The environment is always changing. In order to survive, a species must also adapt to this environment. From bacteria to reptiles, almost all species have undergone evolution according to natural selection. Today, the most information has been found on the evolution of the modern horse, equus.
60 million years ago, a short (40-50 cm) animal roamed the ancient forest. This animal was Hyracotherium-the earliest relative of the modern horse. Hyracotherium had four toes on its front legs and 3 toes on its hind legs and a short snout. Its teeth had rounded projections and its eyes were set more in the front of the head. From the projections of the teeth we can deduce that Hyracotherium was a fruit eater. The eye placement shows that it had good depth perception.
Perhaps Hyracotherium survived by using the depth perception to find fruit to eat. After Hyracotherium (about 36 million years ago) a new species evolved-Mesohippus. This species was taller and now had three toes on both its front and back legs. Unlike Hyracotherium, the snout of Mesohippus was longer and the eyes were spreading further apart. This probably happened because peripheral vision was becoming more important (for spotting predators) than depth perception. Also, the molars had sharper crests.
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Visual sensation differs from visual perception even though the two processes form a sequence. Our brain obtains meaning from what we look at through the process of sensation and perception (Grivas Down & Carter pg 77). Though the two systems interact they both serve a different purpose. Sensation is a physiological process which involves the " reception and transduction of information and its ...
This indicated a possible change in environment that caused Mesohippus to eat a diet of rougher food. Next, Merychippus evolved about 20 million years ago. This species was about 90 cm tall. Also, its feet formed into a hoof and the lower leg bones fused together. This hoof supported most of Merychippus’s weight.
The eyes have moved even further back into the head allowing for even greater peripheral vision. Changing environmental factors could have made food even more difficult to find and Merychippus more vulnerable to predators. Increased height could have helped Merychippus reach more food. Only 14 to 17 million years ago the species Pliohippus evolved. Pliohippus was 2/3 the size of a modern horse and its skull was in the same proportion.
At this time in evolution, each of the legs had lost all the toes and formed into a single hoof. This hoof was better adapted for running fast across hard land away from predators while the toes of earlier species like Hyracotherium were better adapted to walking through dense forests and marshes. Today’s modern species of horse, Equus, was a little bigger than Pliohippus. Both had eyes set so far back in the head that they allowed for great peripheral vision. About 10, 000 years ago Equus thrived in North America. Unfortunately, they became killed off by humans until they became extinct.
However, Equus survived in Europe. While the continents were still together as Pangea, Equus and its relatives migrated to other parts of the world. Therefore, all the horses we own in America today are not native to our soil but brought here from the Spanish. When one looks at one of these beautiful, large creatures it is hard to imagine that its ancestor was a short, fruit eating animal with toes..
