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:
“As a natural process, of the same character as the development of a tree from its seed, or of a fowl from its egg, evolution excludes creation and all other kinds of supernatural intervention.”
—Thomas Henry Huxley (1825–95)
“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 (1921–1990)
“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)