Horse has been a long period in human history and over the period through the domestication of these animals has certainly brought small changes through evolution. The time with human domestication might brought those changes but it certainly not enough time that explains how horses have developed over millions of years. The family Equidae has a single genus called Equus, which has members such as E. caballus (true horses) and non-caballus (asses, zebras) (1). There are three groups of horses which are forest, grassland and savanna which is mixture of small woods and pastures (2).
A horse is the member of the species Equus caballus, the common domestic horse, member of the genies Equus (include domestic and wild horses) and any member of Equidae (and all their ancestors and extinct close relatives like “eohippus”). Equidae is a family that is associated with odd toes ungulate, and recent mitochondrial study have found two genera in this family, one is caballines (modern horse and zebras, donkeys (2). There geological time scale pulls 55 million years of evolutionary period that starts from small dog sized animal Eohippus that strive in grasslands to modern horse (2).
This geological time pulls 55-million-year-old history that has been recorded through fossil records. This has been accomplished with 35 ancient Equid specimen that has been acquired from South and North America, Europe, Asia and Southern Africa (9). It has been important discover which shows a great change in macroevolution. The environment has certainly made a large contribution to the evolution of genus Equus. One of the important example is the changes in teeth structure has been modified through the diet of C4 and C3 plant material. The molars are specially focused to grind cellulose structure that are found in these plants. The evolutionary period can be further subdivided into early equids, true equids and modern equids based on their genus (1).
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The early equids include Eohippus, Orohippus, Epihippus, Mesohippus and Miohippus. The true Equids include Parahippus, Hipparion, Pilohippus and Dinohippus. The lastly it leads to modern Equids that has been evolved from Dinohippus. The evolutionary studies on horses has been extensively done weather considering molecular biology or understanding of remains of early ancestors. One of the most crucial aspect that is preserved for evolution of horses are the feet and the teeth as seen in Figure 1. The anatomy of these two structural bones have made significant changes over time, which allow clear identification on how horses have developed through natural selection and how their environment have changed them over time.
The time period on equids have been studied very well, and the main indication has been developed through the fossil record. One of the most crucial time period to be consider is with early equids starting with genus Eohippus which are early equids found in North America from time period of 52 million years ago. One of the most important finding of Eohippus are its teeth, which are considered hypsodont teeth and complete moralized second and third molar. (3) Hypsodont teeth is pattern that is high crowned teeth which has an enamel passed the gum line, and this kind of pattern provides wear and tear of the food.
The identification of these teeth represent that the animal was available on grasslands. Another very closely related genus Hyracotherium, which has its fossils found in Europe and Asia. It was considered a four towed and low crowned animal, which is very different from modern horse who is single toed (4). The Hyracotherium genus was short animal no larger than a sixty centimeter. As discussed earlier they were four hoofed animals, and each hoofed had pad on its underside. The face of these animals were short when compared to modern horses and their eye socket were in the middle rather than on sides (5).
The second genus was Orohippus which have lived after Eohippus 50 million years ago. The Orohippus was evolved over 2 million years after Eohippus, it was similar in size but had elongated mouth, slimmer fore limbs and longer hind legs (6). The illustrated that it can run longer and jump higher than its previous ancestor. Along with ability to run faster it was still based on four limb pad foot but the main difference was that outer toe that was found in Eohippus was not presented in Orohippus. Furthermore, teeth were also a signicatn finding that indicated a major difference between the Eohippus and Orohippus. The fossil record indicated that Orohippus diet was heavily focused on tougher plant material due to the crest of the teeth was much stronger and premolar were shaped much like molar for grinding (7). After another 3 million period there was a evolutionary change that bring the next genus which was Epihippus.
Although there was not much stronger fossil record on this genus, current work suggested that the genus continued to devolved strong crest teeth for more grinding. Another genus that developed 40 million years ago was Mesohippus. Mesohippus was much taller that its previous ancestors, and just like its ancestors it still had pad foot. The most significant feature about its foot was that its third digit was much larger than other two. This showed a significant change in the foot, that allowed to run in open grassland. The fossil record of the skull also represent some changes specially when it bring from the eyes which were much wider, compare to Eohippus which were in front and closer. Another significant change that front teeth were low crowned and there were total of 6 teeth in mouth that helps with grinding (8).
This was one of the major developmental changes that can bring about and how closely related this genus was to modern horse. The development of four toed padding was continually changing over time toward a single toe that can show that how hoofed toe would develop. This is heavily affected by the open grasslands of both North America and European Central. The strict vegetarian diet and especially toward tough material has brought about changes in the teeth. The crowing of the teeth had made a significant impact when chewing needed to be done to extract the nutrients. The next major change that happened in the genus about thirty six million years ago and genus that had evolved was Miohippus.
The strong evidence suggested that Miohippus have coexisted with Mesohippus which proving that it had anagenetically evolved with Mesohippus, and eventually Mesohippus had disappeared over time (10). Miohippus is much larger that Mesohippus and also had longer skull. It still had three toes but there was much more emphasis on the third digit was much larger, stronger and higher density. The third digit was mostly in contact with the ground and the other two digits came in contact with the ground is when the animal was running. Miohippus has brought one of the significant changes that had lead toward Equus. After Miohippus there is subbranches of evolution, one leading to true Equines which were single toe and high crowned teeth. The other were three toed such as Megahippu, who are not able to survive and continue through the evolutionary tree.
So, focusing on the true Equines, there are many genera were found across many countries such as Asia, Africa and North America. One of the most important genus was Merychippus, that was considered to be found in North America. The fossil study and DNA sample suggested that Merychippus was part of adaptive radiation (1, 11). This is the process from which an organism from previous generation diversify to many new forms, based on environmental challenges, resources and new environment itself. The open grasslands and woodland area, had brought changes that allowed this animal to have hoofs and jaw was longer for grazing.
