Selective breeding of plants and animals to serve humans
Domestication is a multi-generational
mutualistic relationship in which an animal species, such as humans or
leafcutter ants, takes over control and care of another species, such as sheep or fungi, to obtain from them a steady supply of resources, such as meat, milk, or labor. The process is gradual and geographically diffuse, based on trial and error. Domestication affected genes for behavior in animals, making them less aggressive. In plants, domestication affected genes for morphology, such as increasing seed size and stopping the
shattering of cereal seedheads. Such changes both make domesticated organisms easier to handle and reduce their ability to survive in the wild.
The first
animal to be domesticated by humans was the
dog, as a
commensal, at least 15,000 years ago. Other animals, including
goats,
sheep, and
cows, were domesticated around 11,000 years ago. Among birds, the
chicken was first domesticated in East Asia, seemingly for cockfighting, some 7,000 years ago. The horse came under domestication around 5,500 years ago in central Asia as a working animal. Among
invertebrates, the
silkworm and the
western honey bee were domesticated over 5,000 years ago for
silk and
honey, respectively.
The domestication of plants began around 13,000–11,000 years ago with
cereals such as
wheat and
barley in the
Middle East, alongside crops such as
lentil,
pea,
chickpea, and
flax. Beginning around 10,000 years ago, Indigenous peoples in the Americas began to cultivate
peanuts,
squash,
maize,
potatoes,
cotton, and
cassava.
Rice was first domesticated in China some 9,000 years ago. In Africa, crops such as
sorghum were domesticated.
Agriculture developed in some 13 centres around the world, domesticating different crops and animals.
Three groups of insects, namely
ambrosia beetles,
leafcutter ants, and
fungus-growing termites have independently domesticated species of fungi, on which they feed. In the case of the termites, the relationship is a fully obligate symbiosis on both sides.
Definitions
Domestication (not to be confused with the
taming of an individual animal[3][4][5]), is from the
Latindomesticus, 'belonging to the house'.[6] The term remained loosely defined until the 21st century, when the American archaeologist
Melinda A. Zeder defined it as a long-term relationship in which humans take over control and care of another organism to gain a predictable supply of a resource, resulting in
mutual benefits. She noted further that it is not synonymous with agriculture since agriculture depends on domesticated organisms but does not automatically result from domestication.[7]
Michael D. Purugganan notes that domestication has been hard to define, despite the "instinctual consensus" that it means "the plants and animals found under the care of humans that provide us with benefits and which have evolved under our control."[8] He comments that insects such as
termites,
ambrosia beetles, and
leafcutter ants have domesticated some species of
fungi, and notes further that other groups such as weeds and commensals have wrongly been called domesticated.[8] Starting from Zeder's definition, Purugganan proposes a "broad" definition: "a coevolutionary process that arises from a mutualism, in which one species (the domesticator) constructs an environment where it actively manages both the survival and reproduction of another species (the domesticate) in order to provide the former with resources and/or services."[8] He comments that this adds
niche construction to the activities of the domesticator.[8]
Domestication syndrome is the suite of
phenotypic traits that arose during the initial domestication process and which distinguish crops from their
wild ancestors.[9][10] It can also mean a set of differences now observed in domesticated mammals, not necessarily reflecting the initial domestication process. The changes include increased docility and tameness, coat coloration, reductions in tooth size, craniofacial morphology, ear and tail form (e.g., floppy ears), estrus cycles, levels of
adrenocorticotropic hormone and neurotransmitters, prolongations in juvenile behavior, and reductions in brain size and of particular brain regions.[11]
The
domestication of animals and plants was triggered by the climatic and environmental changes that occurred after the peak of the
Last Glacial Maximum and which continue to this present day. These changes made obtaining food by
hunting and gathering difficult.[12] The first animal to be
domesticated was the dog at least 15,000 years ago.[1] The
Younger Dryas 12,900 years ago was a period of intense cold and aridity that put pressure on humans to intensify their foraging strategies but did not favour agriculture. By the beginning of the
Holocene 11,700 years ago, a warmer climate and increasing human populations led to small-scale animal and plant domestication and an increased supply of food.[13]
The appearance of the
domestic dog in the archaeological record, at least 15,000 years ago, was followed by domestication of livestock and
of crops such as
wheat and
barley, the
invention of agriculture, and the transition of humans from foraging to farming in different places and times across the planet.[1][18][19][20] For instance, small-scale trial cultivation of cereals began some 28,000 years ago at the Ohalo II site in Israel.[21]
In the
Fertile Crescent 11,000–10,000 years ago,
zooarchaeology indicates that goats, pigs, sheep, and
taurine cattle were the first livestock to be domesticated. 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 genetically different from those found in the Fertile Crescent.[1] The
cat was domesticated in the Fertile Crescent, perhaps 10,000 years ago,[22] from
European wildcats, possibly to control
rodents that were damaging stored food.[23]
