It is hard to believe that the beautiful horse that we all love started life about 55 million years ago as a little creature no bigger than a pet cat! Officially, it is called Hyracotherium but most people call this creature Eohippus (which means "Dawn Horse" and is much easier to say). It was only 20cm (8in) high at the shoulder — no bigger than a pet cat!
It had four hoofed toes on the front feet and three hoofed toes on the hind feet - in other words, it was an ungulate like modern-day camels and rhinoceros.
Eohippus was the starting point for the long, long journey of evolution and natural selection that resulted in Equus Caballus, the Modern Horse. You can find out more about this journey in the next section ‘From the Beginning', which may also help you to understand horse behaviour today: Why they are a ‘fright and flight' animal; why they have great eyesight; why their sense of smell and hearing are second to none.
During the last ice age (about 20,000 years ago!) there was so much water trapped as ice that sea levels around the world dropped, exposing land that is now far underwater and forming 'bridges' between continents like North America and Asia. Where the Bering Sea is now, the first horses walked across and spread through Russia and East Asia.
Before the first appearance of the domesticated horse it is believed by some that, through the process of natural selection, we had four main types of horse that were well adapted to their environments. This supposedly happened long before people became involved in capturing, breeding and thereby domesticating horses for their own needs. This theory is now disputed. (You can find out more about the original four types of wild horse in the section ‘The First Modern Horse’.)
Domestication of the horse may have occurred at least 6,500 years ago on the steppes (another word for vast grass plains) in Eurasia and the Near East. The earliest evidence of domestication comes from the area we now know as the Ukraine. Here the bones of horses turn up in the garbage heaps of Neolithic farmers. (You can find out more about the domestication of the horse in the section ‘Man and Horse’.)
To understand the development of the horse over such a long period of time it is helpful to understand evolution and the process of natural selection. The most important thing to understand is that the development of the horse is not a straight line from one species to another. It probably looks more like a bush with lots of branches and twigs. Some species were successful and led to more species, while many more were not successful and eventually died out (became extinct). This process took millions and millions of years.
Due to the number of fossils found, the horse has become a focus in many museums around the world to describe and explain Charles Darwin's theory of ‘The Origin of Species through Natural Selection' (other naturalists later changed this theory to ‘Survival of the Fittest').
You can watch a cool David Attenborough video about evolution by clicking the image below:
The most important thing to remember when reading about the long journey from Eohippus to Equus is that at any one time there would have been many similar but different animals living side by side. Over millions of years the ones that were better adapted to their environment survived and those that weren't died out.
From 55m to 36m years ago
55 — 45 million years ago. Early to Mid-Eocene.
Hyracotherium. Also called Eohippus or ‘Dawn Horse’.
Eohippus lived in the forests and jungles that covered the world at this time and its feet were well suited to the soft ground. It had an arched back and a long neck but short muzzle. Because its teeth did not have the hard coating, that modern horses have, it suggests that it fed on soft leaves and fruit. Some scientists also believe that it was also the ancestor of rhinos and tapirs. Eohippus fossils have been found across Asia, Europe and North America.
50 million years ago. Early to Mid-Eocene.
Orohippus - ‘Mountain Horse'
Orohippus evolved either from Eohippus or a similar animal. The earliest fossils found are about 2 million years after the earliest Eohippus fossils. They may well have lived side by side with Eohippus and other similar animals. Orohippus was about the same size but it had a slimmer body and a longer head. Its front feet were slimmer and its back legs were longer, making a good jumper. Its upper back teeth were flatter allowing it to grind its food. This may mean it fed on tougher plants.
About 40 million years ago all equidae (horse family ancestors) died out in Europe and Asia leaving the evolution of the horse to continue in North America.
40 million years ago. Early Oligocene.
Mesohippus - ‘Middle Horse'
The ‘Meso', meaning middle, had longer legs and stood about 60cm (24in) at the shoulder. This is about the height of a large dog. It only had three toes and it stood on the middle toe with the outer toes only being used when it raced away from danger. Its face was longer and larger, the eyes were rounder and set further back than its ancestors. Mesohippus's teeth were also larger and contained a single gap behind the front teeth (this is were the bit goes in the modern horse). It also had an extra grinding tooth. The cranial cavity (were the brains go) was larger and its brain was similar to horse today. During the Oligocene period the climate changed and the warm, damp forests began to die out and were replaced with grass plains. The changes in Mesohippus became a distinct advantage for life on the plains. With their extra height they could see further and run faster while their teeth allowed them to grind the tougher grasses. Mesohippus died out by the middle of the Oligocene period.
