Evolution of the horse

The evolution of the horse, a mammal of the family Equidae, happened over a geologic time scale of 50 million years. It changed from a small, dog-sized, forest-dwelling Eohippus to the large, single-toed, modern-day horse. Paleozoologists have pieced together the outline of the horse's family tree better than any other animal. Much of this evolution happened in North America, where horses originated. Horses died out there about 10,000 years ago and returned in the 15th century.

The horse belongs to the order Perissodactyla (odd-toed ungulates). These animals have hooved feet and an odd number of toes on each foot. They also have mobile upper lips and a similar tooth shape. This means horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls appeared in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. These animals were first suited to life in tropical forests. While tapirs and rhinoceroses kept their forest traits, modern horses adapted to life on the steppes, which are drier and harsher than forests or jungles. Other species of Equus live in many different kinds of places.

The early ancestors of the modern horse walked on several spread-out toes. This helped them on the soft, moist ground of old forests. As grass appeared and grew, the equids began eating grasses instead of leaves. This wore down their teeth, so their teeth grew bigger and stronger. At the same time, as the steppes appeared, running became important to escape predators. Horses got faster by growing longer legs and lifting some toes off the ground. This shifted their weight to one long toe, 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. Over time, scientists have learned that the story of horse evolution is more complex than first thought.

Before odd-toed ungulates

Phenacodontidae

Phenacodontidae is the most recent family in the order Condylarthra believed to be ancestors of 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. They could run well and had tails almost half as long as 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.

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. 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 in many ways.

Miocene and Pliocene: true equines

The Miocene and Pliocene times brought many changes to horses. One important horse was Kalobatippus. It lived in forests and had long front toes for soft ground. From Kalobatippus came Anchitherium. This horse 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 good for eating tough grasses. Merychippus came after it. This horse had wider teeth for grinding hard grasses and legs that helped it run fast. Three groups came from Merychippus: Hipparion, Protohippus, and Pliohippus. Hipparion had special teeth and slim legs with three toes. Pliohippus looked a lot 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 grew colder.

Modern horses

Main article: Equus (genus)

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

Scientists think 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 places. By the last ice age, many kinds of horses lived in North and South America. Recent studies show that most of these were just different versions of one main species, E. ferus.

Genome sequencing

Early studies showed that Przewalski's horse is genetically different from modern horses. This means it is not a wild version of them. Later studies suggested they split from modern horses about 43,000 years ago. In 2013, scientists studied the DNA of a horse that lived over half a million years ago. This was the oldest animal DNA ever decoded. It 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. This happened along with many other large animals. Scientists think this was 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. People began to domesticate them then.

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. They 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 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.