Sheep. Sheep are quadrupedal, ruminant mammals typically kept as livestock. Like most ruminants, sheep are members of the order Artiodactyla, the even-toed ungulates. Although the name sheep applies to many species in the genus Ovis, in everyday usage it almost always refers to Ovis aries. Numbering a little over one billion, domestic sheep are also the most numerous species of sheep. An adult female is referred to as a ewe, an intact male as a ram, occasionally a tup, a castrated male as a wether, and a young sheep as a lamb. Sheep are most likely descended from the wild mouflon of Europe and Asia; one of the earliest animals to be domesticated for agricultural purposes, sheep are raised for fleeces, meat and milk. A sheep's wool is the most widely used animal fiber, and is usually harvested by shearing. Ovine meat is called lamb when from younger animals and mutton when from older ones in Commonwealth countries, and lamb in the United States. Sheep continue to be important for wool and meat today, and are also occasionally raised for pelts, as dairy animals, or as model organisms for science. Sheep husbandry is practised throughout the majority of the inhabited world, and has been fundamental to many civilizations. In the modern era, Australia, New Zealand, the southern and central South American nations, and the British Isles are most closely associated with sheep production. There are a large lexicon of unique terms for sheep husbandry which vary considerably by region and dialect. Use of the word sheep began in Middle English as a derivation of the Old English word sceap; it is both the singular and plural name for the animal. A group of sheep is called a flock. Many other specific terms for the various life stages of sheep exist, generally related to lambing, shearing, and age. Being a key animal in the history of farming, sheep have a deeply entrenched place in human culture, and find representation in much modern language and symbology. As livestock, sheep are most often associated with pastoral, Arcadian imagery. Sheep figure in many mythologies, such as the Golden Fleece, and major religions, especially the Abrahamic traditions. In both ancient and modern religious ritual, sheep are used as sacrificial animals. The exact line of descent between domestic sheep and their wild ancestors is unclear. The most common hypothesis states that Ovis aries is descended from the Asiatic species of mouflon. Sheep were among the first animals to be domesticated by humankind; the domestication date is estimated to fall between 11,000 and 9,000 B.C in Mesopotamia and possibly around 7,000 B.C. in Mehrgarh in the Indus Valley. The rearing of sheep for secondary products, and the resulting breed development, began in either southwest Asia or western Europe. Initially, sheep were kept solely for meat, milk and skins. Archaeological evidence from statuary found at sites in Iran suggests that selection for woolly sheep may have begun around 6000 BC, and the earliest woven wool garments have been dated to two to three thousand years later. Sheep husbandry spread quickly in Europe. Excavations show that in about 6000 BC, during the Neolithic period of prehistory, the Castelnovien people, living around Chateauneuf-les-Martigues near present-day Marseille in the south of France, were among the first in Europe to keep domestic sheep. Practically from its inception, ancient Greek civilization relied on sheep as primary livestock, and were even said to name individual animals. Ancient Romans kept sheep on a wide scale, and were an important agent in the spread of sheep raising. Pliny the Elder, in his Natural History, speaks at length about sheep and wool. European colonists spread the practice to the New World from 1493 onwards. Domestic sheep are relatively small ruminants, usually with a crimped hair called wool and often with horns forming a lateral spiral. Domestic sheep differ from their wild relatives and ancestors in several respects, having become uniquely neotenic as a result of selective breeding by humans. A few primitive breeds of sheep retain some of the characteristics of their wild cousins, such as short tails. Depending on breed, domestic sheep may have no horns at all, or horns in both sexes, or in males only.