Horses

Horses

One common misconception about evolution is that it occurs in a straight line, from an ancestor to a descendant. Although it is possible to trace the lineage, or history, of a certain species, the wider view shows that evolution is actually a very broad process. It may be helpful to visualize evolution as a tree from which many branches sprout, with each branch representative of a line of organisms evolving from the center of the tree. Many of these branches, or lineages, may die out. Other branches continue to grow and branch out further, resulting in the great diversity of life on Earth today. The history of the horse is an excellent example of this evolutionary "tree." Horses did not evolve in a straight line from a common ancestor, through species after species, until the modern wild horse appeared. In fact, the evolution of the horse is a story of great divergence and extinctions that continued through time, until only one species remained.

Paleontologists have discovered a very good record of horse fossils in North America, where the horse first appeared, and have learned much about its early history. Other continents have been subsequently searched for fossils, and the migration and distribution of the horse is now well-known. From the remains of a few small populations of true wild horses in Europe and possibly central Asia, thriving communities of horses now exist in most regions on Earth.

The Beginning

The ancestors of the horse were browsers who fed on the bushy and leafy types of vegetation found in forests. They ate leaves of trees and shrubs and occasionally fed on tender grasses. Scientists deduced what they ate from studying the fossil teeth of animals that had a similar bone structure to the modern horse, but that had bumpy teeth, instead of the flat, grinding teeth horses have today. These early ancestors were small animals, about the size of a fox or a medium-sized dog.

Evolutionary scientists recognize that environment often drives natural selection , and that natural selection leads to evolution. One force causing the transformation in horses was the constant alteration of climate that began about 60 million years ago and continues through the present. As the climate became hotter and drier, two important events occurred. First, the forests shrank and became patchy throughout North America. This reduced the habitat of the forest-dwelling members of the horse group. Second, different forms of grasses, called "C4 grasses," evolved. These grasses were tougher than the fragile forest grasses and better able to withstand harsh conditions. They spread throughout the more arid regions and formed vast plains. These plains created a less nutritious but more stable food source for the animals. The formation of the grasslands provided a new habitat in which many forms of animals, like the camels and horses, thrived. C4 grasses contained a high concentration of a glassy mineral called silica. The horse species that survived this new diet developed stronger, flatter, and more complex teeth than their forest relatives. Most of the evolution of horses was identified and traced by studying these changes in tooth shape and structure.

The second major factor that affected the evolution of the horse was the emergence of more effective competitors, such as the artiodactyls (herbivores like camels, deer, and bison), and swifter predators, especially the cats with their saberlike canine teeth. These swift and efficient predators found the smaller, forest-dwelling horses to be easy prey, resulting in the swift extinction of the horses. This kind of selection pressure favored the swifter and more wary equids of the open grasslands. So as one horse lineage died out, another evolved quickly. This pattern continued until the end of the Pleistocene epoch about 11,000 years ago. Only one species of horse, Equus equus, remains from this once diverse group of animals.

The Story of the Bones

All information scientists know about ancient horses has been gathered from the fossil remains of their skeletons. The skull, and the teeth it contains, can be read almost like a book, revealing how and when physical changes occurred and in what order. While no real "trends" are apparent in the overall picture of horse evolution because so many different species are involved, there were some general changes. As horses evolved, their toes were continually reduced in number until the condition of standing on one toe, like the modern horse, was achieved. The teeth became larger, with a more complex surface. The face became longer. In addition, the overall body size increased, growing from the tiny size of the forest dwellers to the significantly larger modern horse size.

An animal called Eohippus is often cited as the first identifiable horse ancestor. Eohippus lived about 60 million years ago and is nicknamed the "dawn horse." It was a small, dog-sized animal with five toes on its front feet and three toes on its hind feet. The animal stood high on its toes, the tips of which were covered with strong little hooves. The teeth were the browsing type that had small bumps like those on human molars, but which more closely resembled those of a pig. Scientists estimate that this little horse was about 35 centimeters (14 inches) high and weighed a little over 5 kilograms (12 pounds).

The next candidate for selection in the fossil record is Hyracotherium, an animal that lived about 55 million years ago. Hyracotherium was more horselike than Eohippus, with its skeleton showing the characteristics that became unique to horses. The skull was longer and larger, and had a shallow basin at the end of shortened nasal bones where the nose is. The lower jaw was bigger and stronger than its relatives. The top and bottom incisors, or front teeth, met squarely and formed a "nipping-type" set of teeth. The back of the head no longer sloped backward but was now straight up and down. The neck bones were shaped so that the neck not only was longer, but could be rotated upward. Eventually, this feature helped the descendants of Hyracotherium to reach downward for grasses. Scientists believe that Hyracotherium (and many other species of horses at this time) had a short, prehensile proboscis, or snout, that could pick tasty leaves from high up in trees and bushes.

