EVOLUTION

America's Gift to the World

In an address titled "Origin and History of the Horse," given before The New York Farmers at the Metropolitan Club on December 19, 1905, Henry Fairfield Osborn painted the horse as "... the noblest of the domesticated animals ... one of the gifts of America to the world."¹ Since establishing the Department of Mammalian Paleontology at New York City's American Museum of Natural History, in 1891, Osborn had been conducting paleontological explorations to discover the evolutionary antecedents of the modern horse, Equus caballus. Finds resulting from these expeditions had already augmented a growing bank of scientific knowledge pointing to the horse's evolution in North America. In 1889, he and his colleagues had come upon "... a considerable number of skeletons of the ... original North American horse ...," known as eohippus in fossil beds near Mount Blanco in the Texas Panhandle. (Eohippus is a Greek word meaning "dawn horse" and is neither italicized nor capitalized, as it no longer is a valid scientific name.)²

Fossil remains of the horse, representing every phase of evolutionary modification, have been found in North America. Ancestors of the modern horse evolved on the North American continent over 57 million years, with horse evolution being cited as a classic example of the evolutionary process, where natural selection molds characteristics, both biological and behavioral, that promote survival. It was once thought that horse evolution was a straight-line process, with horses becoming progressively larger, with fewer toes, and alterations in teeth structure that changed horses from browsers to grazers. However, horse evolution took place, not in a smooth, gradual, or straight-line fashion, as had been previously thought, but through a complex, branching process -- more fully understood, recently, through improved dating techniques (geochronology) and advances in interpreting evolutionary development and taxonomy. While nearly all natural experiments in horse evolution failed, with almost all side branches of the horse family tree becoming extinct, one branch did survive and kept growing, changing in reaction to ecological challenges. The genus Equus, which includes modern horses, zebras, and asses, is the only surviving genus in a once diverse family of horses that included 27 genera.^2a

Hyracotherium

Hyracotherium was the first distinct taxonomic group of horses. The genus eohippus is most commonly used in referring to the North American "dawn horse." However, eohippus is actually a synonym of the European genus Hyracotherium, the older of the two names and the most correct, under the rules of zoological nomenclature. When a bricklayer discovered Hyracotherium fossils near Suffolk, England in 1838, scientists did not realize, at first, that this tiny creature was related to the horse. As the ancient remains closely resembled the Hyraxes (rock rabbits), similar in size and external appearance to rodents, the paleontologists named the fossil horse after them. Fossil remains of this most ancient, rabbit-sized ancestor of the horse have also been found in rich Eocene beds in the Bighorn Basin of northern Wyoming, just a few miles from the Pryor Mountain Wild Horse Range.³

Species of Hyracotherium lived from 55 to 45 million years ago. The teeth of these earliest of horses are quite primitive, resembling those of monkeys and other primates. In fact, the renowned British anatomist, Richard Owen, originally concluded that Hyracotherium fossils proved that primates once lived in England. However, after studying additional specimens, he corrected his mistake. These primitive horses had four toes on the front feet with three behind. They possessed a primitive, short face, with eye sockets in the middle and a short diastema (space between the front teeth and cheek teeth) and little or no lateral vision. Well-known science artist Charles Knight of the American Museum of Natural History gave the dawn horse a striped coat because, he speculated, it was a browsing animal, and modern browsers often have striped coats as camouflage in the play of light and dark on the forest floor. Scientists have concluded, based on remains, that eohippus had a coat similar to a deer in texture. The tail had short hair, with no mane in evidence. Some scientists hypothesize that species of Hyracotherium are not only ancestors of the modern horse but, also, relatives of rhinos, tapirs, and several extinct animals, such as the strange clawed chalicotheres, huge horned titanotheres, and the "Beast of Baluchistan," the largest land mammal ever to walk the Earth.⁴

The diminutive dawn horse lived in North America and Europe at the same time. This was when the British Isles and the North American continent were attached -- part of a supercontinent called "Laurasia," which included North America, Greenland, and Europe north of the Alps and as far east as the Himalayas. Hyracotherium appeared in Laurasia at the beginning of the Eocene Epoch (lasting 16 million years), during which all major orders of modern mammals appeared. Several species of Hyracotherium developed, varying in size from about 10 to 20 inches tall. Then, this earliest horse, for some unknown reason, became extinct during the Oligocene Epoch, in Europe, which, by this time, had separated from the North American continent.⁵ Therefore, the evolutionary process for the horse in Europe stopped at the alpha-stage, while continuing on in the New World, through a complex branching process of trial and error, success and extinction.

