The evolution of the mammary gland from a sweat gland-like skin organ occurred almost 100 million years ago. This unique development was in a large part responsible for the rapid proliferation of the class of vertebrates that we now know as mammals. The advantage of the mammary gland was that it provided a ready source of nutrition for the still-developing infant, obviating the need for prolonged incubation in a fragile and vulnerable egg encasement. From a functional point of view, there are those who would contend that the human mammary gland has outlived its usefulness; however, the discovery of the multiple valuable and unique properties of human milk for humankind and the devastating results of attempting nonlactational nutrition for infants in many of the developing nations of the world would argue strongly against this point. We have just begun to appreciate the potential long-range sequelae of breast-feeding biologically and behaviorally.
Beesley, R, Johnson, J, Global Library of Women's Medicine, (ISSN: 1756-2228) 2009; DOI 10.3843/GLOWM.10304
Available at: http://works.bepress.com/julia_johnson/14/