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The Eocene horse

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Figure 9. (A) The Eocene horse (Hyracotherium) and representatives of the condylarths, (B) Phenacodus (early Eocene) and (C) Mesonyx (middle Eocene). Note how very carnivore-like Mesonyx is although it possessed small hooves rather than claws and is classified with the ungulates. (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)

Prehistoric Horses
    Writing this week in the journal Science, paleontologist Bruce J. MacFadden said the evolution of horses involved many more twists and turns than previously imagined. Modern steeds did not follow a relatively smooth transition from the diminutive, foxlike forest browsers that were their earliest ancestors to those impressive, open-plains athletes we know today. Rather, horses fluctuated considerably in form and size over time.   MacFadden, who is the vertebrate-paleontology curator at the Florida Museum of Natural History in Gainesville, said horses have proved especially popular with evolutionary scientists.
    Kathleen Hunt, a biologist at the University of Washington in Seattle, said the modern-day horse is "merely one twig on a once flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived."   MacFadden, the study author, agrees that horse evolution was, in fact, a pretty messy affair—a jumble of evolutionary processes such as random genetic variation and natural selection.   "Any changes in morphology [physical form and function], such as in tooth or limb evolution, can be explained within this framework," he said. "Equine mammals are adaptable critters whose size, diet, and range depended on geography and climate."
The modern horse appears to have evolved over three million years ago in North America. Crossing land bridges, it crossed to Asia and Europe.  It disappeared from this western hemisphere around 10,000 years ago.  Around 3,500 years ago, horses were domesticated. and by 1,000 years ago they were widely used throughout Asia, Europe and North Africa.   They were reintroduced to America by Spanish and English colonists.  
About 3,500 years ago, somewhere in the steppe region of Asia, horses were domesticated. The spotted horses were especially eye-catching and so they frequently became models for the local artisans. Some of those relics have endured to present day leaving us some record of horses during ancient times.

The modern horse evolved over three million years ago and then disappeared from this hemisphere 10,000 years ago. The horse returned to North America when explorers Cortes and DeSoto came mounted on magnificent Barbs from Morocco, Sorraia from Portugal and Andalusians from Spain.

The oldest species of "true" horse, Equus stenonis, was discovered in Italy, and is believed to have evolved from Plesippus-like animals at the end of the Tertiary or beginning of the Quaternary periods. Equus stenonis proliferated into two branches, one lighter in body mass and one heavier.

     Equus stenonis crossed into North America, where similar forms known as Equus scotti are common; some types (Equus scotti var. giganteus) exceeded the modern horse in size. However, all the horses in North America ultimately became extinct, approximately 11,000 years ago, perhaps due to climate change or some pandemic. It has also been suggested that humans hunted horses to extinction, as the appearance of humans in the Americas occurred at about the same time as the extinction of most large mammals in the Americas. However, there are no known kill sites of Pleistocene horses in North America, and so this scenario remains unsupported.
     Recent studies by a team of geneticists headed by C. Vila indicate that the horse line split from the zebra/donkey line between 4 and 2 million years ago. Equus ferus, ancestor species to Equus caballus, appeared 630,000 to 320,000 years bp. Equus caballus was formed from several subspecies of Equus ferus by selective breeding widely over Eurasia for an extended time. The details of this process are currently a target of research by archaeologists and geneticists.
Hyracotherium / Eohippus  *
also known as Eohippus. This small forest-dwelling browser, which arose in North America roughly 50 million years ago, was the ancestor of all modern equids (including the common domesticated horse).

     The horse originated in North America.  Its earliest known ancestor was the Hyracotherium, sometimes called Eohippus or "dawn horse."   Thousands of complete, fossilized skeletons have been found, mainly in the Wind River basin in Wyoming.  It was the size of a fox and lived about 55 million years ago.

