The evolution of equines: how did horses evolve?

The evolution of the horse is a fascinating subject: it covers over 50 million years of geographical and biological changes.

While such an expanse of time will always leave some questions open and some puzzles unsolved, the horse is comparatively well studied.

This means that there’s a lot of information for you to learn from.

By understanding how your horse developed over time, you can better understand your modern horse’s species-specific needs.

What are the early equids called?

Each twig on the evolutionary horse tree has its own name, some of the major ones including:

• Eohippus, “the dawn horse”, also known as Hyracotherium
• Orohippus, “the mountain horse”
• Epihippus, “the before horse”
• Mesohippus, “the middle horse”
• Miohippus, “the small horse”
• Parahippus, “the side horse”
• Merychippus, “the ruminant horse”
• Pliohippus, “more- horse”
• the Hagerman horse
• Equus (the modern genus which includes horses, zebra, and donkeys)

Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, zebras, and many other species known only from fossils.

The early equids weren’t like modern horses, they were their own unique animals. Some of them were even alive for longer than the existence of modern horses has been so far.

Eohippus, the dawn horse, was the first.

Fossils of Hyracotherium (Eohippus) have been found in the Western US and in Europe.

When the fossils were first discovered, they were thought to belong to a monkey.

19th century British anatomist Richard Owen initially thought that Eohippus fossils proved that primates once lived in England. He later corrected his mistake after studying more specimens, and today Eohippus is the earliest-known member of the equine family.

Specimens found in the United States were given the name “Eohippus” by O. Marsh in 1876. Palaeontologists later determined that Eohippus was really the same genus as Hyracotherium and, based on scientific procedure, the older name Hyracotherium (1840) took precedence over, and includes, Eohippus (1876).

Eohippus gave rise to many branches of the horse family that are now extinct – and some of which were substantially different from the line that leads to the modern equines.

Over the millennia, the evolution of Eohippus into the modern horse saw some common characteristics emerge:

• a reduction in the number of hooves from 14 to 4
• losing the footpads
• the legs got longer
• the independent bones in the lower legs fused together into one bone
• an increase in overall size
• the muzzle got longer
• development of crested and high-crowned teeth which are better suited to grazing
• the brain became bigger and more complex.

The biggest difference to modern horses’ teeth was that Eohippus had large canine teeth.

Eohippus had long incisors and low crowns on the cheek teeth, which shows that it was a browser, feeding on leaves, fruits and shoots from shrubs rather than grass.

Because of this, and because of its small size, we believe Eohippus lived in the forest’s shelter rather than out on the open plains (like its later descendants).

Eohippus’ feet were drastically different from modern horses; its legs ended in padded feet — with 4 toes on the front feet and 3 toes on the hinds — each toe ending in a small hoof.

That’s 14 hooves compared to your horse’s 4 — plus your horse’s feet aren’t padded.

About the size of a fox, with an arched back and raised hindquarters, Eohippus looked so drastically different from the modern horses that the evolutionary relationship between them wasn’t suspected at all.

It wasn’t until palaeontologists unearthed fossils of later extinct horses that the link between Eohippus and modern horses became clear.

The teeth were the first significant evolutionary change for the horse.

Orohippus (~55 to 44 million years ago) and Epihippus (~46 to 38 million years ago) had more molar-like teeth, with a system of ridges along their length.

This helped them with eating a variety of foodstuffs.

An environmental change from forests to grassy plains meant equines needed to adapt to a grazing diet, and have teeth to match.

We believe Epihippus evolved from Orohippus, and continued the evolutionary trend of increasingly efficient grinding teeth.

Parahippus was the first horse to have this adaptation. It had cheek teeth which were adapted for grinding with a side-to-side action, and long crowns embedded in the gumline.

Eating grass wore down the teeth, which meant that they developed to constantly grow.

This is something that’s still happening in horses today and that’s why they need to have their teeth checked and floated regularly.

Scientists have found evidence on fossilised teeth (as tiny scratches) which show that some of these long-toothed animals continued to eat leaves alongside eating grass.

Equus has straighter and longer teeth than Parahippus. Its teeth were covered in a substance called cementum, which makes them harder and better suited for eating abrasive and fibrous grass.

