MEAT ANIMALS, ORIGIN AND DOMESTICATION

Introduction

Domestication of plants and animals was a pivotal moment in human history. It initiated the Neolithic agricultural revolution some 10 000 years ago, and underpinned the spread of human civilizations. Domestication originated in only a few areas of the world and gave inhabitants of those areas enormous advantages over other peoples. This ultimately transformed human demography and gave rise to the modern world.
Domestication of animals most likely had its roots in the ubiquitous habit of all peoples to capture and tame wild animals, and was at first unintentional. It probably originated as a practice of keeping and raising the young animals captured and spared in hunting. Domestication started at the end of Pleistocene, when increasing unpredictability of climate and rapid reduction of numbers of game animals forced people to seek alternative and reliable food supplies.

The Nature of Domestication

Domesticated animals are those that were ultimately genetically modified from their wild ancestors by artificial selection for use by humans, whose breeding and maintenance is controlled, and whose food supply is provided for the benefit of a community or society. Domestication is thus a different process from mere taming of genetically unmodified representatives of wild species and maintaining them in captivity. The degree of suitability of wild animals for domestication depends largely on the degree of their genetic variability and the match between husbandry conditions and species-specific behavioural patterns expressed in the wild. Domestication has been restricted to surprisingly few species of mammals and birds. Especially astonishing is the lack of domesticated mammals indigenous to sub-Saharan Africa, which is a homeland of the largest world populations of wild ungulates. This points to a very unique suite of physiological and behavioural characteristics defining suitability of a particular species for domestication.
All domesticated mammalian species important for meat production thrive on readily available and renewable plant food that can be harvested and stored as supplies by humans, for later use beyond the main growing period. The ability to digest poor quality plant food limited the scope of mammalian species available for domestication to large and medium-size animals weighing 45 kg or more, and belonging to the Order Artiodactyla. Most of them (including cattle, sheep and goats) are capable of fermenting plant material in the voluminous and highly compartmentalized stomach. The domestic pig has a simple stomach and relies on fermentation in the extended morphological structures of the hindgut. The less efficient digestion of fibre-rich plant food is, however, offset by its extremely opportunistic food habits.
All ancestors of major domesticated species were  precocial (that is, their offspring became mobile and able to feed themselves soon after birth), which was a prerequisite for pastoralism. Their high growth rates made them easily renewable human food resources, and speeded up the process of artificial selection by promoting early sexual maturation and shortening generation time. Equally important for successful domestication are behavioural traits. All domesticated meat animals live in herds with a well-developed dominance hierarchy. In the process of domestication, humans have essentially taken over the dominant position, which enables them to manage the herds. Many species otherwise suitable for domestication are notoriously aggressive (e.g. African buffalo), or have tendency to panic in enclosure (antelopes and gazelles) or are reluctant to breed in captivity (e.g. Andean vicuña). Failure to overcome problems with any of these characteristics is the most plausible reason why only 14 out of 148 mammalian species over 45 kg body mass, potentially suitable for domestication, became important as locally or globally distributed domesticated animals. However, only four of them (sheep, goat, pig and cattle) provide the bulk of world meat production.

Origins of Domesticated Meat Animals

Until recently, documentation of events of domestication in the archeological records has proved difficult because of equivocal discrimination of remains of domesticated animals from their wild ancestors. These difficulties have been largely overcome with the advent of analysis of the mitochondrial genome transmitted from generation to generation in maternal lineages and harboured in the egg cells. Sequences of mitochondrial DNA (mtDNA) characteristic of distinct wild populations subject to domestication events have been transmitted throughout millennia, which allows for discrimination between single and multiple origins of domesticated breeds. It may also be recalledthat, across different species, the mutation rate of the most variable regions of mtDNA is constant and high, relative to generation time. This rate of variation constitutes the pacemaker of the so-called molecular clock and has proved a useful tool in reconstructing the time depth of domestication. These molecular techniques, along with other archaeological evidence, have recently enabled us to reconstruct fascinating histories of domestication and phylogenetic relations of major meat animals.

