THE EVOLUTION OF THE HORSE
How did our horse develop into the animal we see today?

Orthogenesis is the traditional view of evolution. It is the inaccurate theory that evolution runs in a straight line. Its followers believe that each stage of development results in a superior species, and it has been the opinion of many until more recently. Traditionally the following represents the evolution of the horse:

However this theory has been disproved and it is illogical that the development of a species should be so ordered and exact. In fact, over millions of years many species have co existed and died out and "development" has actually been reactionary due to necessity for survival, often as a result of ecological and climate changes. Many species were victims of their own accomplishments and although they survived successfully (some such as the Hyracothercium which existed for most of the Eocene period) they were unable to adapt to the changes in their environment which meant they inevitably died out.

Originally forest dwellers these animals were designed to eat from bushes and trees and are known as browsers. Mostly their diet was leaves and soft fruits, hence their teeth were Bunodont and much like ours today. Their necks were shorter and they had spines which moved and limbs which rotated for easy manoeuvring around the forest. These creatures more closely resembled dogs or the small munt jack deer; they had pads on their feet with 4 toes at the front and 3 on the back.

Significantly as climate changes progressed the forests began to dry up and reduce in size, thus forcing these animals on to the grassy plains. It is here that we see considerable changes begin. The neck became longer as these animals became grazers and had to reach the ground to eat (although Browsers and Grazers do overlap and co exist), the face shape changed to accommodate the muscles required for grinding food and the Hypsodont teeth. The Silica (and mud) in grass was much tougher than the diet of the browsers and the softer Bunodont teeth was unable to cope. Hypsodont teeth were stronger, taller and had cementine which binds the enamel. These teeth also had more cusps which were joined and they over time became chypselodont.

Limbs and the spine also changed; the spine became rigid with interlocking lumber processes (this is first seen in the Parahippus) enabling them to propel forward rather than moving in and out of the forest, darting to hiding places from predators. In the Parahippus we begin to see the "springing foot" which is an arrangement of tendons that allows the energy to be stored and reapplied with every stride, helping them to go faster, again to escape from the predators they were now exposed to. Toes began to reduce which more weight being taken on the central toe, the pads became harder and thicker to cope with the harder ground, and the central pad today still exists as the frog in modern Equus. Again designed for forward propulsion was the legs, in the Merychippus we see the radius and the Ulna fused which prevents the leg rotation, likewise we also see the fibula of the shin reduced. They adapted for the forward straight motion which was now needed for running on the open plains, and the legs become longer for speed.

Another incorrect theory that arises from the traditional view is that of size and that the size of the horse has increased over time to the size we have today. Previous to the Miocene period average bodyweight was approx 25-50 Kilos but around 18mya branches developed that ranged from 75-500 Kilos. Some of these animals grew bigger over the course of evolution while others grew smaller, and like the other changes they can probably be attributed to the need to be faster getting away from the predictors they were now exposed to and also as a way of conserving energy. Horses need to cope with more fibre than ruminates and they depend on a diet heavy in cellulose, this is broken down by bacteria into something that the horse can digest. Large animals lose heat slower than smaller ones and the energy demands of each cell is less so getting larger was a way of coping with the new high fibre low protein diet.

Horses from the beginning had larger brains to their fellow herbivores and it is possible that the development of the larger brain is not only for control of the body and speed etc but also due to the increased need for sensitivity to the lips, necessary for both browsers and grazers to find and select food.

Another development important for both survival and the conservation of energy was the "stay apparatus". Horses or any large animal cannot get up from the floor easily or quickly, by being able to lock thier legs horses can sleep standing but be ready to flee as soon as danger threatens. It has also been shown that a horse sleeping standing conserves approximately 10% more energy.

There appears to be certain trends that lead to the Modern Equus as described above (lengthening of the neck, nose and limbs, changes in teeth, spine and brain etc.), however it should be clear that not all branches of the species followed this trend, and in effect as far as the evolution of the horse is concerned there is no actual trend.

It is the line that leads to and from the Dinohippus that is thought to lead to modern Equus, as it is one of the 1 toed species which had the straighter teeth and facial fossae reminiscent of our horses of today. But as will be shown in a diagram below there were many branches and examples of cladogenesis. The Miocene period being a very important stage for the evolution of the horse and the existence of many genera. Below is a simplified version of the actual evolution of the horse:

As the diagram above shows, the equine hoof has gone through some dramatic changes in the last 55 million years. The "hoof" originally was not actually a hoof at all but had "toes" as shown above, with pads. The limbs were originally rotational allowing them to move easily left, right, forward and sideways in the depths of the forest which was their habitat, enabling them to hide from predators. As their environment changed and they was forced from the forests onto the grassy plains there became a need for speed for survival. No longer did they need their limbs to rotate, ideally they needed to run in a straight line away from their hunter. To do this many changes took place.

Legs increased in length

No muscle is found beneath the knee and hock, making them lighter and faster

The Radius and Ulna fused to prevent rotation of the limbs

Pads became harder and thicker to cope with harder ground - eventually becoming our modern horses frog

Outside toes reduced with more weight being taken on the central toe

The spine became rigid

Interestingly one of the categories that Equids are identified by are as perissodactyls, these are members of hoofed animals that bear their weight on the 3rd central toe, these include tapirs and Rhinos. The most modern Equids are known as Equines and are descendants from the Parahippus.

Many of the changes to hooves and legs can be seen in the Merychippus, the legs of this horse were built for one function, speed when escaping from danger. It is this behaviour we see in our own horses who frequently react with the desire to "take off" when something scares them. The change from pads to our current hoof enables the horse to deal with hard ground, while the composition of the foot absorbs concussive forces. The shape of the foot is weight bearing and supports the horses size. The changes to the hoof and legs makes for a great part of what enables the horse to be such an impressive athlete.

Members of the Equus family still retain the genes for the side toes as seen in their ancestors, although these are normally seen as splint bones, rarely a horse is born with small but fully formed side toes.