Orohippus

Orohippus ("mountain horse") another primitive and early genus of the horse, was first found in middle Eocene beds in Wyoming, about two million years after Hyracotherium first appeared. The two genera coexisted during the Eocene, although fossils of Orohippus are not as numerous or as geographically widespread as those of the dawn horse. Fossils of Orohippus have now been excavated in both Wyoming and Oregon in Eocene sediments dating from about 52 to 45 million years ago. Professor Othniel Charles Marsh did not leave an explanation of why he named this mammal Orohippus or "mountain horse", although it may have to do with fossils found in Wyoming at a site called Grizzly Butte (a butte is an isolated hill or mountain that juts up from an otherwise flat landscape.) However, contrary to its name, the mountain horse did not live in the mountains.⁶

Orohippus was slightly larger than Hyracotherium. However, the two genera shared a somewhat primitive postcranial skeletal structure. For example, lower limb bones of the forelimb (the radius and ulna) of Hyracotherium and Orohippus are distinct and unfused, as is true in humans. This primitive condition for mammals permits rotational movement at the elbow and wrist joints. It is retained by animals, such as small forest dwellers who must maneuver over uneven terrain. "In its postcranial skeleton, Orohippus differs from Hyracotherium by having more enlarged middle digits on its fore and hind feet, and by displaying a complete loss of the first and fifth (thumb and pinkie) toes of the hindlimb."⁷

Mesohippus

Mesohippus ("middle horse") got its name from being an intermediate between the dawn horses of the Eocene and more "modern" horses. Fossils of Mesohippus have been found at many Oligocene sites in Colorado and throughout America's Great Plains (e.g., in Nebraska, South Dakota, and North Dakota) and Canada. This genus existed about 37 to 32 million years ago. While earlier horses of the Eocene had four toes on their forefeet, Mesohippus possessed only three. Also, its premolar teeth evolved to be more like molars. Premolars are said to be "molariform." Primitive triangular premolars pulp food, while the squared molariform teeth crushed and ground food. It is speculated that this reflected a shift in diet from a more diverse form, including fruit, to a more limited diet of leaves and, perhaps, grasses. "The first upper premolar is never molarized. It is popularly called the "wolf-tooth" by horse breeders."⁸

Miohippus

During the Oligocene Epoch, 32 to 25 million years ago, the genus Miohippus gave rise to the first burst of diversity in the horse family. Until this "little horse" appeared, few side branches had developed, but numerous descendants of Miohippus evolved, becoming distinct genera. Consequently, during the following epoch, the Miocene, over a dozen horse genera existed. (By contrast, today, only one genus of the horse family survives -- Equus.) Fossils of Miohippus are found at many Oligocene sites on the Great Plains, in the western United States, and in a few places in the state of Florida. While one might expect Miohippus to have existed within the Miocene Epoch, owing to its name, it did not. When Professor O.C. March (who also named eohippus and Pliohippus) gave Miohippus its name, in 1974, he believed these fossils were from Miocene rocks. However, more recent work indicates that nearly all species of Miohippus existed during the Oligocene.⁹

Parahippus

Parahippus (meaning "side horse") seems to be the evolutionary link beween older forest-dwelling horses and modern plains-dwelling grazers. This genus of horses had three toes, like primitive horses, but with smaller side toes. Parahippus is long-headed, with eye sockets situated well back from the middle of the skull. Fossils of Parahippus have been excavated from many early Miocene sites on the Great Plains and in Florida. Species of this genus lived 24 to 17 million years ago. While the name Parahippus might lead one to believe that "side" toes inspired this appellation, that is unclear, and the origin of this genus' name was never well explained. However, the name may refer to the "side" branches of the posterior crest of the upper molars that were noted by the paleontologist who coined the name. This feature differentiated Parahippus species from those of Anchitherium.¹⁰

Some species of this German Shepherd-sized horse developed teeth that were tall, or "high-crowned." High-crowned teeth may have permitted this horse to have a certain amount of grass in its diet. Because grass is abrasive and wears teeth down, short teeth in grass-eating animals would result in early death from starvation. Other advancements in the teeth of this genus included the appearance of cementum, a protective coating on teeth, and molar ridges that are found in modern Equus caballus. One species, Parahippus leonensis, is believed to be a close relative to the group from which modern horses evolved.¹¹