Domestication of Cattle

Among all meat-producing domesticated animals, cattle have had the most economically important role in the evolution of human cultures. There are two major types of cattle: western cattle (Bos taurus) lacking the shoulder hump, and the humped Indian zebu cattle (Bos indicus). Both types interbreed fully and, therefore, their status as separate species is questionable. The continued existence of many of the 800 extant cattle breeds is severely threatened by modern agricultural practices. There is little doubt that all modern cattle breeds (with the exception of mithan and Bali cattle) were derived from the auroch or wild ox (Bos primigenius). Three subspecies of the auroch formerly roamed over vast areas of North Africa (Bos primigenius opisthonomus), Asia (B. primigenius nomadicus) and Europe (B. primigenius primigenius). The auroch became extinct around 2000 years ago within most of its geographical range. Small populations survived in the forested parts of Central Europe, but, despite active protection, the last individual succumbed in 1627 in Jaktorowska Forest, near Warsaw, Poland.
A survey of mtDNA variation revealed that the most recent common ancestor of western and Indian breeds of cattle lived 200 000 to 1 million years ago – much earlier than the appearance of modern humans. Separation of African and European cattle ancestors occurred 22 000–26 000 years ago and, therefore, predates domestication of cattle. This suggests that each continental set of extant breeds originated as a result of separate domestication events in North Africa, the Middle East and Southwest Asia. Furthermore, the genetic affinity of European cattle breeds is much closer to the breeds from Anatolia and the Middle East than to now-extinct European populations of the auroch B. primigenius primigenius. The extant European breeds are, therefore, derivatives of cattle expanding some 5000 years ago from a centre of domestication located in the region of the Fertile Crescent (the area encompassing southern Turkey, northern parts of Jordan, Syria and Iraq). Thus, it is unlikely that a local domestication event contributed significantly to the establishment of European agriculture.
Taurine cattle (B. taurus) were domesticated from the B. primigenius nomadicus 8000–10 000 years ago. The earliest, 7800 year-old, archeological evidence of B. taurus has been found in Anatolia (Turkey). Remains of Bos indicus dated to be at least 4500 years old have been unearthed in Iran, Mesopotamia and the Indus valley. However, mtDNA analysis clearly suggests that zebu cattle must have been domesticated much earlier, some 8000 – 10 000 years ago. Archeological evidence of cattle herding from 7000 years ago points to Pakistan as a potential domestication centre of zebu cattle.
The oldest (9000 years ago) African Bos remains that can be putatively associated with domestication were found in eastern Sahara. Although African cattle are humped and have the distinct morphological characteristics also present in Indian breeds, their mtDNA sequence is much closer to that of Bos taurus. In contrast, the nuclear DNA of African breeds bears the signature of Indian zebu cattle. The apparent lack of mtDNA of zebu cattle in African breeds along with the presence of zebu cattle sequences in nuclear DNA of African breeds strongly suggests a deliberate breeding of African zebu-type females, bearing Bos taurus mtDNA sequences, with zebu males of Asian origin. These males were most likely imported into Africa during the Arab invasions of the eighth century AD. Interestingly, as a secondary consequence of the slave trade, North African mtDNA sequences have been found in cattle from southern Portugal and isolated populations from the Americas.