36m to 11m years ago
36 million years ago. Early to Mid-Oligocene.
Miohippus – ‘Small Horse’
Miohippus lived in North America. It is believed to have branched off from Mesohippus and they lived side by side for about 4 to 8 million years. Miohippus became larger with longer legs, a longer skull and a slight change to the ankle joint. It weighed about 55kg but compared to the modern horse, which weighs around 500kg, it was still small. Miohippus had two forms, one was a forest dweller and the other remained suited to life on the grass plains. The forest dweller evolved into Kalobatippus whose second and fourth finger again grew longer for travel on the softer forest ground.
24 – 19 million years ago. Early to Mid-Miocene.
Kalobatippus – ‘Stilt Horse’
The Kalobatippus got its name from the elongated bones between the ankle/wrist and the toes. This horse was a ‘browsing’ horse living in the forest and eating leaves and fruit. After crossing the land bridge from North America through the Bering Straits, its descendants gave rise to Anchitherium in Europe and Sinohippus in East Asia. Its height made Kalobatippus stand out from all other horses that came before it.
20 million years ago. Miocene.
Parahippus – ‘Near Horse’
During this period the earth was undergowing huge change. Great grass plains were taking over from forests, which were thinning out or disappearing. Mountains were being pushed up by plate tectonics.
In the theory of tectonics, the outermost part of the Earth's interior is made up of two layers: above is the lithosphere, which includes the crust and the rigid uppermost part of the mantle. Below the lithosphere lies the asthenosphere. And although solid, the asthenosphere can flow like a liquid on geological time scales. The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are eight major and many minor plates and these plates ride on the asthenosphere, moving in relation to one another. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 centimeters per year.
20 million years ago, the swampy ground was drying out and it is believed that all these environmental changes had a huge impact on horse’s ancestors. Leafy foods were becoming scarce therefore the horses that were better adapted to eating grass or grazing, survived and flourished.
Parahippus was similar to Miohippus but slightly larger. Parahippus inherited the extra molar crest on its teeth. This was sometimes found in Miohippus but became a permanent feature of Parhippus. Its molars (back teeth) also developed high crowns and a hard covering. This allowed them to grind grass, which was covered in grit and sand, without wearing their teeth down too quickly. Parahippus had longer legs and face. The leg bones were fused (joined together) so that Parahippus could move forward and back very quickly. Flexible leg rotation was eliminated, as side-to-side movement through dense forests was no longer an advantage to escape predators. It could also stand on its middle toe, which was supported by ligaments under the fetlock to the central toe. With fused leg bones and the ability to stand and move on one toe the Parhippus could run faster across the hard flat ground of the plains. Its height and speed meant it had a good chance of escaping predators and surviving.
17 – 11 million years ago. Mid to Late Miocene.
Merychippus ‘Ruminant Horse’
A ruminant is one of the various hoofed, even-toed mammals such as cattle, sheep, goats, deer, and giraffe who have a stomach divided into four compartments. In the first two compartments, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids then clump together to form a cud. The cud is then regurgitated and chewed slowly to completely mix it with saliva and to break down the particle size.
Merychippus still had three toes on each foot with ligaments supporting the middle toe. It was the first horse to be a true grazer living on grasses. It lived in herds and was the tallest equidae/horse, (48in or 122cm or 12hh, about the size of a small pony today), to have existed up till this time. Merychippus had a longer muzzle, deeper jaw and its eyes were wide apart, much like horses today. Its brain was larger – making it smarter and more agile. Merychippus was the first equine to have the distinctive head of today’s horse. Fossils indicate that there was an explosion of different equidaes at this time.
11m to 4m years ago
13 – 5 million years ago. Mid Miocene to Early Pliocene.
Dinohippus ‘Powerful Horse’
Some references translate the Greek word ‘Dino’ as meaning ‘Terrible’ rather than ‘Powerful’ but we will stick with Powerful. Dinohippus was the most common horse in North America during the late Pliocene period. It is also believed that Dinohippus is the closest relative to Equus, which is the genus of animals that include horses, asses, and zebras. Today all animals in the Equus group have a distinctive ‘stay apparatus’ formed by bones and tendons. This allows the horse to rest and conserve energy while standing up. Dinohippus was the first horse to show a very basic form of this characteristic. From the fossils it would seem that individuals with three toes lived alongside ones with one toe.
6 - 2 million years ago.Late Miocene to late Pliocene.