Merychippus

The genus Merychippus ("ruminant horse") represents a milestone in horse evolution. Although it had three toes, like primitive horses, its appearance was that of the modern horse. This genus had a long face and long legs. Its longer legs allowed it to escape from predators and to migrate long distances to graze. Its high-crowned cheek teeth made it the first known grazing horse. It was the ancestor of all later horse lineages. Fossils of Merychippus are found in many late-Miocene sites throughout the United States. Species within this genus lived from 17 to 11 million years ago. While paleontologists rarely find fossilized digestive tracts and can only speculate about the digestive physiology of extinct animals, so far as is known, no extinct horses had a ruminant digestive system. The name, therefore, came from Merychippus' strong crests on its teeth that reminded the paleontologist who names this genus of the teeth of ruminants.¹²

Pliohippus

Grandfather to the modern horse, Pliohippus ("more horse") appears to be the source of the latest radiation in the horse family. It is believed to have given rise to Hippidion and Onohippidion, genera that thrived, for a time, in South America, and to Dinohippus, a genus that, in time, led to Equus, the modern horse. Fossils of Pliohippus have been found at many late-Miocene sites in Colorado, the Great Plains (Nebraska, North Dakota, and South Dakota) and Canada. Species of this genus lived from 12 to 6 million years ago. While its name would imply an origin during the Pliocene Epoch, this is not the case. Refinements in the understanding of the Pliocene managed to shrink this epoch and placed Pliohippus within the Miocene, instead.¹³

Taxonomists have, more recently, split the genus Pliohippus into two separate genera. Pliohippus now includes horses with large facial depressions in front of their eyes that lived during the Miocene. The second genus, Dinohippus, includes horses with smaller facial depressions that lived into the Pliocene Epoch. (Equus lacks these depressions.)¹⁴

Hippidion

Oddly enough, Hippidion means "pony." However, this genus consisted of Clydesdale-size browsing horses that lived in South America, during the Ice Age (Pleistocene Epoch), between 2 million and 10,000 years ago. They possessed a highly distinctive, long, domed nasal bone. Some scientists have speculated that because of this unusual feature, Hippidion had an elongated, flexible snout.¹⁵

Kalobatippus

This browsing horse, dubbed the "stilt-walking horse," had exceptionally long legs, so it must have stood out, at a time when horses were all intermediate in size and structure. Descendants of this horse circled the globe, with Kalobatippus giving rise to the European Anchitherium and the East Asian Sinohippus. Fossils of Kalobatippus have been found in Miocene sites in the western United States. Species of this genus lived from 24 to 19 million years ago. Members of the genus had low-crowned teeth, useful for browsing on leaves, sprouts, and, perhaps, fruits. Had they been grass eaters, their teeth would have quickly worn down by the abrasive grasses and could have led to starvation.¹⁶

Hypohippus

The name Hypohippus means "low horse." This genus was a highly specialized browsing horse, with three spreading toes that would have been well-adapted to soft ground within forests where it roamed. For its time, it was a large horse -- about the size of a modern pony. Fossils of Hypohippus have been excavated in Nebraska, Colorado, and Montana. Species of this genus lived from 17 to 11 million years ago. Its common name, "low horse," is based on some "milk molars" (baby teeth, sometimes called deciduous teeth). It was noted that the middle cusp was” lower" than in the teeth of others within its taxonomic grouping. Perhaps this explains the name. As it turns out, "lower horse" was an appropriate appellation. Nearly 50 years after its discovery, a skeleton of this genus was described. The description revealed that Hypohippus was a long-faced, long-necked, and long-bodied animal with short legs. Compared to the modern horse, and even to other horses of its time, it was, indeed, a "low horse."¹⁷

Megahippus

Megahippus ("big horse") was the last of the browsing horses in North America. This large horse (about 585 pounds) was a highly specialized leaf eater (browser) at a time when most horses were becoming, exclusively, grass eaters (grazers). Fossils of this genus are not common. They are known from the Miocene in sites located on the Great Plains. Species of this genus lived from 15 to 11 million years ago.¹⁸

