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Goat Evolution

Billy Goat

Goat evolution is the process by which modern domestic goats came to exist through evolution by natural election. Goats are part of the family Bovidae, a group which began to diverge from Deer and Giraffids during the early Miocene epoch, making Goats closely related to Cows and Sheep. The subfamily Caprinae, which includes Goats, Ibex and Sheep, are considered to have diverged from the other Bovidae as early as the late Miocene, with the group reaching its greatest diversity in the ice ages. The tribe Caprini would subsequently develop from Caprids who arrived in the mountainous areas of Eurasia and split into Goats and Sheep in response to a further geographic separation. This divergence resulted in the adaption of the ancestors of Goats to a mountainous environment, producing many of the traits considered peculiar about the species. During the ice ages a genus called Capri evolved which would then diverge into the modern goat species along with several species of Ibex. Modern goats were one of the first species domesticated by modern humans, with the date of domestication generally considered to be 8,000 BCE. It is commonly held that the earliest domestication was of the bezoar ibex in the Zagros Mountains. These earliest domesticated Goats were used to produce meat and milk for neolithic farmers, along with providing many of the materials required to built residences and tools. Following the domestication of goats over 300 breeds have been established for a variety of purposes, including for the maximisation of milk production and for meat.

Evolution of the Bovidae family

Eotragus, the earliest known Bovid genus

It is generally considered that the Bovidae family separated from Deer and Giraffids approximately 20 million years ago, in the early part of the Miocene epoch. These early bovids likely lived in woodland environments in the Old World and were small in size and deer-like. The earliest known Bovid was Eotragus, a genus of small antelope-like animals which are closely related to the modern nilgai and four-horned antelope. Although little is known directly about Eotragus many of the traits characteristic to modern Bovids have been observed, including horns, crowned teeth and the presence of hooves. The characteristic large size of modern members of the Bovidae family also became evident in Bovids found soon after Eotragus.

Early in the natural history of the family there was a divergence into the Boodontia and Aegodontia clades, which are from Africa and Eurasia respectively. This divergence is generally attributed to a momentary continental divide between the two landmasses. The two clades began to coexist after the continents were subsequently rejoined, as the geographic divide disappeared. Modern goats are descended from the early members of the Aegodontia tribe, which encompasses all bovids outside the Bovinae subfamily.

Miocene period, separation of subfamily Caprinae

By approximately 15 million years ago, during the late Miocene epoch, the Bovidae family had radiated into around 15 different genera, predominantly concentrated in Asia. Following this the family's diversity increases dramatically and by the end of the Miocene a total of 70 genera are said to have existed. The success of the Bovidae is generally attributed to their ability to rapidly move across plains and to cope with the tough grass found in them due to their crowned teeth. It was during this period of rapid diversification in the mid-late Miocene that the Caprinae diverged from the other Bovidae. These early Caprids are generally considered to have resembled the Serow, a genus of medium sized goat-like mammals. Caprids were forced to find their niche away from the plains which were heavily populated by Cervidae (Deer) and thus developed the characteristic agility required to survive in harsh environments. Their dependence on alpine habitats meant that the subfamily was much more successful in Asia than Africa, one of the main Bovid groups to to so.

Natural history following the divergence of the subfamily Caprinae

The subfamily Caprinae has also been subdivided into the tribe Caprini, the group which includes Goats and Sheep. Selective pressure is generally considered to be the cause for the split between the Caprini and other Caprinae, with early members of the group moving into mountainous regions and developing particular traits in order adapt and to escape predators. The subsequent split between what would become Goats and Sheep occurred due to a further separation in geography, with the latter occupying the foothills and the former moving into higher altitudes. This separation and subsequent specialisation is attributed to the need to escape predators, as the adaption to higher altitudes allowed for easier avoidance. In contrast Sheep developed herding behavior in order to combat the threat posed by predation. Those who remained in the mountains would subsequently form the Capra genus, which encompasses modern Goats along with several species of Ibex, by the most recent ice age. The need to adapt to higher altitudes pushed for the development of browsing behavior due to the lack of low and easily accessible grass, a factor which likely contributed to the evolution of curiosity in Goats and their ability to digest plants which would otherwise be poisonous.  