  The earliest animal to bear recognizably horse-like anatomy was the Hyracotherium ("hyrax-like beast"). Its scientific name is derived from initial confusion over early partial fossils' relationship with living species: Richard Owen likened early Hyracotherium fossils "to a hare in one passage and to something between a hog and a hyrax in another". A later name for the Hyracotherium, "eohippus" ("dawn horse"), is also popular, though the earlier name takes precedence due to scientific naming conventions.
Hyracotherium evolved in the early Eocene (54–34 million years ago). It was an animal approximately the size of a fox (250–450 mm in height), with a relatively short head and neck and a springy, arched back. It had 44 low-crowned teeth, in the typical arrangement of an omnivorous, browsing mammal: 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of the jaw. Its molars were uneven, dull, and bumpy, and used primarily for grinding foliage. The cusps of the molars were slightly connected in low crests. The Hyracotherium browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern muntjac: the Hyracotherium had a small brain, and possessed especially small frontal lobes. Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in the Wind River basin in Wyoming
     This small dog-sized animal is the oldest found horse ancestor that lived about 55 million years ago.  It had a primitive short face, with eye sockets in the middle and a short diastema (the space between the front teeth and the cheek teeth). Although it has low-crowned teeth, we see the beginnings of the characteristic horse-like ridges on the molars.
     The origin of equines can be traced to the Eocene period, between 60 and 50 million years ago. Eohippus, or Dawn Horse, was about the size of a Cocker Spaniel - 14 inches at the shoulder - and is thought to have weighed about twelve pounds. He had four toes on the front legs and three on the back, which were padded like those of a dog and allowed easy movement over wet ground. These toes and pads are now the ergots and splint bones found on the legs of the modern horse. Eohippus was a browsing animal that lived on soft leaves growing on low shrubs. He was well equipped to survive in what were then the semi-tropical forests of the U.S. Midwest.
    Approximately 50 million years ago, in the early-to-middle Eocene, Hyracotherium smoothly transitioned into Orohippus over a gradual series of changes.  Although its name means "mountain horse", Orohippus did not live in the mountains. It resembled Hyracotherium is size, but had a slimmer body, an elongated head, and slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. Although Orohippus was still pad-footed, the vestigial outer toes of Hyracotherium were not present in the Orohippus; there were four toes on each forelimb, and three on each hind leg.  The most dramatic change between Hyracotherium and Orohippus was in their teeth: the first of the premolar teeth were dwarfed, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. Both of these factors gave the teeth of Orohippus greater grinding ability, suggesting that Orohippus was subsisting on tougher plant material.
    The earliest evidence of this “little horse” is found in the middle Eocene of Wyoming, about 2 million years after the first appearance of Hyracotherium.  The two genera coexisted during the Eocene, although Orohippus fossils are not as numerous or as geographically widespread as those of Hyracotherium. Fossils of Orohippus have been found in Eocene sediments in Wyoming and Oregon, dating from about 52-45 million years ago.
In the mid-Eocene, about 47 million years ago, Epihippus, a species which continued the evolutionary trend of increasingly efficient grinding teeth, evolved from Orohippus. Epihippus had five grinding, low-crowned cheek teeth with well-formed crests. A late form of Epihippus, sometimes called Duchesnehippus, had teeth similar to Oligocene equids, although slightly less developed. Whether Duchesnehippus was a subspecies of Epihippus or a single species is disputed.
Mesohippus  * 