The domestication of vertebrate animals is the relationship between non-human vertebrates and humans who have an influence on their care and reproduction.[7] In his 1868 book The Variation of Animals and Plants Under Domestication,
Charles Darwin recognized the small number of traits that made domestic 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, in which traits evolve as a by-product of
natural selection or from selection on other traits.[26][27][28]
There is a difference between domestic and wild populations; some of these differences constitute the
domestication syndrome, traits presumed essential in the early stages of domestication, while others represent later improvement traits.[9][29][30] Domesticated mammals in particular tend to be smaller and less aggressive than their wild counterparts; other common traits are floppy ears, a smaller brain, and a shorter muzzle.[25] 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 proportion of domesticates, though they may be fixed in individual breeds or
regional populations.[29][30][31]
Certain animal species, and certain individuals within those species, make better candidates for domestication because of their behavioral characteristics:[32][33][34][35]
The size and organization of their social structure[32]
The availability and the degree of selectivity in their choice of mates[32]
The ease and speed with which the parents bond with their young, and the maturity and mobility of the young at birth[32]
The degree of flexibility in diet and habitat tolerance[32]
Responses to humans and new environments, including reduced flight response and reactivity to external stimuli.[32]
The beginnings of mammal domestication involved a protracted
coevolutionary process with multiple stages along different pathways. There are three proposed major pathways that most mammal domesticates followed into domestication:[32][30][36]
Humans did not intend to domesticate mammals from either the commensal or prey pathways, or at least they did not envision a domesticated animal would result from it. In both of those cases, humans became entangled with these species as the relationship between them intensified, and humans' role in their survival and reproduction gradually led to formalized
animal husbandry.[30] Although the directed pathway for draft and riding animals proceeded from capture to taming, the other two pathways are not as goal-oriented, and archaeological records suggest that they took place over much longer time frames.[37]
Unlike other domestic species selected primarily for production-related traits, dogs were initially selected for their behaviors.[38][39] The dog was domesticated long before other animals,[40][41] becoming established across
Eurasia before the end of the
Late Pleistocene era, well before
agriculture.[40]
The archaeological and genetic data suggest that long-term bidirectional
gene flow between wild and domestic stocks – such as in
donkeys,
horses, New and Old World camelids, goats, sheep, and pigs – was common.[30][36] 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 apply to other domesticated animals.
[42][43]
The 2023
parasite-mediated domestication hypothesis suggests that
endoparasites such as
helminths and
protozoa could have mediated the domestication of mammals. Domestication involves taming, which has an endocrine component; and parasites can modify endocrine activity and
microRNAs. Genes for resistance to parasites might be linked to those for the domestication syndrome; it is predicted that domestic animals are less resistant to parasites than their wild relatives.[44][45]
The
chicken was domesticated from the red junglefowl, apparently for
cockfighting, some 7,000 years ago.[16]
Domesticated birds principally mean
poultry, raised for meat and eggs:[46] some
Galliformes (
chicken,
turkey,
guineafowl) and
Anseriformes (waterfowl:
ducks,
geese, and
swans). Also widely domesticated are
cagebirds such as
songbirds and
parrots; these are kept both for pleasure and for use in research.[47]
The
domestic pigeon has been used both for food and as a means of communication between far-flung places through the exploitation of the pigeon's homing instinct; research suggests it was domesticated as early as 10,000 years ago.[48] Chicken fossils in China have been dated to 7,400 years ago. The chicken's wild ancestor is Gallus gallus, the red junglefowl of Southeast Asia. The species appears to have been kept initially for
cockfighting rather than for food.[16]
Two
insects, the
silkworm and the
western honey bee, have been domesticated for over 5,000 years, often for commercial use. The silkworm is raised for the silk threads wound around its
pupal cocoon; the western honey bee, for
honey, and, from the 20th century, for
pollination of crops.[17][49]
Several other invertebrates have been domesticated, both terrestrial and aquatic, including some such as Drosophila melanogaster fruit flies and the freshwater cnidarian Hydra for research into genetics and physiology. Few have a long history of domestication. Most are used for food or other products such as
shellac and
cochineal. The
phyla involved are
Cnidaria,
Platyhelminthes (for
biological pest control),
Annelida,
Mollusca,
Arthropoda (marine
crustaceans as well as insects and spiders), and
Echinodermata. While many marine mollusks are used for food, only a few have been domesticated, including
squid,
cuttlefish and
octopus, all used in research on
behaviour and
neurology. Terrestrial snails in the genera Helix are raised for food. Several parasitic or parasitoidal insects, including the fly Eucelatoria, the beetle Chrysolina, and the wasp Aphytis are raised for biological control. Conscious or unconscious artificial selection has many effects on species under domestication; variability can readily be lost by inbreeding, selection against undesired traits, or genetic drift, while in Drosophila, variability in eclosion time (when adults emerge) has increased.[50]