Divergence of domestic goat from wild goat and recent developments since domestication

Although estimates vary it is generally held that Goats were first domesticated approximately 9,500 - 9,900 years ago. This occurred in south-eastern Anatolia, although separate instances of domestication happened in Iran approximately 6,500 years ago and in eastern Turkey 2,500 years ago. The majority of domesticated Goats today are descended from the latter two cases and not from the first. Other evidence for domestication exists in Western Asia, dated approximately 8,000 years ago. Early Goat domestication was able to provide meat, milk, clothing and fuel for neolithic farmers and their remains could also have been used to build shelters and weapons. The domestication process has rapidly increased both the rate of evolutionary development and the genetic diversity of the Goat population, with their currently being recorded 300 breeds catered for a variety of purposes.

Sources i would like to use

Savage, R 1986 Mammal evolution: An illustrated guide, Random House Value Publishing, New York City

This book is an account of the evolutionary history of multiple mammal groups, within my article I will specifically use it within the section which documents the evolution of the Bovidae family. This book provides a detailed account through, both illustrations and writing, of the history of mammalian evolution beginning with the divergence of mammals from reptiles before discussing the evolution of multiple mammal groups. The book specifically will be used to explain the divergence of the Bovidae family from the related giraffids and cervids in the early Miocene era in Africa and Eurasia. It illustrates the appearance and environment which these animals which resembled small antelopes and were found in forests and the subsequent diversification of the family.

Kingdon, J 1989 East African Mammals: An Atlas of Evolution in Africa (Volume III, Part C), University of Chicago press, Chicago

This book is a part of a widely acclaimed series which looks at a wide variety of East African mammals, discussing evolutionary history as well as ecology and physical features. The book provides a strong analysis of the various features of goats and the evolution of such traits, allowing a greater level of detail within my study of goat evolution. I will thus utilise this within both the sections detailing separation of the subfamily Caprinae and the natural history following this transition with some mention during the first section on the creation of the Bovidae family. Particular goat traits which will be analysed within the book which will be discussed within my article include tooth structure, horns and body shape and size.

Macdonald D 1984, The Encyclopedia of Mammals, Facts on File, New York City

This book is an illustration which includes detailed coloured photographs and articles which discuss the behaviour, physiology and evolution of a variety of mammals, including a section on goats and related species. Within this the evolution of Caprinae is discussed and hence I will utilise within my section on that topic. Beyond this I will also likely use it within the section on the subsequent evolution of goat-like animals until the development of the modern species. The article on goats describes the origin of the Caprinae subfamily and how the earliest Caprid species would have likely resembled the modern Serow. In regards to subsequent evolution it describes how Caprinae forms likely developed traits which would make them accustomed to particular environments, with the goats becoming adapted to a mountainous lifestyle. Such analysis and its broad application to the study of goat evolution makes this source particularly useful for the article.

Naderi, Saeid; Rezaei, Hamid-Reza; Pompanon, François; Blum, Michael G. B.; Negrini, Riccardo; Naghash, Hamid-Reza; Balkiz, Özge; Mashkour, Marjan; Gaggiotti, Oscar E.; Ajmone-Marsan, Paolo; Kence, Aykut; Vigne, Jean-Denis; Taberlet, Pierre 2008, The goat domestication process inferred from large-scale mitochondrial DNA analysis of wild and domestic individuals, PNAS

This article analyses the history of goat domestication, in particular the locations in which it first occurred and how much these regions contribute to the overall goat population. It reveals that the earliest domestication of goats occurred in south eastern Anatolia 9,500 to 9,900 years ago. However, those descended from these earliest examples of domestication make up a small percentage of the total goat population. Thus, the article concluded that multiple independent sites of domestication must have occurred. The article states that three of these events must have occurred. The other two examples are said to occur in Iran 6,500 years ago and eastern turkey 2,500 years ago. This article will be used in conjunction with other sources to write the final section of the article.

Hassanin, D. Douzery, EJ 1999, The tribal radiation of the family Bovidae (Artiodactyla) and the evolution of the mitochondrial cytochrome b gene, Molecular Phylogenetics and Evolution

This article examines the early development of the Bovidae family and will thus by discussed in the first section. Particularly the article attempts to explain the radiation, both genetically through the creation of new species and through geographical change. This it is helpful for exploring the relationship between Caprinae and other members of the Bovidae family, particularly its divergence in relation to other subfamilies and which of these are its closest relatives.