Beginning about 32 million years ago, the climate became drier and early grasses began to evolve.  Several million on years later, Mesohippus or "middle horse" appeared and was one of the most widespread mammals in North America. .  It had six grinding cheek teeth suitable for chewing the grasses.    Longer legs permitting it to run faster, thus avoiding predators.  Fossils have been found throughout the Great Plains.
In the late Eocene and the early stages of the Oligocene epoch (32–24 million years ago), the climate of North America became drier, and the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses and various kinds of brush. In a few areas these plains were covered in sand, creating the type of environment resembling the present-day prairies.
    In response to the changing environment, equids, too, began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in non-wooded areas. About 40 million years ago, the Mesohippus ("middle horse") suddenly developed in response to strong new selective pressures to adapt, beginning with the species Mesohippus celer and soon followed by Mesohippus westoni.
    In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs, Mesohippus was an agile animal.
     Mesohippus was slightly larger than Epihippus, about 610 mm (24") at the shoulder. Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which in later horses became quite detailed, and serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all later equid species would retain. Mesohippus also had the sharp tooth crests of Epihippus, improving its ability to grind down tough vegetation.
     The "middle horse" earned its name.  Mesohippus is intermediate between the eohippus-like horses of the Eocene, (which don't look much like our familiar "horse") and more "modern" horses. Fossils of Mesohippus are found at many Oligocene localities in Colorado and the Great Plains of the US (like Nebraska and the Dakotas) and Canada.  This genus lived about 37-32 million years ago.
    By the Oligocene period, about 38 million years ago, Eohippus had evolved into Mesohippus and Miohippus and had achieved the size of a German Shepherd. Both these evolutions were taller and heavier, with teeth that allowed them to eat a wider variety of plants. They were still browsers living in forests and swamps. Their front feet were reduced to three toes, still padded, but the middle toe carried most of the weight.
Miohippus  *
Miohippus was a genus of prehistoric horse that lived in what is now North America during the Oligocene Period some 25 to 40 million years ago. It is believed to have branched off from Mesohippus, and the two coexisted for about four-eight million years.
     Around 36 million years ago, Soon after the development of the Mesohippus, the Miohippus ("lesser horse") emerged, the earliest species being Miohippus assiniboiensis. Like Mesohippus, Miohippus's evolution was relatively abrupt, though a few transitional fossils linking the two genera have been found. It was once believed that the Mesohippus had anagenetically evolved into the Miohippus by a gradual series of progressions, but new evidence has shown that Miohippus's evolution was cladogenetic: a Miohippus population split off from the main Mesohippus genus, coexisted with Mesohippus for around 4 million years, and then over time came to replace Mesohippus.> 
     The Miohippus was significantly larger than its predecessors, and its ankle joints had subtly changed. Its facial fossa was larger and deeper, and it also began to show a variable extra crest in its upper cheek teeth, a trait that became a characteristic feature of later equid teeth.
     The Miohippus ushered in a major new period of equid diversification. While Mesohippus died out in the mid-Oligocene, Miohippus continued to thrive, and in the early Miocene (24–5.3 million years ago), it began to rapidly diversify and speciate. It branched out into two major groups, one of which adjusted to the life in forests once again, while the other remained suited to life on the prairies.
    Species of Miohippus gave rise to the first burst of diversity in the horse family.  Until Miohippus, there were few side branches, but the descendants of Miohippus were numerous and distinct.  During the Miocene, over a dozen genera existed. Fossils of Miohippus are found at many Oligocene localities in the Great Plains, the western US and a few places in Florida. Species in this genus lived from about 32-25 million years ago.
Miocene and Pliocene: True equines
    The forest-suited form was Kalobatippus (or Miohippus intermedius, depending on whether it was a new genus or species), whose second and fourth front toes were long, well-suited travel on the soft forest floors. Kalobatippus traveled to Asia via the Bering Strait land bridge, and from there to Europe, where its fossils were formerly described under the name Anchitherium. Kalobatippus is believed to be ancestral to another European species known as Hyohippus, which became extinct near the beginning of the Pliocene.


The Miohippus population that remained on the steppes is believed to be ancestral to Parahippus, a North American animal about the size of a small pony, with a prolonged skull and a facial structure resembling the horses of today. Its third toe was stronger and larger, and carried the main weight of the body. Its four premolars resembled the molar teeth and the first were small and almost nonexistent. The incisive teeth of Parahippus, like those of its predecessors, had a crown as humans do; however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup. =D
     Parahippus appears to be the evolutionary “link” between the old forest-dwelling horses and the modern plains-dwelling grazers.   It has 3 toes, like primitive horses, but the side toes are smaller.   They are "horse-faced," or long-headed with the eye socket well back from the middle of the skull. Fossils of Parahippus are found at many early Miocene localities in the Great Plains and Florida.   Species in this genus lived from 24-17 million years ago.
     The watershed in the development of the horse occurred in the Miocene period, about 26 million years ago, when he moved out of the forests and swamps and onto the plains. As he adapted to changing conditions, his neck and head became longer, the incisors moved forward in the skull and the form and position of the eyes altered to allow the horse to view the horizon while grazing. His legs became longer, giving him speed to escape from predators. These horses, Parahippus and Merychippus, stood firmly on a single toe with semi-functional side toes, and were about 10.5 hands (42") high.

     Merychippus was a milestone in horse evolution.  Though it retained the primitive character of 3 toes, it looked like a modern horse.  It had high-crowned cheek teeth, making it the first known grazing horse and the ancestor of all later horse lineages.  Fossils have been found throughout the United States.  The species in this genus lived from 17-11 million years ago. 


In the middle of the Miocene epoch, an animal called Merychippus was alive. Merychippus had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running

   Merychippus represents a milestone in the evolution of horses.   Though it retained the primitive character of 3 toes, it looked like a modern horse.   Merychippus had a long face.   Its long legs allowed it to escape from predators and migrate long distances to feed.   It had high-crowned cheek teeth, making it the first known grazing horse and the ancestor of all later horse lineages.  Fossils of Merychippus are found at many late Miocene localities throughout the United States.   Species in this genus lived from 17-11 million years ago. 