Another major change was the shape of the head.

The earliest equids had short heads, with snout-like noses.

The lengthening of the nose is linked to the change in the teeth.

As the equids needed more space for the new dentition, the shape of the skull expanded to accommodate that and the equids began looking more like our idea of a horse.

The change was gradual (over several million years) but by Merychippus the skull was very similar to today’s horses.

Changing from toes to hooves.

Of all the changes in appearance which have occurred over the millions of years of equine evolution, the toes merging into a single hoof is perhaps best known.

As equines moved to live from forests to open grasslands, the ability to run away from predators became more critical. In the forest, hiding is a common tactic for small prey animals, but for a larger animal with nothing but open space for miles, running is the best option.

This meant that having a single hoof and long legs, rather than several toes and shorter legs, was more advantageous.

The first loss of functional toes and changes in leg anatomy happened pretty early.

Eohippus had four toes on the front legs and three on the hinds. Mesohippus had three toes on each foot with a fourth toe present on the forelimbs, but it wasn’t functional.

In Parahippus, the bones of the leg had already fused together, and while the extra toes were still present, Parahippus bore most of its weight on the central toe.

This change in the lateral toes is where Parahippus gets its name from: “para” is Greek for “side”.

By the time Merychippus arrived on the scene, the footpads had disappeared completely and the lateral toes were minuscule.

The central toe was attached to the lower leg with strong ligaments, which formed the familiar spring mechanism for pushing the hoof forward after absorbing the impact of hitting the ground.

These lateral toes aren’t obvious in modern horses as they’ve been incorporated into the anatomy of the horse; the short splint bones are remnants of the prehistoric lateral toes, and recent research suggests that the frog is also a vestigial toe that found a new function in modern horse’s hoof anatomy.

Merychippus had several three-toed descendants, but it was the one-toed trait that survived. This led to Equus, the group that includes all modern-day equines. Equus displays a further adaptation in the spring mechanism of the hoof, which gives equines the characteristic speed of the modern galloping horse.

The other crucial change occurred in size, and while there was some back-and-forth growth with species decreasing in size and then increasing again, the overall trend was always for a taller, heavier animal.

Merychippus had several three toed descendants, but crucially for the development of the modern horse one inherited the one-toed feature. This led to Equus, the group which includes all modern day equids. Equus shows further adaptation in the spring mechanism, which gives it the characteristics of speed which we associate with galloping horses.

Where did ancient horses live?

The very first equine ancestors lived in a world that was quite different from today. Laurasia was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around 335 to 175 million years ago. This is where eohippus lived, although most of equine evolution happened in what today is North America – also the best place to discover equine fossils.

The environment of early horses was wetter and warmer than today, as there were tropical conditions across much of the world. Horses lived in the forests for tens of millions of years before the landscape finally turned into open grasslands.

A study of ancient DNA from horse fossils found in North America and Eurasia shows that horse populations on the two continents remained connected through the Bering Land Bridge, moving back and forth, interbreeding multiple times over hundreds of thousands of years.

The study highlights the importance of the Bering Land Bridge as an ecological corridor for the movement of large animals between the continents during the Pleistocene, when massive ice sheets formed during glacial periods. Dramatically lower sea levels uncovered a vast land area known as Beringia, extending from the Lena River in Russia to the MacKenzie River in Canada, with extensive grasslands supporting populations of horses, mammoths, bison, and other Pleistocene fauna.

The findings of the study demonstrate the genetic continuity between the horses that disappear from the known fossil record in North America at the end of the last Ice Age, and the horses that were eventually domesticated in Eurasia. As the Bering Land Bridge disappeared, the populations were cut off from each other, and horses weren’t reintroduced to North America by Europeans until the Spanish brought horses with them in the early 1500s. The study has been accepted for publication in the journal Molecular Ecology and is currently available online.

Taylor et al. looked at the genetics of horses across the Old and New Worlds and studied archaeological samples. They found no evidence for direct Pleistocene ancestry of North American horses, but they did find that horses of European descent had been integrated into indigenous cultures across western North America long before the arrival of Europeans in that region.

