Equus caballus, Horse: Origin & Evolution
A major part of the evolution of horses occurred in North America.
The first equid was Hyracotherium, a small, 10-20" at the shoulder, dog-like forest animal of the early Eocene (~55.8 - 33.9 million years ago). Other Eocene horses were Orohippus and Epihippus.
Late Eocene and Oligocene (33.9 - 23 million years ago) horses, Mesohippus and Miohippus, had tougher teeth and were larger and leggier in adaptation to drier climate and life in open grasslands.
In Early Miocene (~23.3 - 16.3 million years ago) Miohippus lineage gave rise to several families: 3-toed browsers called "anchitheres"; a dead-end line of small "pygmy horses", e.g. Archeohippus, and a line of open-country grass grazers, swift runners with long legs, which became ancestors of the modern horses (best studied genera Parahippus and Merychippus).
As the Miocene climate continued to become cooler and drier, grasslands spread dramatically during the Miocene Epoch, 23 to 5 million years ago. Miocene mammals responded with further adaptations to eating grass and with a move to speed, both as defense and offense in the open grasslands.
The mural at National Museum of Natural History presents an idealized picture of this active period, about 25 million years ago in what is now Nebraska.
By the late Miocene (17 - 11 million years ago), Merychippus, with its primitive 3 toed feet, was the one of the first bona-fide speedy plains grazers. It also had the distinguishable head of today's horses. It was about twelve hands (48 inches) tall, which was the tallest equine yet. Merychippus represents a milestone in the evolution of horses. It gave rise to at least nineteen different species of grazers, which can be categorized into three major groups, one of which led to "true equines". This burst of evolution is often known as the Merychippine radiation.
About 10 million years ago, the horse family reached an apex of diversity (of species and of genera) and sheer numbers.
The first Equus were 13.2 hands tall (pony size) with a classic horse body. The earliest known three Equus species collectively known as the Equus simplicidens (The Hagerman horse) first appeared about 3.5 million years ago. They quickly diversified into at least 12 new species in 4 different groups, in a burst of evolution reminiscent of the great merychippine radiation.
During the first major glaciations of the late Pliocene (2.6 million years ago), some Equus species migrated
via land bridges to other continents where they diversified
into the modern zebras in Africa; into as desert-adapted onagers and asses in Asia,
the Mideast, and North Africa. Other Equus species, now extinct migrated into South America.
The modern horse, E. caballus, diverged from the lineage of
extant zebras and asses at least 2 millions years ago, as the fossil record suggests, or about
3.9 million ago, according to molecular data.
Until about 1 million years ago, there were Equus species all over Africa, Asia, Europe, North America,
and South America.
A set of devastating extinctions in the late Pleistocene (~11,000 years ago) killed off most of the large mammals in North and South America. All the horses of North and South America disappeared. These extinctions are attributed to climatic changes as well as overhunting by humans, who had just reached the New World.
For about eleven thousand years there were no horses in North America. Spaniards reintroduced horses to North America starting with Columbus in 1493 and continuing into the 16th century. Escaped horses established feral population that by the early 1800' reached 2 million animals. The population declined with increased expansion of civilization and exploitation by man. There were only 15,000 to 30,000 feral horses remaining in the United States and 2,000 to 4,000 in Western Canada by 1959.
Wild horses were common throughout the Eurasian steppe during the Upper Paleolithic (35,000 to 10,000 years ago). Fossil records indicate that about 10,000 yeas ago, wild horse population decreased sharply and by the Iron Age almost disappeared. Today, only one putative wild population, Przewalski's horse, remains. Early horse domestication events possibly started taking place about 6,000 - 5,000 years ago, and remains of horses that were not only different from wild horses in their morphology but also had dental damages consistent with bridling, became increasingly frequent in archeological sites of southern Ukraine and Kazakhstan. A rapid and substantial increase in the number of coat colorations found in Siberia and East Europe, which best can be explained by selective breeding, roughly coincides with this time.
Equus caballus evolved during the Pleistocene about 1.8 million years ago and survives today as domestic horse.
The large and slender horse skull has long, broad, tapering nasals. At the front of the mouth are large incisors, at the back of the mouth are six large, squared molars on either side of the maxilla and the mandible. The area between the incisors and the molars is called the bar. In that space a canine tooth may be located in some horses. High-crowned teeth extend so far into the skull so that there is no room for the eye socket above them.
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