Horse. Evolution. Modern Equids

The horse is one of the most important animals ever domesticated. Horses belong to the taxonomic order Perissodactyla, animals with an odd number of toes. Perissodactyla contains three families: Tapiridae (tapirs), Rhinocerotidae (rhinoceroses), and Equidae (horses, asses, and zebras).

Evolution. Equidae evolution begins in the Eocene epoch with the appearance of the Pliolophus in Eurasia some 50 million years ago. Pliolophus was a slightly built, dog-sized mammal with short legs and a curved back. Rather than single, unpadded hooves characteristic of modern equids, Pliolophus had four padded toes on its forefeet, three on its hind feet.

Its teeth indicate it subsisted by browsing on leafy forest vegetation. Pliolophus migrated into North America during the Eocene where its Oligocene descendants evolved larger, more horselike forms but continued to subsist as generalized browsers.

Equid subsistence changed radically during the Early Miocene. Natural selection on the emerging grasslands of North America produced Parahippus with tooth morphology adapted to specialized grazing not browsing. A descendent, Protohippus, continued this evolutionary trajectory during the Middle and Late Miocene. Accelerated grassland expansion in North America led to the Late Miocene evolution of Hipparion, a tall, long-limbed, three-toed descendant of Protohippus.

Hipparion was the first equid to develop high-crowned, hard, enamel-edged, hyposodont cheek teeth able to efficiently chew hard, silica-rich, abrasive grass fibers. Declining sea level and reemergence of the Bering Straits land bridge permitted Hipparion's spread into the Old World. The three-toed Hipparion equid body-plan persisted in the New and the Old Worlds into the Early Pleistocene.

The first true horse, Pliohippus, evolved in the Early Pliocene—a small, pony-sized animal. Natural selection on the grasslands lengthened its legs, enlarged the grinding surfaces on its molars, expanded its muzzle to accommodate larger teeth, and reduced the number of its toes to a single functional digit. Pliohippus spread from North America to Eurasia until severance of the Bering Straits land bridge during the Pliocene. This severance contributed to the differentiation of Old and New World equine taxa.

Modern Equids. Equus, the genus of modern horses, donkeys, and zebras, evolved from Pliohippus in North America during the Late Pliocene. A key evolutionary development in Equus is evident in both their fore and hind legs. Whether quadruped or biped, the knee joints of most mammals are structured in such a way that muscular effort must be exerted for them to remain standing.

In the absence of such effort, gravity forces the knee joint to flex or collapse. Modem equid legs have "passive stay-apparatus," a complex arrangement of bone, muscle, and ligaments that passively "lock" the knee in extension without calling forth continued high levels of muscular activity. By enabling them to stand for long periods with reduced muscular activity, this anatomical feature contributes to their energetic efficiency.

Skeletal evidence for the passive knee lock is first seen in the hind leg of Protohippus around 11 or 12 million years ago. Analogous passive locking mechanisms do not appear in the shoulder region until the evolution of the modern equines between about 3 to 5 million years ago.

Equus dispersed widely over North America during the Pleistocene epoch, reaching South America via the Isthmus of Panama and Eurasia via the Bering Straits. Extinct in New World around 8000 все, at the Pleistocene's close, the horse persisted in the Old World where it was eventually domesticated.

Horses Domesticated.The great diversity of mitochondrial DNA in domesticated horses suggests that wild horse domestication was not confined to a restricted geographic area or prehistoric culture. Rather this diversity indicates that wild horses were utilized and domesticated at many different times and places throughout prehistoric Eurasia.

One important early center of horse domestication was among peoples of the Tripolye culture who, around 3000 все, were located north of the Biack Sea on the steppes between Ukraine and Turkestan. They used domesticated horses for milk, meat, and funeral offerings; their use of horses as beasts of burden, draft animals, and mounts came later.

Horses were pulling wheeled carts in Eurasia by 2600 все. But evidence that horses were mounted and ridden is not present in the archaeological record until about 2000 все (but see Brown and Anthony 1997). Afterwards, "classic" pastoral nomadism—mounted riders driving livestock and moving their belonging on pack animals—emerged in southwestern Eurasia.

Efficient use of the horse in hauling demands a special kind of harness. As horses hold their necks erect, they are choked by the oxen's neck yoke. Horse harnesses must fit across the animal's chest. Since their invention sometime before 800 CE in Europe, such harnesses placed formidable traction power at the disposal of humankind and ultimately opened the great grasslands of the world to plow agriculture.

War Horses. Domesticated horses spread rapidly over the Old World and their dispersal profoundly affected patterns of warfare. Mounted warriors strike or flee quickly; mounted bowmen are particularly dangerous opponents. Yet, light cavalry had only limited effectiveness against well-drilled or fortified infantry.

Decisive military use of the horse in ancient times came from its traction power. Perfection of two-wheeled, horse-drawrn war chariots around 1800 все brought radical change to warfare in Eurasia. Mobility, firepower, and battlefield carnage were raised to new levels by teams of galloping horses drawing fast, light, sturdy chariots containing archers and drivers.

The peoples best able to use chariot warfare were steppe dwellers, whose way of life assured familiarity with and access to horses. Accordingly, between 1800 and 1.500 все, waves of barbarians equipped with chariots overran the civilized lands of the southwestern Asia. The effectiveness of cavalry-mounted warriors (as opposed to charioteers) was not much enhanced until the invention of the foot stirrup in China about 500 ce.

With this device, cavalrymen could use the shock of their horses' charge to drive their lances against an enemy without themselves being catapulted from the saddle by the force of the blow. Stirrups, arriving in Europe around 700 ce, effected a military revolution.

Infantry was eclipsed for five hundred years during the Age of Chivalry and mounted knights dominated Europe's battlefields until the Age of Gunpowder. As its military significance declined, the horse's role in society contracted to symbol of authority, beast of burden, and source of recreation.