Domestication of vertebrates
The domestication of vertebrates is the mutual relationship between vertebrate animals, including birds and mammals, and the humans who influence their care and reproduction.
Charles Darwin recognized a small number of traits that made domesticated species different from their wild ancestors. He was also the first to recognize the difference between conscious selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of natural selection or from selection of other traits. There is a genetic difference between domestic and wild populations. There is also a genetic difference between the domestication traits that researchers believe to have been essential at the early stages of domestication, and the improvement traits that have appeared since the split between wild and domestic populations. Domestication traits are generally fixed within all domesticates, and were selected during the initial episode of domestication of that animal or plant, whereas improvement traits are present only in a portion of domesticates, though they may be fixed in individual breeds or regional populations.
Domestication should not be confused with taming. Taming is the conditioned behavioral modification of a wild-born animal when its natural avoidance of humans is reduced and it accepts the presence of humans, but domestication is the permanent genetic modification of a bred lineage that leads to an inherited predisposition toward humans. Certain animal species, and certain individuals within those species, make better candidates for domestication than others because they exhibit certain behavioral characteristics: the size and organization of their social structure; the availability and the degree of selectivity in their choice of mates; the ease and speed with which the parents bond with their young, and the maturity and mobility of the young at birth; the degree of flexibility in diet and habitat tolerance; and responses to humans and new environments, including flight responses and reactivity to external stimuli.
It is proposed that there were three major pathways that most animal domesticates followed into domestication: commensals, adapted to a human niche ; animals sought for food and other byproducts ; and targeted animals for draft and nonfood resources. The dog was the first to be domesticated, and domestic dogs were established across Eurasia before the end of the Late Pleistocene era, well before the first cultivation and before the domestication of any other animals. Unlike other domestic species, which were primarily selected for production-related traits, dogs were initially selected for their behaviors. Archaeological and genetic data suggest that long-term bidirectional gene flow between wild and domestic stocks was common is some species, including donkeys, horses, New and Old World camelids, goats, sheep, and pigs. One study has concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars into pigs and created domestication islands in the genome. The same process may also apply to other domesticated animals. Some of the most commonly domesticated animals are cats and dogs.
Definitions
Domestication
Domestication has been defined as "a sustained multi-generational, mutualistic relationship in which one organism assumes a significant degree of influence over the reproduction and care of another organism in order to secure a more predictable supply of a resource of interest, and through which the partner organism gains advantage over individuals that remain outside this relationship, thereby benefitting and often increasing the fitness of both the domesticator and the target domesticate." This definition recognizes both the biological and the cultural components of the domestication process and the effects on both humans and the domesticated animals and plants. All past definitions of domestication have included a relationship between humans with plants and animals, but their differences lay in who was considered as the lead partner in the relationship. This new definition recognizes a mutualistic relationship in which both partners gain benefits. Domestication has vastly enhanced the reproductive output of crop plants, livestock, and pets far beyond that of their wild progenitors. Domesticates have provided humans with resources that they could more predictably and securely control, move, and redistribute, which has been the advantage that had fueled a population explosion of the agro-pastoralists and their spread to all corners of the planet.Domestication syndrome
is a term often used to describe the suite of phenotypic traits arising during domestication that distinguish crops from their wild ancestors. The term is also applied to animals and includes increased docility and tameness, coat color changes, reductions in tooth size, changes in craniofacial morphology, alterations in ear and tail form, more frequent and nonseasonal estrus cycles, alterations in adrenocorticotropic hormone levels, changed concentrations of several neurotransmitters, prolongations in juvenile behavior, and reductions in both total brain size and of particular brain regions. The set of traits used to define the animal domestication syndrome is inconsistent.Difference from taming
Domestication should not be confused with taming. Taming is the conditioned behavioral modification of a wild-born animal when its natural avoidance of humans is reduced and it accepts the presence of humans, but domestication is the permanent genetic modification of a bred lineage that leads to an inherited predisposition toward humans. Human selection included tameness, but without a suitable evolutionary response then domestication was not achieved. Domestic animals need not be tame in the behavioral sense, such as the Spanish fighting bull. Wild animals can be tame, such as a hand-raised cheetah. A domestic animal's breeding is controlled by humans and its tameness and tolerance of humans is genetically determined. However, an animal merely bred in captivity is not necessarily domesticated. Tigers, gorillas, and polar bears breed readily in captivity but are not domesticated. Asian elephants are wild animals that with taming manifest outward signs of domestication, yet their breeding is not human controlled and thus they are not true domesticates.History, cause and timing
The domestication of animals and plants was triggered by the climatic and environmental changes that occurred after the peak of the Last Glacial Maximum around 21,000 years ago and which continue to this present day. These changes made obtaining food difficult. The first domesticate was the domestic dog from a wolf ancestor at least 15,000 years ago. The Younger Dryas that occurred 12,900 years ago was a period of intense cold and aridity that put pressure on humans to intensify their foraging strategies. By the beginning of the Holocene from 11,700 years ago, favorable climatic conditions and increasing human populations led to small-scale animal and plant domestication, which allowed humans to augment the food that they were obtaining through hunter-gathering.The increased use of agriculture and continued domestication of species during the Neolithic transition marked the beginning of a rapid shift in the evolution, ecology, and demography of both humans and numerous species of animals and plants. Areas with increasing agriculture underwent urbanization, developing higher-density populations and expanded economies, and became centers of livestock and crop domestication. Such agricultural societies emerged across Eurasia, North Africa, and South and Central America.
