Evolution
It is believed that the mammary gland is a transformed sweat gland, more closely related to Apocrine sweat glands. There are many theories of how they evolved, but since they do not fossilize well, supporting such theories is difficult. Many of the current theories are based on comparisons between lines of living mammals- monotremes, marsupials and eutherians. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist and free from infection (monotremes still lay eggs). Other theories propose that early secretions were used directly by hatched young, or that the secretions were used by young to help them orient to their mothers.
Lactation developed long before the evolution of the mammary gland and mammals, see evolution of lactation.
Read more about this topic: Mammary Gland
Famous quotes containing the word evolution:
“Historians will have to face the fact that natural selection determined the evolution of cultures in the same manner as it did that of species.”
—Konrad Lorenz (1903–1989)
“Like Freud, Jung believes that the human mind contains archaic remnants, residues of the long history and evolution of mankind. In the unconscious, primordial “universally human images” lie dormant. Those primordial images are the most ancient, universal and “deep” thoughts of mankind. Since they embody feelings as much as thought, they are properly “thought feelings.” Where Freud postulates a mass psyche, Jung postulates a collective psyche.”
—Patrick Mullahy (b. 1912)
“By contrast with history, evolution is an unconscious process. Another, and perhaps a better way of putting it would be to say that evolution is a natural process, history a human one.... Insofar as we treat man as a part of nature—for instance in a biological survey of evolution—we are precisely not treating him as a historical being. As a historically developing being, he is set over against nature, both as a knower and as a doer.”
—Owen Barfield (b. 1898)