Humans
foraged for wild cereals, seeds, and nuts thousands of years before they were domesticated; wild wheat and barley, for example, were gathered in the
Levant at least 23,000 years ago.[51][14]Neolithic societies in West Asia first began to cultivate and then domesticate some of these plants around 13,000 to 11,000 years ago.[14] The
founder crops of the West Asian Neolithic included cereals (
emmer,
einkorn wheat,
barley),
pulses (
lentil,
pea,
chickpea,
bitter vetch), and
flax.[15][52] Other plants were independently domesticated in 13
centers of origin (subdivided into 24 areas) of the Americas, Africa, and Asia (the Middle East, South Asia, the Far East, and New Guinea and Wallacea); in some thirteen of these regions people began to cultivate grasses and grains.[53][54] Rice was first cultivated in East Asia.[55][56]Sorghum was widely cultivated in sub-Saharan Africa,[57] while peanuts,[58] squash,[58][59] cotton,[58]maize,[60]potatoes,[61] and
cassava[62] were domesticated in the Americas.[58]
Continued domestication was gradual and geographically diffuse – happening in many small steps and spread over a wide area – on the evidence of both archaeology and genetics.[63] It was a process of intermittent trial and error and often resulted in diverging traits and characteristics.[64]
Whereas domestication of animals impacted most on the genes that controlled behavior, that of plants impacted most on the genes that controlled morphology (seed size, plant architecture, dispersal mechanisms) and physiology (timing of germination or ripening),[32][19] as in the
domestication of wheat. Wild wheat
shatters and falls to the ground to reseed itself when ripe, but domesticated wheat stays on the stem for easier harvesting. This change was possible because of a random mutation in the wild populations at the beginning of wheat's
cultivation. Wheat with this mutation was harvested more frequently and became the seed for the next crop. Therefore, without realizing it, early farmers
selected for this mutation. The result is domesticated wheat, which relies on farmers for its reproduction and dissemination.[14]
Farmers with wheat and cattle –
Ancient Egyptian art 3,400 years ago
Wild
wheat ears shatter when ripe, but domesticated wheat has to be
threshed and
winnowed (as shown) to release and separate the grain. Photograph by
Harold Weston, Iran, 1920s
less efficient
breeding system (e.g. without normal
pollinating organs, making human intervention a requirement), larger seeds with lower success in the wild,[14] or even sterility (e.g.
seedless fruits) and therefore only vegetative reproduction[67][68]
Plant defenses against herbivory, such as
thorns, spines, and prickles, poison, protective coverings, and sturdiness may have been reduced in domesticated plants. This would make them more likely to be eaten by herbivores unless protected by humans, but there is only weak support for most of this.[69] Farmers did select for reduced bitterness and lower toxicity and for food quality, which likely increased crop palatability to herbivores as to humans.[69] However, a survey of 29 plant domestications found that crops were as well-defended against two major insect pests (
beet armyworm and
green peach aphid) both chemically (e.g. with bitter substances) and morphologically (e.g. with toughness) as their wild ancestors.[72]
Changes to plant genome
During domestication, crop species undergo intense artificial selection that alters their genomes, establishing core traits that define them as domesticated, such as increased grain size.[14][74] Comparison of the
coding DNA of
chromosome 8 in rice between fragrant and non-fragrant varieties showed that aromatic and fragrant rice, including
basmati and
jasmine, is derived from an ancestral rice domesticate that suffered a deletion in
exon 7 which altered the coding for betaine aldehyde dehydrogenase (BADH2).[75] Comparison of the potato genome with that of other plants located genes for resistance to potato blight caused by Phytophthora infestans.[76]
In wheat, domestication involved repeated
hybridization and
polyploidy. These steps are large and essentially instantaneous changes to the genome and the
epigenome, enabling a rapid evolutionary response to artificial selection. Polyploidy increases the number of chromosomes, bringing new combinations of genes and alleles, which in turn
enable further changes such as by
chromosomal crossover.[73]
Several species of
fungi have been domesticated for use directly as food, or in fermentation to produce foods and drugs. The cultivated mushroom Agaricus bisporus is widely grown for food.[86] The yeast Saccharomyces cerevisiae have been used for thousands of years to ferment
beer and
wine, and to leaven
bread.[87] Mould fungi including Penicillium are used to mature
cheeses and other dairy products, as well as to make drugs such as
antibiotics.[88]
Effects
On domestic animals
Selection of animals for visible traits may have undesired consequences for the genetics of domestic animals.[89] A side effect of domestication has been
zoonotic diseases. For example, cattle have given humanity various
viral poxes,
measles, and
tuberculosis; pigs and ducks have contributed
influenza; and horses have brought the
rhinoviruses. Many
parasites, too, have their origins in domestic animals.[90] Alongside these, the advent of domestication resulted in denser human populations, which provided ripe conditions for pathogens to reproduce, mutate, spread, and eventually find a new host in humans.[91]
On society
Scholars have expressed widely differing viewpoints on domestication's effects on society.