Plesippus is often considered an intermediate stage between Dinohippus and the present day horse, Equus. At the moment it is being hotly debated whether Plesippus should just be considered a species of Equus. These animals are estimated to have weighed about 425kg, which is roughly the size of an Arabian horse. Although some still had small side toes they lived with other individuals where the side toes were absent and only the splint bone remained, as in modern horses.
Towards the end of the Miocene and throughout the Pliocene period the climate became cooler, drier and more seasonal, similar to our climate today. Ice sheets grew on Antarctica and it was completely ice bound by the beginning of the Pliocene period. Forests continued to recede and were replaced with open grass plains and deserts. Continents continued to drift towards their present positions. South America became linked to North America through Panama and Africa collided with Europe to form the Mediterranean Sea.
Sea level changes exposed the land bridge between Alaska and Asia. At this time, with forests disappearing, the earliest human ancestors (hominids) appeared in the Rift Valley of northeast Africa. They spread throughout most of Africa giving rise to Homo erectus, the common ancestor of both Neanderthal and Homo sapiens: us.
To escape the cooling weather and advancing ice some Plesippus crossed the land bridge from Alaska to Asia and Europe while others escaped into South America across the newly formed land of Panama. A portion also remained in the southern part of North America.
4m Years ago to Modern
4 million – 100,000 years ago. Mid Pliocene to Late Pleistocene.
The oldest species of ‘true horse’ was discovered in Italy and is believed to have evolved from a Plesippus-like animal. They were about 13.2hh (pony size) with a rigid spine, long legs, fused leg bones with no rotation, long nose, flexible muzzle and a deep jaw. Equus had become one-toed with side ligaments that prevented the hoof from twisting and its teeth were adapted to grazing on grasses. Equus Stenonis proliferated into two branches, one lighter bodied and one heavier. About 2 million years ago the lines that would evolve into horses split from the donkey and zebra lines. The evolution of the horse continued throughout America until, it is believed, all horse died out, in the Americas, about 11,000 years ago. At this time many horses escaped over the land bridge from Alaska to Asia for the last time. The reason for the extinction of those that remained is not known, however research is being done to establish whether it was a severe climate change event, disease or human hunting.
630,000 – 100 years ago. Mid Pleistocene to Recent.
This is the direct ancestor to Equus ferus caballus – the modern domesticated horse. It is believed that the various breeds of Equus caballus were developed from several subspecies of Equus ferus across Europe and Asia. This process took a very long time and is currently being studied by archaeologists and geneticists to determine the origins of the modern horse.
Two of these wild subspecies,Equus ferus ferus (the Tarpan) and Equus ferus przewalski (Mongolian Wild Horse) survived until recent times. It was believed that all domesticated horses were bred from possibly four types of these wild horses, but this theory is now being questioned by some geneticists.
(You can find out more about the ‘First Modern Horse ’ in the next section.)
Update: News Articles
Schoolboy Finds Three-toed Horse
First published here: 30 January 2011
Story from horsetalk.co.nz
Gavin Sutter, aged eight from Auburn, California, found a remarkable horse fossil dating back 15 million years. The fossil was of a three-toed horse that lived in the Northern Nevada area during the Miocene period. Gavin was on an expedition with his parents as well as scientists from the Sierra College Natural History Museum when he made the discovery of a three-toed horse fossil.
The bones have now been prepared for display at the museum, however Gavin is allowed to take them to school for ‘show and tell’.
Gavin has always been a keen rock collector, and has declared his find “very cool”.
This fossil fills in a crucial gap in the evolutionary journey of the horse from four hoofed-toes (Eohippus) to the one hoofed-toe that we see today in Equus caballus (the modern domesticated horse).
The team also found bones from early rhinos, turtles, beavers, canaids (the dog family), and badgers. Even though this area is now a desert, with little rain, 15 million years ago it was wetter and had oak and maple trees, and redwood forests. About 4 million years ago tectonic forces started to push the granite rock up to form the Sierra Nevada mountain range, which stretches through California and Nevada. The ‘High Sierras’ are 644 kms (400 miles) long, 105 kms (65 miles) wide and 4,421 metres (14,505 feet) at its highest point. As these mountains were uplifted they blocked the rain from the Northern Pacific Ocean. The change in climate led to a change in the vegetation, which resulted in many extinctions – lions, saber-toothed cats, dog-bear, camels and primitive elephants (gomphotheres).
Horses became extinct in the Americas, about 10,000 years ago at the end of the last Ice Age. Before this time, North America was the centre of horse evolution.