Accessibility is one reason why wild equines have been found in Asia, Europe, and Africa, but not other continents. The mustangs of North America and brumbies of Australia are feral populations originating from domesticated horses, not indigenous wild animals.

When did modern horses first evolve?

The genus which includes modern horses, Equus, first evolved somewhere between 4 and 4.5 million years ago. Most of equine evolution happened in North America, but horses were domesticated in Europe and Asia, after crossing the land bridge.

During the Pleistocene, horses were common and widespread across Eurasia, with three populations believed to be the ancestral origins of our domestic horses.

The most well-known of these is the Przewalski’s horse, a sub-species which some regard as the only remaining truly wild horse. There is debate around the Przewalski’s horse, as some scientists believe it’s a separate species, and some state that there is evidence it descended from domesticated horses.

Another population is called the tarpan (Equus ferus ferus). This name is of Turkic origin, and was a local term for free-roaming horses in the Russian steppe that came up in the literature in the 18th and 19th century. It’s unknown whether the tarpan represents wild horses, feral domestic horses, or some kind of hybrid of the two. The last individual believed to be a tarpan died in captivity in the Russian Empire during 1909.

In the 1930s there were several attempts to recreate the tarpan. The Heck horse was created by German zoologist brothers Heinz and Lutz Heck. They were unsuccessful at creating a genetic copy of the extinct horse, but instead developed a breed with grullo colouration and primitive markings.

The other population is the forest horse of northern Europe, which may have given rise to the heavy, cold-blooded breeds of that region. The Estonian horse is one of the last surviving breeds that descend directly from the northern type of horse. The breed is relatively pure, as it has not been crossed with many other breeds.

The Estonian horse is considered to be the progenitor of breeds such as the Norwegian Dole Gudbrandsdal and North Swedish Horse.

The domestication of horses.

Around 6,000 years ago, the domestication of horses is believed to have occurred in an area ranging from Ukraine to Kazakhstan, above the Black Sea. However, it is possible that domestication took place independently in different regions, such as Spain and Portugal. This area was a vital refuge for horses during the Ice Age as it remained habitable.

Initially, domestication was for food, meaning humans have been in a predator role in the human-horse relationship much longer than as riders. As domestication spread, genetic analysis indicates that humans incorporated wild horses into their breeding program, introducing new genetic material into the gene pool.

Human selection for desirable traits has resulted in the vast diversity of horses we see today. From miniature horses no larger than early equids to the heavy horses that play a critical role in agriculture and industry.

How are zebras and donkeys related to modern horses?

Zebras and donkeys are closely related to modern horses, as all of them belong to the Equidae family. They share a common ancestry and are believed to have diverged from a common ancestor about four to four-and-a-half million years ago.

While horses, zebras, and donkeys are separate species, they can interbreed to produce hybrid offspring, such as the zorse (a cross between a zebra and a horse) and the mule (a cross between a donkey and a horse), this offspring isn’t fertile. This is why mules and zorses are sterile.

However, despite their close relation, horses, zebras, and donkeys have distinct physical and behavioural differences that reflect their evolutionary history and adaptations to different environments.

What can we learn from the evolution of the horse?

Although domesticated horses differ from their wild ancestors and earlier equids, their evolutionary adaptations have implications for how we manage them. According to researcher Goodwin (2002), wild horse populations live in stable groups and spend most of their time grazing, up to 16 hours a day, covering large areas to find food that varies with the seasons.

The teeth of horses have evolved over millions of years to grind grass, with teeth continuously growing to compensate for the tough food source. However, modern horse ownership often involves long periods of housing, isolation, or frequently changing groups, along with concentrated feed and limited forage.

Therefore, it is not surprising that horses can experience behavioural and physical problems when compared to the conditions they evolved to thrive in over 50 million years. Horses have adapted and evolved successfully over time, developing social structures and physical characteristics that allow them to survive in various environments.

As horse owners, it is crucial to consider their evolutionary needs and replicate their natural conditions as much as possible. Providing ad-lib hay, longer turnout access, species-appropriate companionship and sufficient exercise (both mental and physical) offers several benefits to modern horses.