Domestication of Sheep

The taxonomy of wild species of sheep is still poorly resolved and has become a source of major controversies related to the origins of domestic sheep. Depending on the adopted criteria of classification, such as morphological traits or the number of chromosomes, the genus Ovis comprises between three and nine species. All members of the genus produce fertile and viable offspring when bred in captivity, and many of them also interbreed in the wild. For this reason, unlike the other domesticated animals, whose presumed ancestors are usually well-recognized and limited to one or two wild species, the relative contribution of numerous members of the genus Ovis to the gene pool of domesticated sheep is still debatable. All three major groups of Eurasian wild sheep – mouflon (O. musimon), urial (O. vignei) and argali (O. ammon) – have been suggested as potential progenitors of domestic sheep. Earlier studies indicated that one of the oldest domesticated forms of sheep probably originated some 8000 years ago from urial in the region of the Caspian sea and was subsequently adopted by the peoples of the Middle East, and later also by early European herders. However, a frequent and mixing with argali as well as mouflon greatly complicated the history of its domestication.
Recent mtDNA analysis separated the phylogenetic tree of the genus Ovis into four distinct branches. Two of them contain breeds of domestic sheep (O. aries) and resemble brush-like structures whose fine details still remain to be determined. One of these branches consists of domestic sheep and the representatives of the mouflon (O. musimon/O. orientalis). The other one appears to group domestic sheep, and in stark disagreement with earlier suppositions, does not contain urial. One of the two remaining branches encompasses various subspecies of urial (O. vignei) and argali (O. ammon), whereas a separate, distant branch groups bighorn (O. canadensis). The two distinct branches of phylogeny of domestic sheep suggests that they have been derived from two separate ancestral populations, as in other meat producing domesticated animals. The topology of phylogenetic relationships suggests that mouflon (O.musimon/O. orientalis), rather than argali or urial is the most probable candidate progenitor of domestic sheep.

Domestication of Goats

Domestication of goats (Capra hircus) may have played a key role in the Neolithic agricultural revolution and the spread of agriculture from its earliest homelands. The extreme ability of goats to thrive on poor-quality fodder and to cope with harsh environmental conditions makes them the most geographically widespread, domesticated herbivorous species, ranging from cold Siberian mountains to the driest parts of North Africa.
Earlier archaeological evidence suggested that the bezoar (Capra aegagrus), the wild progenitor of the domestic goat, was the first wild ungulate to be domesticated. Domestication most likely took place in the region of the Fertile Crescent. However, recent analyses of genetic diversity of the domestic goat have revealed three distinct mtDNA lineages. Using the genetic distance between goat and sheep mtDNA sequences as a calibration, the most recent common ancestor of these three lineages can be traced back to 200 000 years ago. This vastly predates the earliest archaeological evidence of time of domestication. Because of such genetic distinctiveness, it is unlikely that the goat lineages have evolved from a single, ancestral population.
The geographic distribution of one of the three goat lineages is limited to eastern and southern Asia (Mongolia, Pakistan, India and Malaysia), whereas the other two lineages primarily occur in Europe, Africa and the Middle and Near East. Both genetic distance between the lineages and their geographic distribution therefore strongly imply multiregional domestication of the goat. Most recent archeological data suggest up to five distinct centres of goats domestication: the Euphrates valley (ca 11 000 years ago); the Zagros Mountains (Iran) (ca 10 000 years ago); the Indus Valley (Beluchistan, India, ca 9000 years ago); and the Southern Levant and the central Anatolia (ca 9–10 000 years ago).
Goats must often have been human companions, both in commercial trade as well as during migrations and explorations. The geographic distribution of genetic variation of the extant lineages of goats is much less diversified than that in cattle. Intercontinental differences between goat populations account for only 10% of the total mtDNA variation, whereas genetic differences between cattle breeds on different continents explain over 80% of the variation. This attests to an intensive intercontinental gene flow between goat populations, which resulted from longdistance transportation of goats along ancient trading routes. Differences in mtDNA variation within each of the lineages suggest a long history of expansion of one of the lineages. Its geographic distribution overlaps with primarily Asian distributions of the other two lineages, which underwent a relatively recent geographic expansion.

