The domestication of animals for agricultural purposes dates back to the beginning of the Neolithic period, 9,000 years ago. Early agriculturalists in the Fertile Crescent of the Near East began breeding goats first, then sheep, pigs, and cattle. The stimulus for this advance was probably global warming at the end of the Ice Age, which caused drought in the Near East and forced people to congregate around reliable sources of water. The subsequent increase in population density strained the ability of hunting and gathering to meet the demand for food. Herding animals provided a reliable source of protein-rich food during times of scarcity.

A domestic animal is characterized by several attributes. First, it is bred in captivity for economic profit. Second, humans control its breeding, territory organization, and food supply. Animals bred in captivity tend to have different anatomies and behavior from their wild ancestors. Stress and...