viviparity (live birth). Subsequent to these developments, the cutaneous glands would have become more specialized and grown in size, and eventually permitted the copious production of a nutritious secretion -- milk.
Now, you should be aware that that is a largely speculative account. To get a better idea, you might want to look at the actual literature, for example this paper (Blackburn, D.G., V. Hayssen, and C.J. Murphy (1989). The origins of lactation and the evolution of milk: a review with new hypotheses. Mammal Review 19: 1- 26.), or this paper (Blackburn, D.G. (1991). Evolutionary origins of the mammary gland. Mammal Review 21: 81-96.)
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