Evolution of the horse

The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the large, single-toed, modern-day horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal. Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago, before being reintroduced in the 15th century.

The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life in the climatic conditions of the steppes, which are drier and much harsher than forests or jungles. Other species of Equus are adapted to a variety of intermediate conditions.

The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist ground of primeval forests. As grass species began to appear and flourish, the equids' diets shifted from foliage to silicate-rich grasses; the increased wear on teeth selected for increases in the size and durability of teeth. At the same time, as the steppes began to appear, selection favored increase in speed to outrun predators. This ability was attained by lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.

History of research

Wild horses have been known since ancient times in places like central Asia and Europe. When Europeans first arrived in the Americas, they brought horses with them, and these horses soon formed large wild groups. Early scientists thought this showed that animals in the Americas were less advanced, but later ideas changed this view.

Scientists have found many fossils that help us understand how horses changed over millions of years. One important early discovery was made in Paris in the 1820s, where a tooth was found and identified as belonging to an ancient horse-like animal. Later, during a trip to South America, Charles Darwin found horse fossils, which helped show that horses had lived there long ago before disappearing and then returning with Europeans. Over time, scientists have learned that the story of horse evolution is more complex than first thought, with many different kinds of horses existing at the same time and changes happening in different ways.

Before odd-toed ungulates

Phenacodontidae

Phenacodontidae is the most recent family in the order Condylarthra believed to be ancestral to the odd-toed ungulates. It includes several types of small animals that lived long ago in Europe from the Early Paleocene to the Middle Eocene. These animals were about the size of a sheep and had good running skills, with tails that were almost half the length of their bodies.

Eocene and Oligocene: early equids

Main article: Eohippus

Main article: Orohippus

Main article: Epihippus

Long ago, about 52 million years ago, a small animal named Eohippus lived in the forests of North America. It was about the size of a fox, with a short head and neck, and it had long legs that were just starting to be good for running. Eohippus had many teeth that helped it chew soft leaves and fruits. Over time, its teeth changed a little to help it eat different kinds of plants.

Soon after, around 50 million years ago, Eohippus changed into another animal called Orohippus. Orohippus looked similar but had a slimmer body and longer legs, which made it good at jumping. Its teeth also changed, becoming better at grinding tougher plants.

A little later, around 47 million years ago, Epihippus appeared. This animal had even better teeth for grinding plants, and it was only about 2 feet tall.

As the climate in North America became drier and forests turned into open areas with grasses, the animals changed too. By about 40 million years ago, Mesohippus appeared. It had three main toes on each foot and longer legs, which helped it run faster to avoid danger. Mesohippus was a bit larger than Epihippus and had teeth that could chew tough grasses.

Not long after, around 36 million years ago, Miohippus showed up. It was larger than Mesohippus and had even better teeth for grinding plants. Miohippus marked the start of a time when these early horse-like animals began to change and diversify in many ways.

Miocene and Pliocene: true equines

The Miocene and Pliocene epochs saw many changes in horse evolution. One important animal was Kalobatippus, which lived in forests and had long front toes good for soft ground. From Kalobatippus came Anchitherium, which traveled to Asia using the Bering Strait land bridge and then to Europe. Larger horses like Sinohippus, Hypohippus, and Megahippus evolved from Anchitherium.

Another important horse was Parahippus, about the size of a small pony. It had stronger legs and teeth better for eating tough grasses. Merychippus followed, with wider teeth for grinding hard grasses and legs that helped it run quickly. Three groups came from Merychippus: Hipparion, Protohippus, and Pliohippus. Hipparion had unique teeth and slim legs with three toes. Pliohippus looked very much like modern horses but had extra toes and curved teeth. Later, Dinohippus became common in North America, and Plesippus was an early form close to today’s horses, Equus. Some Plesippus moved to Eurasia when the climate cooled.

Modern horses

Main article: Equus (genus)

The genus Equus, which includes all living horses, donkeys, and zebras, evolved from an earlier form called Dinohippus. One of the oldest known species is Equus simplicidens, which lived about 3.5 million years ago and was found in Idaho. This genus quickly spread around the world.

Scientists believe the common ancestor of all modern horses lived between 5.6 million and 4.0 million years ago. Over time, horses changed and adapted to different environments. By the time of the last ice age, many different kinds of horses lived in North and South America. Recent studies show that most of these were just variations of one main species, E. ferus.

Genome sequencing

Early studies showed that Przewalski's horse is genetically different from modern horses, meaning it is not a wild version of them. Later studies suggested they split from modern horses about 43,000 years ago. In 2013, scientists sequenced the DNA of a horse that lived over half a million years ago, the oldest animal DNA ever decoded. This helped confirm that all modern horses, donkeys, and zebras share a common ancestor from about 4.5 million years ago.

Pleistocene extinctions

Horses lived in North America until about 12,000 years ago when they went extinct, along with many other large animals. Scientists think this happened because of sudden climate change, which made their favorite grasses disappear, or because the first humans hunted them. In Europe and Asia, horses reappeared in archaeological sites about 6,000 years ago, and people began to domesticate them.

Return to the Americas

Horses returned to the Americas with Christopher Columbus in 1493. They were brought to many places, including Mexico, Brazil, and Peru. Over time, some of these horses escaped or were lost and formed wild herds called mustangs.

Details

See also: Limbs of the horse

The ancestors of today's horses walked on the end of their third toe, with the second and fourth toes smaller and on the sides. We can see evidence of these smaller toes in the bones of modern horses, called "splint bones." These bones help support the joints in the legs.

The teeth of horses also changed a lot over time. Early horses had short, bumpy teeth for eating many types of food. Over millions of years, their teeth grew longer and flatter, which helped them grind plants better. At the same time, the shape of their heads changed, their necks grew longer, and their bodies became bigger.

Further information: Horse teeth

Further information: Equine coat color

Early horses might have had a single color coat, like a sandy brown. Over time, many different colors appeared, including black and spotted patterns. This variety in color became more common after humans began to raise horses.