In the Fertile Crescent 10,000-11,000 years ago, zooarchaeology indicates that goats, pigs, sheep, and taurine cattle were the first livestock to be domesticated. Archaeologists working in Cyprus found an approximately 9500 year old burial ground containing an adult human with a feline skeleton. Two thousand years later, humped zebu cattle were domesticated in what is today Baluchistan in Pakistan. In East Asia 8,000 years ago, pigs were domesticated from wild boar that were genetically different from those found in the Fertile Crescent. The horse was domesticated on the Central Asian steppe 5,500 years ago. The chicken was domesticated in Southeast Asia 4,000 years ago.
Universal features
The biomass of wild vertebrates is declining relative to the biomass of domestic animals, with the calculated biomass of domestic cattle alone now being greater than that of all wild mammals combined. Because the evolution of domestic animals is ongoing, the process of domestication has a beginning but not an end. Various criteria have been established to provide a definition of domestic animals, but all decisions about exactly when an animal can be labelled "domesticated" in the zoological sense are arbitrary, although potentially useful. Domestication is a fluid and nonlinear process that may start, stop, reverse, or go down unexpected paths with no clear or universal threshold that separates the wild from the domestic. However, there are universal features held in common by all domesticated animals.Behavioral preadaption
Certain animal species, and certain individuals within those species, make better candidates for domestication than others because they exhibit certain behavioral characteristics: the size and organization of their social structure; the availability and the degree of selectivity in their choice of mates; the ease and speed with which the parents bond with their young, and the maturity and mobility of the young at birth; the degree of flexibility in diet and habitat tolerance; and responses to humans and new environments, including flight responses and reactivity to external stimuli. Reduced wariness to humans and low reactivity to both humans and other external stimuli are a key pre-adaptation for domestication, and these behaviors are also the primary target of the selective pressures experienced by the animal undergoing domestication. This implies that not all animals can be domesticated, e.g. a wild member of the horse family, the zebra.Jared Diamond in his book Guns, Germs, and Steel enquired as to why, among the world's 148 large wild terrestrial herbivorous mammals, only 14 were domesticated, and proposed that their wild ancestors must have possessed six characteristics before they could be considered for domestication:
- Efficient diet – Animals that can efficiently process what they eat and live off plants are less expensive to keep in captivity. Carnivores feed on flesh, which would require the domesticators to raise additional animals to feed the carnivores and therefore increase the consumption of plants further.
- Quick growth rate – Fast maturity rate compared to the human life span allows breeding intervention and makes the animal useful within an acceptable duration of caretaking. Some large animals require many years before they reach a useful size.
- Ability to breed in captivity – Animals that will not breed in captivity are limited to acquisition through capture in the wild.
- Pleasant disposition – Animals with nasty dispositions are dangerous to keep around humans.
- Tendency not to panic – Some species are nervous, fast, and prone to flight when they perceive a threat.
- Social structure – All species of domesticated large mammals had wild ancestors that lived in herds with a dominance hierarchy amongst the herd members, and the herds had overlapping home territories rather than mutually exclusive home territories. This arrangement allows humans to take control of the dominance hierarchy.