     Three new equids are believed to be descended from the numerous varieties of Merychippus: Hipparion, Protohippus and Pliohippus. The most different from Merychippus was Hipparion. The main difference was in the structure of tooth enamel: in comparison with other equids, the inside, or tongue side, had a completely isolated parapet. A complete and well-preserved skeleton of the North American Hipparion shows an animal the size of a small pony. They were very slim, rather like antelopes, and were adapted to life on dry prairies. On its slim legs, Hipparion had three toes equipped with small hooves, but the side toes did not touch the ground.
     An American form of Hipparion, also known as Neohipparion, proliferated into many kinds of equids several of which managed to migrate to Asia and Europe during the Pliocene epoch. (The European Hipperia differs from the American Hipparion in the smaller body size – the best-known discovery of these fossils was near Athens.)  Recent research suggests that Hipparion is an ancestor of the zebra and the donkey, rather than the horse.

     This was the first truly single-hoofed horse-like animal evolved about seven million years ago.  The side toes became the splint bones found in modern horses. For many years, it was believed to be the "grandfather" of the horse, but recent studies indicate that it was from a parallel line that died out. 

arose from Calippus in the middle Miocene, around 15 million years ago. It was very similar in appearance to Equus, though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs. The long and slim limbs of Pliohippus reveal a quick-footed steppe animal.
    Until recently, Pliohippus was believed to be the ancestor of present-day horses because of its many anatomical similarities. However, though Pliohippus was clearly a close relative of Equus, its skull had deep facial fossae, whereas Equus had no fossae at all. Additionally, its teeth was strongly curved, unlike the very straight teeth of modern horses. Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is a likely candidate for the ancestor of Astrohippus.
    "Grandfather" to the modern horse, Pliohippus 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 American, and to Dinohippus which in turn led to Equus. Fossils of Pliohippus are found at many late Miocene localities in Colorado, the Great Plains of the US (Nebraska and the Dakotas) and Canada.  Species in this genus lived from 12-6 million years ago.
     The first truly single-hoofed horse was Pliohippus, which evolved about seven million years ago in the Pliocene period. The side toes became the splint bones found in modern horses. This small, lightly built horse was the prototype for the Equus caballus, the first true horse, which evolved during the Pleistocene period, almost two million years ago. Equus had a rigid spine, with short, powerful and well-muscled bones in the upper limbs and long, slender unmuscled lower limbs. He was well equipped for life on the open plain and had a well developed defense system. The foot pad of earlier evolutions became the frog of modern horses.
Dinohippus  *
     Dinohippus was the most common horse in North America during the late Pliocene. It was originally thought that Dinohippus was a monodactyl horse, but a 1981 fossil find in Nebraska shows that some were tridactyl.
    Dinohippus is believed to be the closest relative to Equus, the genus that includes the living horses, asses and zebras.  Dinohippus fossils are found in the Upper Miocene of North America and date from 13 - 5 million years ago.
Plesippus is often considered an intermediary stage between Dinohippus and the present day horse, Equus.  At the end of the Pliocene, the climate in North America began to cool down significantly and the animals were forced to move south. One group of the Plesippus species escaped to South America, and the other moved across the land bridge around the Bering Strait into Asia and Europe. A portion also remained in the southern part of North America. The Ice Age spread five times over Europe and North America and five times again receded; it is estimated that approximately one million years elapsed from the Ice Age (the Quaternary period) to our era.
    In South America a form named Hippidium developed from Plesippus. Hippidium was relatively short-legged with a deeply recessed nasal notch, very thin and delicate nasals, and long ectoflexids in the lower premolars. It continued to live on the South American pampas for a long time, but eventually died out.
    These are the horse-like animals, sometimes called the horse family. It includes horses, the donkey, the four zebra and the onager.   They first appear in the fossil record 4 million years ago.  Some of these animals crossed the Bering land bridge into Asia.  Their descendents migrated to Europe.  The horse became extinct in North America about 11,000 years ago. 

     The oldest species of "true" horse, Equus stenonis, was discovered in Italy, and is believed to have evolved from Plesippus-like animals at the end of the Tertiary or beginning of the Quaternary periods. Equus stenonis proliferated into two branches, one lighter in body mass and one heavier.

