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
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Famous quotes containing the word evolution:
“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 naturefor instance in a biological survey of evolutionwe 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)
“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)
“The evolution of humans can not only be seen as the grand total of their wars, it is also defined by the evolution of the human mind and the development of the human consciousness.”
—Friedrich Dürrenmatt (19211990)