Studies of living hoofed herbivores have revealed that those with wide muzzles usually eat grass, while those with narrow snouts tend to eat soft vegetation, such as leaves, sprouts, and fruits. Megahippus possessed a very narrow snout that was used to select rich food items, such as fresh sprouting leaves, rather than twigs and old leaves -- food items with lower nutritive quality. This genus had front teeth that curved strongly in a narrow "U-shape" -- an adaption for browsing. These horses also had low-crowned teeth, so it would have been difficult for them to eat abrasive grasses. Because Megahippus was a three-toed browser, it is sometimes considered to have been "stuck" in an evolutionary dead-end, relative to its grazing contemporaries. This genus, however, evolved just as rapidly as the grazing horses. The grinding surfaces of their cheek teeth became larger, as did their general body size.¹⁹

Archaeohippus

Archaeohippus means "ancient horse." It was a small, sleek, grazing animal with an elongated snout. This tiny browsing horse from the Miocene Epoch was about the size of a collie. It co-existed with grazing horses that were diversifying throughout the drier, open savannahs of that time. Fossils have been found in Nebraska, Oregon, California, and Florida, but skulls are quite rare. Species of Archaeohippus lived from 21-13 million years ago. Although many horses became progressively larger than their ancestors, this genus became considerably smaller. Archaeohippus descended from the larger genus Miohippus. (Nannippus is another example of a horse becoming smaller than its ancestors.)²⁰

The shape of its molars (crown height) of Archaeohippus suggests that this horse inhabited either a forest environment or an open woodland. It consumed a seasonally variable diet of leaves and other non-fibrous foods. Individuals were either solitary or formed territorial breeding pairs, with small home ranges. Their life-span was probably 4-5 years. ²¹

Neohipparion

"New better horse" is the translation from the Latin for this genus. Neohipparion was the most successful hipparion horse, with the largest number of species in the fossil record. Fossils have been found in sediments dating from the mid-Miocene through the early Pliocene of North America and Central America. Species of this genus lived from 16 to 5 million years ago.²²

The hipparion horses have been excavated in North America, Europe, and Asia. However, Neohipparion appears to have been strictly a New World genus -- hence the "neo-" in its name. The hipparions were a group of three-toed horses found throughout the world during the Miocene and Pliocene Epochs. "They are distinguished from contemporary non-hipparion horses by the condition of the protocone, a structure in the upper cheekteeth. In hipparions, the protocone and protoloph typically are not connected, whereas in other horses of the late Miocene and Pliocene, they are connected."²³

Hipparions are the most common large mammal found in many 10-million-year-old fossil sites. Despite their success, hipparions were a side branch of horse evolution, with no modern survivors. By 6 million years ago, only a few species had survived. The last hipparion species, Cormohipparion emsliei, remained in Florida until about 2 million years ago.²⁴

Dinohippus

Dinohippus, the "powerful horse," is thought to be the closet relative of Equus -- the genus that includes modern horses, zebras, and asses. Fossils of this genus are found in the Upper Miocene of North America and date from 13 to 5 million years ago. This horse seems to be an intermediate between Pliohippus and Equus. In fact, the genus was established from species initially included in Pliohippus (for example, P. leidyanus).²⁵

While Pliohippus had a couple of depressions (or fossae) in the bones in front of its eyes, Equus does not. The fossae in the face of Dinohippus are shallow, suggesting it to be an intermediate form between the two genera. Fossae may have served as a location for glands (as in some antelope) or may have been sites for muscle attachment.²⁶

Equus species have a distinctive passive "stay apparatus," formed by bones and tendons, to help conserve energy while standing for long periods. Dinohippus is the first horse to show a rudimentary form of this character, giving additional credence to the close relationship between Dinohippus and Equus. Variation exists in the number of toes among Dinohippus individuals. "An exquisitely preserved primitive population of Dinohippus from the famous Ashfall Beds in northeastern Nebraska suggests that some individuals had three toes, while others had only one toe."²⁷

Equus

Equus simply means "horse." This genus first appeared in the late Miocene Epoch, 5 million years ago, until present. Fossils of Equus have been found on every continent on earth, except Australia and Antaractica. Domesticated about 6,500 years ago, the horse has greatly impacted human history, like no other animal, in such areas as migration, hunting, agriculture, war, sport, communication, and, simply, companionship.²⁸

The genus Equus originated on the North American continent. This genus now includes African zebras and asses and, in recent times, included extinct forms such as the quagga and tarpan (or Kiang), By 30,000 years ago, in North America, Equus had been transformed into an animal closely resembling the modern horse we know today.²⁹ Wild horses (E. caballus) are uniquely suited to the environment of the western United States. Many of the adaptations of Equus, including monodactyly (single-toed hoof) and very high crowned teeth (to eat dry grass, covered with grit) were the result of this type of environment. To this day, members of this genus are anatomically, ecologically, and physiologically well adapted to very dry habitats. ³⁰