The legs of Hyracotherium were longer than those of Eohippus and other horses. The front legs of its ancestors had been about 40 percent longer than the hind legs, but the two sets of legs of Hyracotherium were more evenly lengthened. The feet began to change shape as their function for running became greater. The carpals and tarsals (wrist and ankle bones) became smaller and more square. The metacarpals and metatarsals (equivalent to the bones of a human palm) were longer and more slender. The wrist and ankle became more stabilized to prevent side-to-side motion and aid in more efficient running. One of the ankle bones, the astragalus, formed a unique notch where it met the lower leg bone, the tibia. This permitted greater force to be exerted on the foot when pushed against for running. In the wrist, the carpals interlocked with the lower row, providing a stronger pull stroke when running.

These trends continued in species named Orohippus and Mesohippus. The fossil record reveals a divergence of evolution around the time of Mesohippus, about 34 million years ago. One line contained the species Kalobatippus and Hypohippus, and died out with Megahippus. The other line, which leads to Equus, contained Miohippus and a tiny Archaeohippus and continued the skull and leg transformation. Archaeohippus was not highly successful, and its lineage died out relatively quickly. The ankle and wrist of Miohippus and its ancestors continued to strengthen, and the legs finally lengthened so that the animal stood higher in front than in the back. Mesohippus was the first ancestor of the horse to have one fewer front toe, although all the remaining horses eventually had three toes on each foot.

By the Oligocene (the end of the Tertiary epoch), major changes in the horses began to take place. The forest dwellers were no longer dominant, and horses who ate the newly rising C4 grasses began to spread into the great grasslands. These horses were larger, with increasingly long legs, and were able to explore new territory. The wrist and ankle bones continued to become more square and flat so that the force of running would not destabilize the foot. The side-to-side motion of the wrist and ankle was reduced to prevent wobbling, with the back-and-forth motion becoming stronger.

The trend toward an enlarged skull continued for the rest of horse history. The teeth, which were so important for grazing on tougher grasses, lost their roots and became hypsodont, or very high-crowned. One of the most identifiable characteristics of the horse is complex enamel, the tough outer coating of the tooth. Enamel resists the grinding actions of chewing. During the evolution of the horse, the enamel on the molars infolded from the sides, increasing the number of bumpy grinding surfaces. This trend continued for millions of years as horses ate more and more fibrous food. Many scientists believe that horses have the most complex and resistant teeth of almost all the large mammals. Some rodents have complex teeth, but like horses, they eat tough, fibrous foods, like seeds and silica-containing grasses.

The skeleton of the horse continued to grow from about 24 million years ago to the present. While the legs got longer and longer, the scapular (shoulder bone) and pelvis (hip bones) stayed relatively the same size. The neck and back elongated. This change resulted in two advantages for the horse. First, it allowed the horse's head to flex down to the ground to get the grasses. Second, the longer back gave greater flexure for a fast running pace. When an animal like a horse or cheetah runs, one of the important parts of the running pace is the springlike flexure of the back. Some species of horses that showed these changes were Parahippus, Merychippus, Neohipparion, Pliohippus (the first single-toed horse), Dinohippus, Hippidion, and Equus.

Hippidion and Equus lived at the same time, but Hippidion became extinct sometime in the Pleistocene. By this time, horses were gone from North America and it is believed that only a few populations continued in remote places. One population—a small, stocky, pony-type horse resistant to cold and wet—was discovered in northwest Europe. Another population—horses that were larger and resistant to heat—was found in central Asia. These central Asian horses were the ancestors of the desert horses of today.

The horse was reintroduced to North America by the Spanish during their explorations in the early sixteenth century. Many escaped or were let go and are the ancestors of the wild mustang. Horses now thrive all over the world. They are considered animals of beauty and grace and many cultures, such as the Native Americans of the North American plains and nomadic peoples in Mongolia, depend on the horse.

see also Biological Evolution; Physiology.

Brook Ellen Hall

Bibliography

MacFadden, Bruce. Fossile Horses: Systematics, Paleobiology, and Evolution of the Family Equidae. Cambridge, U.K.: Cambridge University Press, 1994.

Simpson, George Gaylord. Horses: The Story of the Horse Family in the Modern World and Through Sixty Million Years of History. New York: Oxford University Press, 1951.

Internet Resources

The International Museum of the Horse. Lexington, Kentucky. <http://www.imh.org/imh/kyhpl1a.html#xtocid224361>.

The Equine Studies Institute. Ed. Deb Bennett. Smithsonian Institution. <http://www.equinestudies.org/historical.htm>.

Horses. PBS/Nature. <http://www.pbs.org/wnet/nature/horses/>.

The Florida Museum of Natural History. <http://www.flmnh.ufl.edu>.