       Equus stenonis crossed into North America, where similar forms known as Equus scotti are common; some types (Equus scotti var. giganteus) exceeded the modern horse in size. However, all the horses in North America ultimately became extinct, approximately 11,000 years ago, perhaps due to climate change or some pandemic. It has also been suggested that humans hunted horses to extinction, as the appearance of humans in the Americas occurred at about the same time as the extinction of most large mammals in the Americas. However, there are no known kill sites of Pleistocene horses in North America, and so this scenario remains unsupported.
     Recent studies by a team of geneticists headed by C. Vila indicate that the horse line split from the zebra/donkey line between 4 and 2 million years ago. Equus ferus, ancestor species to Equus caballus, appeared 630,000 to 320,000 years bp. Equus caballus was formed from several subspecies of Equus ferus by selective breeding widely over Eurasia for an extended time. The details of this process are currently a target of research by archaeologists and geneticists.
     The Spanish began to import Iberian horses to breeding ranches on Cuba, Haiti, and other large islands offshore of the Americas beginning with Columbus' second voyage in 1493. The first Conquistador horses to land on the main continent were most likely spare stallions and expendable (infertile) mares from these island ranches. Later, as Spanish missions were founded on the main land, horses and cattle would eventually be lost, and would proliferate into large feral herds.
     At the end of the 15th century, when the first Europeans came to America, no native horses were reported observed by them in the regions where they landed, or as they began to explore the interior. The natives of post-Colombian Mexico and Peru did not have a specific name for the animal, calling it in their language a "sun dog" or "deer". In one incident, a lame horse was left behind by a Spanish expedition to be cared for by a local tribe. It was reported that the indigenous people of that area attempted to feed it meat, and were surprised that it preferred to graze on vegetation instead. For these reasons, it has long been assumed that no one, anywhere, on the continent had ever seen a horse before. Though, to this day, some Native Americans (in particular those of the northwestern United States, and southwestern Canada) by way of their oral traditions, disagree.
    Equus spread across the Bering Strait from America to Asia. Primitive man, starting to evolve in Asia, followed horse herds back across the Bering Strait into America, some staying to become the first Americans. When the glaciers retreated about ten thousand years ago, the land bridges between what is now Alaska and Asia disappeared. Soon after that the horse became extinct in North America. No one knows why. They were later re-introduced to the continent by Spanish explorers, and became the progenitors of the Mustang.


Introduction. This chapter talks about the evidence that modern day horses, donkeys and zebras (grouped in the genus Equus) evolved from a dog sized mammal that lived 60 million years ago. The chapter will key in on three major traits that changed over time through mutation, variation and environmental challenge (natural selection). These traits are: 1. thickness and length of teeth, 2. number and size of toes and 3. length of bones. The chapter will show evidence for how mutation can produce variation within a group, and natural selection can cause an advantageous trait to become predominant by making its possessors able to have more offspring.
I have selected the evolution of horses because the evidence for it is extensive with large numbers of fossils of both the animals and of plants that lived at the same time. There is also a significant amount of evidence for how genes work in tooth, toe and bone formation.

Tracing the evidence back in time, the evidence points to an animal called “Hyracotherium” as the ancestor of today’s horses. Hyracotherium was a very successful animal in its time. The plant fossils with it show it lived in forests. Its feet were slightly different than “typical” mammals. It only had four toes on its front feet and had three regular toes and two vestigial toes on the back foot. This is a good time to talk about how toes form.

A group of genes called “Hox” genes controls fingers and toes. They affect the number and length of bones in a toe or finger and also the number of toes. Experiments show that a change in a Hox gene can change the number of toes or the number of bones in the toe. If we look at humans, the human thumb has only two bones while the other fingers have three bones. Some humans have six fingers. Humans also vary in the length of their finger and toe bones. These variations are due to slight differences (caused by gene mutation) in their Hox genes. A vestigial (small, functionless) toe can result from a mutated Hox gene. A Hox gene mutation can lead to a missing digit (the embryo makes four toes instead of five). The picture on the right shows one experiment where the loss of Hox gene 11 produces an animal with one less bone in the first toe. Genetics thus easily explains how Hyracotherium could have four toes in front and three in back.