The fossil indicated that molar has more oval shape and the anterior posterior enamel are more rounded (11). The hard woods were changing to grasslands, this means that animal has to run which gave hoof to increase the surface area. The hoof did not have the pad at the bottom, and the foot was attached to ligaments. As disused earlier that Merychippus was going through adaptive radiation, this had given rise to three verities which was Hipparion, Protohippus and Pilohippus (1). Hipparion fossils have been discovered throughout North America, Asia and Europe. One of the most important fossil came from Mediterranean region which is 11 million year old (12). Hipparion has been existing for 22 million years, dating from 23 million years to 780,000 years ago.
The fossil dating using carbon 13 and oxygen 18 had determine that the enamels were shaped in such a way that Hipparion was grazing on C4 plants (13). The C4 plants are those plants that uses calvin cycle to fix carbon dioxide into four carbon sugar and the plants itself is highly nutritial growing hot climate such as Mediterranean and Spain. Hipparion is still has small hoof and two vestigial toe that does not touch the ground. Another Miohippus that was further developed was Protohippus, that was about the size of donkey and had lived 6 million years ago. One of the specimen from fossil record that has been pulled came from South Dakota, suggesting that Protohippus had large orbital fossa and lacrimal is also large (14). This indicating that they had large skull, there was increase in height on the tooth crown, suggesting more toward grazing. They also had broader muzzles, and this pushed toward dietary selectivity due to the changes in the muzzle length.
Pliohippus is another genus that has evolve through adaptive radiation, and it has existed twelve million years ago. It was similar to Equus, although there were some general characteristics such as stubs that are extra toes that are visible externally near the hoofs. The legs were slim, which shows that it was running animal that can quickly made dodge when escaping as a prey. It had curved teeth compare to modern horse had straight teeth, and furthermore the skull had deep facial fossa which can also be compared since modern horse don’t have that deep facial fosses. Another genus that diverged from Pliohippus and form new genus was Dinohippus about 10 million years ago and eventually Pliohippus became extinct genus (15). One of the most important finding from North America in Santa Maria was Dinohippus mexicanus who fed on C4 and C3 plants and lived in open zones (16).
Dinohippus were the first in the linage of the Equus that had monodactyl, which means that it had a single toe with hoof. The reason why the monodactyl would occur is due to the increase in limb length in order to increase the stride for longer distance (17). The single complete digit allows stability and improve efficiency while running increasing flexion and extension. This is the reason why Dinohippus are direct descendent to Equus. There is still large differences, such as the foot padding is much smaller in Dinohippus, it had stay apparatus formed from bones to help stay up to conserve energy. Dinohippus has been a successful through the evolutionary period, which had allowed it transient toward modern Equus. There have been two subgenus that has been found that allowed this transaction one in North America is Plesippus and in Eurasia was Allohippus (18).
One of the important fossil that has been found was in China of Equus eisenmannae that is 1.85 million years old. The fossil allows direct comparison to the Equus that are currently seen. E eisenmannae is found to be of very large size that had skull of around 590 millimeter, and the skull had shallow orbital fossa. The molar teeth had deeper valley and more elongated teeth this is very close description to current Equus. The palatal length has been changing from Plesippus to E. eisenmannae and then to current Equus, this is suggesting that there is transition and evolution in the lineage of the Equus. Although E. eisenmannae is not an immediate ancestor of modern Equus but the transition can be seen and how the anatomical structure had aligned with the environment of Equus.
This genus leads forward E. caballus which is modern horse with foot padding that has four toes fused into one toe. The ligaments and longer muscles have improved the stride of the horse to run longer and faster. The molar teeth are much more thicker enamel than its previous ancestors and also molars are wider for grazing. The changes in skull and fossas had allowed to date back to previous ancestors and how the linage had continued. There are however other changes that has not been fully discussed for example how did climate came into effect the evolutionary period, the changes in skin color and did the domestication had any impact since the domestication is being done since 3000 BCE.
These are some of the analysis that can be further applied and studied. The structure of the hoof is certainly that has developed over time, the structure is made out of protein called keratein. The keratin structure of the wild horse is maintained through their terrain. The domesticated horses however does need to be maintained. The second anatomical structure that has progressed over time is teeth, there are 12 incisors that allows the grass to be pulled and molar of the horse has certainly changed that allowed more grinding. The major diet of the Equus caballus has been in focus on C4 plant material, the reason for that is provides a lot of sugar that horse muscles heavily work on. The instent sugar can be easily converted in metabolism especially in electron transport chain cycle to maximize the build of ATP.
The grass which is readily available source of C4 plant has definitely impact the evolution of horses and it is certain that the bone structure of the teeth need to be specifically designed to break the cellulose structure of the grass. In summary, the genus Equus had able to survive and evolve through environmental changes that had allowed them to benefit and fit according to evolutionary rules. The time period required for the change was very extensive that allowed 55 million years of changes. Although molecular analysis is extremely crucial, but fossil record has also given a direct comparison on how the organism has changed over time.
One of the most important factor that had allowed this direct comparison of the bone structure was the comparison of the legs and toes. The early ancestor started as four toed animals and over the time period bones have fused to single toe with padding to allow improvement to run in open grasslands. The bone structure of teeth had also been an important factor, since animals diet on C3 and C4 plants had given the teeth to crush and extract maximum amount of nutrients. The macro evolution of the Equus has what made the developments and changes that allowed modern Equus to survive 55 million year of history.