Anarcho-primitivism critiques domestication as destroying the supposed primitive state of harmony with nature in hunter-gatherer societies, and replacing it, possibly violently or by enslavement, with a social
hierarchy as property and power emerged.[92] The
dialectal naturalistMurray Bookchin has argued that domestication of animals, in turn, meant the domestication of humanity, both parties being unavoidably altered by their relationship with each other.[93] The sociologist
David Nibert asserts that the domestication of animals involved violence against animals and damage to the environment. This, in turn, he argues, corrupted human ethics and paved the way for "conquest, extermination, displacement, repression, coerced and enslaved servitude, gender subordination and sexual exploitation, and hunger."[94]
Domesticated ecosystems provide food, reduce predator and natural dangers, and promote commerce, but their creation has resulted in habitat alteration or loss, and multiple extinctions commencing in the Late Pleistocene.[95]
Domestication reduces
genetic diversity of the domesticated population, especially of alleles of genes targeted by selection.[96] One reason is a
population bottleneck created by artificially selecting the most desirable individuals to breed from. Most of the domesticated strain is then born from just a few ancestors, creating a situation similar to the
founder effect.[97] Domesticated populations such as of dogs, rice, sunflowers, maize, and horses have an increased
mutation load, as expected in a population bottleneck where genetic drift is enhanced by the small population size. Mutations can also be fixed in a population by a
selective sweep.[98][99] Mutational load can be increased by reduced selective pressure against moderately harmful traits when reproductive fitness is controlled by human management.[25] However, there is evidence against a bottleneck in crops, such as barley, maize, and sorghum, where genetic diversity slowly declined rather than showing a rapid initial fall at the point of domestication.[98][97] Further, the genetic diversity of these crops was regularly replenished from the natural population.[98] Similar evidence exists for horses, pigs, cows, and goats.[25]
Domestication by insects
At least three groups of insects, namely ambrosia beetles, leafcutter ants, and fungus-growing termites, have domesticated species of
fungi.[8][100]
Ambrosia beetles in the
weevil subfamilies
Scolytinae and
Platypodinae excavate tunnels in dead or stressed trees into which they introduce fungal gardens, their sole source of nutrition. After landing on a suitable tree, an ambrosia beetle excavates a tunnel in which it releases
its fungal symbiont. The fungus penetrates the plant's
xylem tissue, extracts nutrients from it, and concentrates the nutrients on and near the surface of the beetle gallery. Ambrosia fungi are typically poor wood degraders and instead utilize less demanding nutrients.[101] Symbiotic fungi produce and detoxify ethanol, which is an attractant for ambrosia beetles and likely prevents the growth of antagonistic pathogens and selects for other beneficial symbionts.[102] Ambrosia beetles mainly colonise wood of recently dead trees.[103]
The leafcutter ants are any of some 47 species of leaf-chewing ants in the genera Acromyrmex and Atta. The ants carry the discs of leaves that they have cut back to their nest, where they feed the leaf material to the fungi that they tend. Some of these fungi are not fully domesticated: the fungi farmed by Mycocepurus smithii constantly produce spores that are not useful to the ants, which eat fungal
hyphae instead. The process of domestication by Atta ants, on the other hand, is complete; it took 30 million years.[104]
Some 330 fungus-growing termite species of the subfamily
Macrotermitinae cultivate Termitomyces fungi to eat; domestication occurred exactly once, 25–40
mya.[8][100] The fungi, described by
Roger Heim in 1942, grow on 'combs' formed from the termites' excreta, dominated by tough woody fragments.[105] The termites and the fungi are both obligate
symbionts in the relationship.[106]
Domestication by insects
Gallery of the ambrosia beetle Xylosandrus crassiusculus split open, with pupae and black fungus. The fungus decomposes materials in the wood, providing food for the beetles.
Leafcutter ants Atta cephalotes carrying discs of leaf material back to their nest to feed to their domesticated fungus
Inside the nest of the fungus-cultivating termite Ancistrotermes
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