Hyracotherium had the typical mammalian “omnivorous” jaw. This means that, like humans, it had a variety of teeth that allowed it to eat a wide variety of foods. Its jaw had incisors, cuspids (fangs) molars and premolars. Incisors and molars allow the animal to cut and grind up plants. I will concentrate on the molars and a similar tooth called a premolar. The difference between molars and premolars is their size and the number and height of bumps (called crests or cusps) on the surface of the tooth. Genes control the thickness, height and number of these cusps. Premolars are smaller and have fewer cusps than molars. The picture of a Hyracotherium skull on the right shows this as well as the fact that Hyracotherium had three molars and three premolars on each side of its jaw. A gene mutation can cause a premolar to make another cusp and thus become a molar. Whether this is an advantage or a disadvantage depends on the main source of food. Premolars are generally better for eating meat, but molars are better for eating plants.
About 55 million years ago, the world around Hyracotherium changed.
Diet Selects – as plants change having more molars is an advantage. Fossils show that berries and easier to chew plants were replaced by tougher plants. In the Hyracotherium herd there would have been a few individuals where one premolar had become a molar. This meant they had four molars per side rather than three. With 33% more grinding teeth, they would have been able to chew 33% more of the tough plants than the rest of the herd. As the food source changed to tough plants, they would have gotten more food. They would have had more children, each of whom inherited the trait for four molars.

This led to one branch from Hyracotherium called Orohippus. Orohippus lived 50 million years ago and was very like Hyracotherium. It was the same height with the same skull and body shape. It had two differences from Hyracotherium. It had four molars and two premolars and the vestigial toe on the hind foot was now gone. Thus Orohippus had one more "grinding tooth" on each side. Also, the cusps on the molars had become more pronounced. Both changes let Orohippus eat tougher plants. Now only the toughest plants survived and the Orohippus with the toughest teeth ate them.

In the next several million years, Orohippus gave rise to Epihippus when another premolar became a molar. Thus Epihippus had five grinding molars on each side of the jaw. This is a typical evolution pattern of mutual natural selection. Tougher plants selected animals with tougher teeth, and these animals eating the plants promoted survival of even tougher plants. Epihippus had one other change, it had longer legs. How would evolution produce longer legs?

Anyone studying anatomy knows that there are two major genetic controls of bone length. One affects the growth of individual bones; the other regulates growth hormone which affects the size of all the bones. In this figure, A, B and C are different heights only because of differences in the genes determining the leg growth. D and E are different heights because of differences in the gene for growth hormone. In E both trunk and legs are smaller. Humans are a good example of how a species can vary in size. Just look around to see the variety of sizes of people around you. Besides differences in total height you will see that some people have long legs and others have short legs. One environment can mean that being short or tall has no advantage, in such a case the population shows a wide variety in height. Another environment may make shortness an advantage. In such an environment, the population will become short as short persons survive better and have short children. The Pygmies of Africa are an example of the environment selecting for shortness. Shortness was an advantage in their thick jungle environment. Isolation kept them from sharing genes with other populations.

Climate Selects – climate change causes plant change – makes longer legs an advantage. The climate of North America was becoming drier, and grasses were just evolving. The vast forests were starting to shrink.

An animal able to take advantage of the open grasslands would get more food and be more successful. Epihippus, mentioned above, was such an animal. Its longer legs gave it speed in the open and its grinding teeth could handle grass. (Grass is hard on teeth because it has silica that wears them down). The theory of evolution explains how Epihippus could have evolved from Orohippus. Those Orohippus with longer legs and five grinding teeth would have possessed greater speed and better chewing. This would give them an advantage. They could run out into the grassland, grab a lot of food, and then run back to the forest. Their children would inherit the longer legs and extra molar. Natural selection would have then gradually caused a population of animals with long legs and five molars, a new species we call Epihippus.

Predators Select. Having longer legs is great for increased speed, but it makes one taller. It was thus harder for them to hide from predators that also moved out into the grassland. Some populations of these “pre horses” continued to work out from the forest, but nature offered another possible option. Those Epihippus with even greater speed would be able to outrun these predators. This led to Mesohippus.

Mesohippus was slightly larger than Epihippus with even longer legs. The back was less arched, which gave it more speed and greater endurance at speed. The neck and face became longer as the legs lengthened, thus it could still reach food without bending down. Fossils show Mesohippus still lived in the forest part of the time like its ancestors, but it could venture further out into the expanding grasslands.

One group of Mesohippus evolved by becoming larger with longer legs. It became Miohippus. A typical Miohippus also had a slightly longer skull than Mesohippus,. In addition, the ankle joint is changed slightly. This change made it faster in grass. Mesohippus and Miohippus lived at the same time for millions of years. Mesohippus probably spent more time in the forest and Miohippus spent more time in the grasslands.