Domestication of Pigs

There are two major forms of domestic pigs, European and Asian, whose distinctiveness was recognized by earlier authors, including Charles Darwin. Because of marked, morphological differences, both forms were assumed to originate from different species of wild boar. The progenitor of European form was assumed to be the wild boar (Sus scrofa), whereas Asian form has been thought to originate from Sus indicus, presumed to exist, but unknown from the  wild. Although comprehensive analysis of both mitochondrial and nuclear DNA sequences has revealed a considerable genetic divergence of European and Asian breeds of pigs, it is now clear that both forms are closely related to various subspecies of the wild boar (S. scrofa), widespread in Eurasia and northwest Africa.
As in the case of the other meat-producing animals, the time since divergence from the common ancestor of European and Asian forms of pig falls well outside the known history of animal domestication and has been estimated at 500 000 years ago. This provides strong evidence for independent domestication of pigs in Europe (or rather the Near East) and Asia (probably China), around 9000 years ago. However, some European breeds (for example, European Large White) are characterized by high frequency of mtDNA haplotypes of Asian origin. This is most likely a legacy of well-documented European breeding practices of the eighteenth and nineteenth centuries, when Asian sows were used to improve the contemporary breeds.

Domestication of Poultry

Although the meat yield of wild birds was far lower than that of much bigger mammals, attempts to domesticate fowl have a long history and have been independently undertaken on all continents inhabited by humans. Various breeds of duck and geese species were successfully domesticated in Eurasia, turkeys in Mesoamerica and guinea fowl in Africa, whereas the extant breeds of Muscovy duck originated in South America. The earliest remains of domestic chickens were excavated in numerous archeological sites along the Yellow River in China and dated to be at least 7500 years old. They were also found in the Indus Valley in Pakistan. The 4000-year-old remains unearthed in Spain and Ukraine attest an incredibly rapid spread of the domestic chicken.
Around 3600 years ago, chickens were introduced to New Guinea and quickly reached Pacific islands during Austronesian expansion. This fast initial dispersion can be attributed primarily to the ease of transportation of the fowl. Another important factor could have been a religious significance attached to the chicken as a divine offering, widespread in different parts of the world. The most likely ancestor of domestic chickens is the red junglefowl (Gallus gallus) inhabiting vast areas of the Asian mainland and adjacent islands, from the northeast of India to the western coast of China. Recent molecular analysis indicates that among five subspecies of the junglefowl, Gallus g. gallus is the most likely wild ancestor of all the diverse extant breeds. Furthermore, there is a strong possibility that the domestication of chickens was a single event, taking place in the area of Thailand, followed by dispersion to West Sumatra and Indonesia.

Changes in Species under Domestication

Behavioural Changes of Animals under Domestication

Domestic breeds diverged from their wild ancestors in many ways. Since heritabilities of behavioural traits are usually higher than heritabilities of anatomical and physiological traits, one can speculate that the development of domestic phenotypes started with changes of the behaviour of animals undergoing domestication. The most obvious change was the loss of fear of humans. Equally important behavioural changes involved the raising of the threshold of within-species and between-species aggression. This has become essential for the successful maintenance of stocks of domesticated animals living under population densities far exceeding maximum densities tolerated under natural conditions, often next to large stocks of unrelated species. Perhaps the most important  effect of domestication on behaviour was a reduction of the sensitivity to changes in the unfamiliar environment. This stemmed from reduced emotional reactivity to handling by humans and ease of adaptation to novel conditions, which greatly contributed to the high reproductive rates essential for the success of artificial selection.

Morphological and Anatomical Changes of Animals under Domestication

Domestication has also resulted in profound changes in the morphology and anatomy of animals. Primitive breeds of domestic pigs, sheep, goats and cattle were generally smaller than their wild ancestors, which most likely make them more manageable, as pointed out by Francis Galton in 1865. Chickens, in turn, were selected to be larger. The whole brain volume of domesticated animals is 10% less than in their wild relatives. The decrease of brain sensory centres is particularly clear-cut and corresponds well with the observed behavioural differences between domestic animals and their wild relatives. An incredible increase of growth rate of modern meat-type strains of domestic fowl is one of the best examples of both the power of intense, directional selection and its negative side-effects. During the late 1940s, broilers took about 90 days to grow to slaughter body mass of 1800 g. Now it takes less than half of this time to reach the slaughter mass of 2500 g. Surprisingly, most of this progress has arisen through an increase of growth rates during the first two weeks of postembryonic development. However, this impressive selection progress also incurred unavoidable costs associated with increasing incidence of metabolic diseases, such as ‘heart failure syndrome’, sometimes killing 10% of a broiler flock. In addition, changes in body proportions, such as that resulting from selection for large breast size in domestic turkeys, has severely impaired their mating behaviour, and selection for intense egg production resulted in total loss of incubation and brooding behaviour in laying hens.

