Types of consumption

There are two distinct types of milk consumption: a natural source of nutrition for all infant mammals; and a food product for humans of all ages derived from other animals.

Nutrition for infant mammals

Food product for humans

In many cultures of the world, especially the Western world, humans continue to consume milk beyond infancy, using the milk of other animals (in particular, cows) as a food product. For millennia, cow milk has been processed into dairy products such as cream, butter, yogurt, kefir, ice cream, and especially the more durable and easily transportable product, cheese. Industrial science has brought us casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additive and industrial products.

Humans are an exception in the natural world for consuming milk past infancy. Most humans lose the ability to fully digest milk after childhood (that is, they become lactose intolerant). The sugar lactose is found only in milk, forsythia flowers, and a few tropical shrubs. The enzyme needed to digest lactose, lactase, reaches its highest levels in the small intestines after birth and then begins a slow decline unless milk is consumed regularly. On the other hand, those groups that do continue to tolerate milk often have exercised great creativity in using the milk of domesticated ungulates, not only of cows, but also sheep, goats, yaks, water buffalo, horses, and camels.

The term milk is also used for whitish non-animal substitutes such as soy milk, rice milk, almond milk, and coconut milk. Even the regurgitated substance secreted by glands in the mucosa of their upper digestive tract which pigeons feed their young is called crop milk though it bears little resemblance to mammalian milk.

History

Milking has its advent in the very evolution of placental mammals. While the exact time of its appearance is not known, the immediate ancestors of modern mammals were much like monotremes, including the platypus. Such animals today produce a milk-like substance from glands on the surface of their skin, but without the nipple, for their offspring to drink after hatching from their eggs. Likewise, marsupials, the closest cousin to placental mammals, produce a milk-like substance from a teat-like organ in their pouches. The earliest immediate ancestor of placental mammals known seems to be eomaia, a small creature superficially resembling rodents, that is thought to have lived 125 million years ago, during the Cretaceous era. It almost certainly produced what would be considered milk, in the same way as modern placental mammals.