The forests continued to get smaller while grasslands became more vast. One population of Miohippus underwent a transformation that let it change from browsing to grazing, taking advantage of the new grasses. Grazing means spending all the time in the grasses. Open-country grass grazers can benefit from being swift runners with long legs. Certain trait variations were thus selected.

One variation in these animals was a slight bone change that locked the two lower legs bones together. This eliminated leg rotation but gave stronger forward-and-back strides. Another variation was to stand permanently on tiptoe (another adaptation for speed). Animals with these variations survived to have children who inherited the variation.

Standing on tip toe meant those with stronger toes and stronger ligaments to the toe had a better chance. Fossils show that the ligament to the big central toe became much thicker and became like a spring that was cocked when the foot stepped down and released to push forward. The doglike pads became a strong hoof on the middle toe. This could dig into hard ground. It also made a formidable weapon. The other toes are no longer useful. When a finger is no longer useful, animals where the toe is shrunken have an advantage. The unneeded toes become vestigial toes or disappear. Miohippus had one main toe hoof that bore its weight. All these changes made legs specialized for just one function: rapid running over hard ground. Thus, as carnivores got faster, so did the horses.

All of the above happened from 55 million to 35 million years ago. Evolution never stops, however, so changes continued.

Diet Selects again – One population of Miohippus had variations giving them higher tooth crowns. Small crests on the teeth were larger and connected together in a series of ridges for grinding. These molars, besides being longer, could grow continuously. This means that as the tops were worn down, tooth growth kept the tooth the same length. The picture on the left shows the modern horse version of such teeth. In addition, the tooth crowns became harder due to the development of a cement layer on the teeth. Thus, the teeth changed to be better suited for chewing harsh, abrasive grass. These changes gave rise to the group Parahippus.

The rest of horse evolution in North America is a series of small changes that increase their speed and make them better able to eat grasses. Many variant populations developed and were very successful. During the first major glaciations of the late Pliocene (2.6 Million years ago), certain Equus species crossed to the Old World. Some entered Africa and diversified into the modern zebras. Others spread across Asia, the Mideast, & N. Africa as desert-adapted onagers and asses. Still others spread across Asia, the Mideast, and Europe as the true horse, E. caballus. Other Equus species spread into South America. The Equus genus was perhaps the most successful “one toed” genus that ever lived -- even before domestication by humans.

Much of this information came from:, and as well as Hen’s Teeth and Horse’s Toes by Stephen Jay Gould and Functional Anatomy of the Vertebrates by Walker & Liem.
Here are the fossil skeletons of four of these animals showing comparative sizes.


Things to know:

1. Do not memorize names

2. Know how a gene change can change the number of toes or the size of bones or teeth

3. Know how a population can have a variety of traits (toe numbers, tooth sizes, bone length) and nature can select which ones become predominant

4. Know how a succession of small changes can lead to a very different looking animal

5. Note how the changes in horse ancestors took 20 million years

6. Know how the change in plants led to changes in teeth and the change from forest to grassland led to longer, “one toed” limbs

7. Know how a vestigial trait can arise (e.g. a toe that is not needed becomes smaller)


"Eohippus" was a very small (some species only 18 inches long) and generalized herbivore (probably a browser). Besides the well-known difference in toe number (four toes at front, three at back), "Eohippus" had a narrow elongate skull with a relatively small brain and eyes forward in the skull. It possessed small canine teeth, premolars, and low-crowned simple molars. Over geologic time and within several lineages, the skull became much deeper, the eyes moved back, and the brain became larger. The incisors were widened, premolars were altered to molars, and the molars became very high-crowned with a highly complex folding of the enamel (Evander, 1989; McFadden, 1988).

Figure 5. Fossil horse series from Hyracotherium ("Eohippus") to Equus showing changes in skull proportions associated with an adaptive shift from browsing to grazing. This sequence shows a chronological sequence of genera within the perissodactyl family Equidae from the Eocene to the Recent. (From MacFadden [1992], reprinted with permission of Cambridge University Press).


(After Marsh and Lull.)

1 and 1A, fore-limb and hind-limb of Eohippus; 2 and 2A, Orohippus; 3 and 3A, Mesohippus; 4 and 4A, Hypohippus; 5 and 5A, Merychippus; 6 and 6A, Hipparion; 7 and 7A, the modern horse. Note how the toes shorten and disappear.


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