Consequences of Domestication for Meat Composition

Ample anthropological and ethnographic evidences indicate that humans are evolutionarily pre-adapted to a diet that includes meat. There is also little doubt that scavenged or hunted ruminants were the main source of meat throughout early human history. Human dietary lipid requirements are, therefore, likely to match the lipid composition of wild ruminant tissues. This composition is qualitatively and quantitatively different from the lipid profiles of meat of domesticated cattle, which may have important consequences for the health of modern consumers. The most significant difference between meat composition of wild and domesticated ruminants is the relative amount of fat per unit mass of muscle tissue. Meat of grain-fed beef (trimmed of all adherent fat) contains 2–3 times more fat (mean 5.6 g per 100 g tissue) than the muscle of wild ungulates, such as antelope, deer or buffalo (mean 2.2 g per 100 g tissue). The high fat content of beef muscle tissues is mainly associated with the formation of intramuscular fat deposits – a phenomenon known as marbling of meat – which is largely absent in wild ruminants. This intramuscular fat is rich in triacylglycerols and resembles subcutaneous fat with respect to the profile of fatty acids.
Another important difference is that muscle tissue of wild ruminants contains a higher proportion of polyunsaturated fatty acids (PUFAs) than muscles of domestic cattle. Up to 30% of all fatty acids contained in game meat are polyunsaturated, whereas PUFAs account for only 10% of FAs in beef. Increased levels of saturated fat in beef (particularly 12:0, 14:0 and 16:0 fatty acids) have substantially contributed to increased dietary fatty acid levels in the modern westernized diet. This in turn, may be associated with an increased risk of cardiovascular disease if not taken into account in a diet. Moreover, muscles of domesticated cattle are much poorer in long-chain PUFAs (particularly n-3 long-chain PUFAs), as compared to muscles of wild ruminants. It is important to note, however, that there are also significant differences in the muscle fat content and composition between pasture-fed and grain-fed cattle. Muscles of grain-fed cattle are particularly rich in saturated and monounsaturated fats, whereas the lipid profiles of pasture-fed cattle resemble those of game meat. Thus, the differences in meat composition between wild and domesticated ungulates can be largely attributed to the practice of feeding cattle grain, rather than to physiological changes incurred by the process of domestication.

The Future of Domestication of Meat Animals

The incredible progress of modern biology has made it possible not only to maintain but also to breed in captivity almost all terrestrial mammals. Some of them, such as moose (Alces alces), red deer (Cervus elaphus) or American bison (Bison bison) have been domesticated to some extent in the last century. They are, however, still unsuitable for intense meatproducing farming, they cannot be herded for a long time, and it is unlikely they will soon join a very short list of major meat-producing species. However, together with primitive breeds of already domesticated animals, they can serve as a source of meat of very desirable protein and fat profiles. Growing health concerns of consumers may paradoxically give rise to a new trend in the selection of meat animals: towards emulation of the meat composition of their wild ancestors.

By M. Konarzewski, (Mammal Research Institute, Polish Academy of Sciences, Białowieza, Poland) in "Encyclopedia of Meat Sciences" 3 volumes set,  editors Warner K. Jensen, Carrick Devine & Michael Dikeman, Elsevier, 2004, excerpts p. 681-686. Adapted and illustrated to be posted by Leopoldo Costa.