In 1993, Yukon gold miners and, subsequently, scientists found " ... the world's best-preserved example of an extinct 26,000-year-old Ice Age horse." The freeze-dried carcass, which included the pelt and a well-preserved foreleg, was surprisingly similar to modern horses. When archeologist Ruth Gotthardt, who works for the Yukon government, went to investigate the find, she first thought the carcass might be the remains of a horse that had died during the Yukon Gold Rush about 100 years before. Upon climbing into the trench where the horse was unearthed, however, she was "... greeted by the unmistakable smell of horse droppings. ... I really had my doubts (that it was an ancient specimen) because it looked so fresh," she said, questioning how "... something from the Ice Age could keep that smell of horse." Nevertheless, she sent the carcass to Dr. Richard Harington, a paleontologist and Curator of Quaternary Zoology at the Canadian Museum of Nature in Ottawa, and a bone sample to Miami for radiocarbon dating. Scientists ultimately concluded this to be an extinct Equus lambei. The curious and astute gold miners had provided scientists with one of the best Ice Age fossil horses in North America -- a find better than gold. The stomach contents and pelt of this specimen will, in time, be analyzed by the Canadian Museum of Nature to determine the diet and composition of the pelt of E. lambei.³¹

Overview

Horse species were constantly branching off the family tree and evolving along a variety of different evolutionary paths. It is clear that many horse species, possessing diverse characteristics, were present at the same time and that no straight-line, ladder-like evolution took place. However, in tracing the line of descent from Hyracotherium to Equus, one would be able to see the following broad trends:

• reduction in the number of toes
• increase in body size
• increase in size of cheek teeth
• lengthening of the face
• browsing slowly giving way to grazing

All morphological changes within the family of horses can be attributed to Darwin's theory of microevolution that includes genetic variation, natural selection, genetic drift, and speciation. George G. Simpson wrote, "The history of the horse family is still one of the clearest and most convincing for showing that organisms really have evolved, for demonstrating that, so to speak, an onion can turn into a lily."³²

F o o t n o t e s

1: Henry Fairfield Osborn, "Origin and History of the Horse," Address presented before The New York Farmers, Metropolitan Club, New York, 19 December 1905, p. 1

2: Patricia Mabee Fazio, "The Fight to Save a Memory: Creation of the Pryor Mountain Wild Horse Range (1968) and Evolving Federal Wild Horse Protection through 1971," doctoral dissertation, Texas A&M University, College Station, 1995, p. 21; Osborn, "Origin and History of the Horse," p. 1; Hope Ryden, America's Last Wild Horses (New York: Lyons & Burford, 1990), p. 19

2a.: "Horse Evolution" by Kathleen Hunt from www.onthenet.com.au/~stear/horse_evolution.htm; Bruce J. MacFadden, Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae (New York: Cambridge University Press, 1992), p. 205

3: ibid.

4: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History; Pure Illusion Arabians and Crossbreeds Web site, "Evolution of the Horse Presentation" (site has moved)

5: Heather Smith Thomas, The Wild Horse Controversy (New York: A.S. Barnes and Co., 1979), pp. 17, 18; Ryden, America's Last Wild Horses, p. 19

6: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

7: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

8: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

9: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

10: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

11: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

12: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

13: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

14: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

15: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

16: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

17: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

18: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

19: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

20: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

21: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

22: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

23: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

24: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

25: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

26: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

27: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

28: Fossil Horses in Cyberspace Web site, Florida Museum of Natural History

29: Susan M. White, "Evolution of the Horse in America," (typewritten), San Francisco, California, n.d., pp. 1, 2.

30: "Unbroken Spirit: The Wild Horse in the American Landscape" Web site (original site of 1998), Buffalo Bill Historical Center, Cody, Wyoming

31: "Yukon miners unearth Ice Age horse carcass," Billings Gazette, 10 February 1994, p. 6B

32: "Horse Evolution" by Kathleen Hunt from www.onthenet.com.au/~stear/horse_evolution.htm; George G. Simpson, The Story of the Horse Family in the Modern World, and through Sixty Million Years of History (London: Oxford University Press, 1951), p. 168