Because the earliest domestication of plants and animals took place long before the existence of written records, we cannot be sure how and when humans first learned to plant crops and make use of tamed animals. Historians usually link the two processes as part of a Neolithic Revolution, but they were not necessarily connected.
The domestication of plants is much better understood than the domestication of animals. Foraging bands of humans primarily lived on wild seeds, fruits, and tubers. Eventually some humans tried planting seeds and tubers, favoring varieties that they particularly liked, and a variety that may have been rare in the wild became more common. When such a variety suited human needs, usually by having more food value or being easier to grow or process, people stopped collecting the wild types and relied on farming and further developing their new domestic type.
In the case of animals, the basis of selection to suit human needs is less apparent. Experts looking at ancient bones and images interpret changes in hair color, horn shape, and other visible features as indicators of domestication. But these visible changes did not generally serve human purposes. It is usually assumed that animals were domesticated for their meat, but even this is questionable. Dogs, which may have become domestic tens of thousands of years before any other species, were not eaten in most cultures, and cats, which became domestic much later, were eaten even less often. As for the uses most commonly associated with domestic animals, some of the most important, such as milking cows, shearing sheep, and harnessing oxen and horses to pull plows and vehicles, first appeared hundreds and even thousands of years after domestication.
Cattle, sheep, and goats became domestic around ten thousand years ago in the Middle East and North Africa. Coincidentally, wheat and barley were being domesticated at roughly the same time in the same general area. This is the main reason historians generally conclude that plant and animal domestication are closely related. Yet other major meat animals, such as chickens, which originated as jungle fowl in Southeast Asia, and pigs, which probably became domestic separately in several parts of North Africa, Europe, and Asia, have no agreed-upon association with early plant domestication. Nor is plant domestication connected with the horses and camels that became domestic in western Asia and the donkeys that became domestic in the Sahara region around six thousand years ago. Moreover, though the wild forebears of these species were probably eaten, the domestic forms were usually not used for meat.
In the Middle East humans may have originally kept wild sheep, goats, and cattle for food, though wild cattle were large and dangerous and must have been hard to control. It is questionable whether, in the earliest stages, keeping these animals captive for food would have been more productive than hunting. It is even more questionable whether the humans who kept animals for this purpose had any reason to anticipate that life in captivity would cause them to become domestic.
Human motivations for domesticating animals can be better assessed after a consideration of the physical changes involved in going from wild to domestic. Genetically transmitted tameness, defined as the ability to live with and accept handling by humans, lies at the core of the domestication process. In separate experiments with wild rats and foxes in the twentieth century, scientists found that wild individuals with strong fight-or-flight tendencies reproduce poorly in captivity, whereas individuals with the lowest adrenaline levels have the most offspring in captivity. In the wild, the same low level of excitability would have made these individuals vulnerable to predators and kept their reproduction rate down. However, early humans probably preferred the animals that seemed the tamest and destroyed those that were most wild. In the rat and fox experiments, after twenty generations or so, the surviving animals were born with much smaller adrenal glands and greatly reduced fight-or-flight reactions. Since adrenaline production normally increases in the transition to adulthood, many of the low-adrenaline animals also retained juvenile characteristics, such as floppy ears and pushed-in snouts, both indicators of domestication.
Historians disagree about whether animal domestication was a deliberate process or the unanticipated outcome of keeping animals for other purposes. Some assume that domestication was an understood and reproducible process. Others argue that, since a twenty-generation time span for wild cattle and other large quadrupeds would have amounted to several human lifetimes, it is unlikely that the people who ended up with domestic cows had any recollection of how the process started. This would also rule out the possibility that people who had unwittingly domesticated one species would have attempted to repeat the process with other species, since they did not know what they and their ancestors had done to produce genetically transmitted tameness.
Historians who assume that domestication was an understood and reproducible process tend to conclude that
Humans domesticated every species that could be domesticated. This is unlikely. Twentieth-century efforts to domesticate bison, eland, and elk have not fully succeeded, but they have generally not been maintained for as long as twenty generations. Rats and foxes have more rapid reproduction rates, and the experiments with them succeeded.
Animal domestication is probably best studied on a case-by-case basis as an unintended result of other processes. In some instances, sacrifice probably played a key role. Religious traditions of animal sacrifice rarely utilize, and sometimes prohibit, the ritual killing of wild animals. It is reasonable to suppose that the practice of capturing wild animals and holding them for sacrifice eventually led to the appearance of genetically transmitted tameness as an unplanned result.
Horses and camels were domesticated relatively late, and most likely not for meat consumption. The societies within which these animals first appeared as domestic species already had domestic sheep, goats, and cattle for meat, and they used oxen to carry loads and pull plows and carts. Horses, camels, and later reindeer may represent successful experiments with substituting one draft animal for another, with genetically transmitted tameness an unexpected consequence of separating animals trained for riding or pulling carts from their wilder kin.
Once human societies had developed the full range of uses of domestic animals—meat, eggs, milk, fiber, labor, transport—the likelihood of domesticating more species diminished. In the absence of concrete knowledge of how domestication had occurred, it was usually easier for people to move domestic livestock to new locations than to attempt to develop new domestic species. Domestic animals accompanied human groups wherever they ventured, and this practice triggered enormous environmental changes as domestic animals, and their human keepers, competed with wild species for food and living space.
World History
