Three million years later, after the first Orohippus, evolved a new species called the Epihippus. It continued the trend of greater teeth grinding ability. It had five grinding, low-crowned teeth with defined crests.
As the climate became drier in North America 40 million years ago, new environments began to form and grasses evolved (Critters-2-Go, 2007). About 40 – 37 million years ago the Mesohippus developed in response to these new environmental pressures. It was one of the most widespread mammals in North America and was slightly larger than the Epihippus, at 61 cm in height. It had a straighter back and walked on three toes on each foot, but carried the weight on its middle toe. It had a longer face, snout and neck and had larger cerebral hemispheres. There was a small shallow depression on their skull, called a fossa, which in later horses became quite detailed. The Mesohippus has six grinding teeth with a single premolar in the front, a trait that all equids would have, and retained the sharp tooth crests to grind tough vegetation.
Around 35 million years ago, the Miohippus evolved. The population split off from the Mesohippus population and coexisted for around 4 million years before eventually replacing the Mesohippus. The Miohippus was larger and had a deeper facial fossa. It continued to thrive in population and rapidly evolved and branched into two major groups, one that lived in forests and the other remained on grasslands (Florida Museum of Natural History, 2007).
The Miohippus population that remained on grasslands developed into the Parahippus, which was approximately the size of a small pony. They lived in North America and had a long skull with a facial structure similar to that of modern horses (Tufts University, 2007). Its third toe was still the strongest and largest, carrying the weight of the animal. The four premolars resembled the molars, though were relatively small. The incisors had crowns with some having a shallow depression.
Approximately 17 million years ago, the Merychippus evolved from the Parahippus. It had wider molars believed to be used for crunching harder grasses (Draper, J. 1999). It grew to around 100 cm tall and was a grass eater, not a leaf eater, as it completed the transition from living in forests to grassy plains. The hind legs were relatively short and the two side toes had very small hooves that probably only touched the ground when running (Florida Museum of Natural History, 2007). The legs became more rigid and more effective at carrying weight as equids evolved into a larger animal.
The Merychippus descended into three equids: the Hipparion, Protohippus and Pliohippus. The Pliohippus evolved around 15 million years ago and is believed to be the ancestor of modern horses. It retained the two extra toes on both sides of the hoof, thought to have been barely visible externally. It had a deep facial fossa similar to that of the Mesohippus (Critters-2-Go, 2007).
Thirteen million years ago, the Dinohippus evolved from the Merychippus and was the most common horse of its time. It still had the two outer vestigial toes and the facial fossae shallowed slightly. However, later fossils showed that the Dinohippus had only one toe, suggesting it evolved during this period (Florida Museum of Natural History, 2007).
The Plesippus progressed from the Dinohippus and was the species before Equus evolved. Around 2 million years ago, the North American climate began to cool and animals were forced to migrate south. A group of Plesippus journeyed to South America, another crossed the Bering Strait to Europe and Asia and the other stayed in southern North America (Critters-2-Go, 2007). The ice age of that time occurred, spreading and receding five times (Wikipedia, 2007).
The first Equus progressed from the Plesippus in Europe, roughly 4 – 2 million years ago. It crossed to North America; however, all horses that lived in North America became extinct approximately 11 000 years ago along with many other American mega fauna (Critters-2-Go, 2007). Possibilities for extinction include climate change, pandemic or hunting from the newly evolved humans. The earliest Equus had a rigid spine, long neck, legs and nose and a deep jaw. It had a slightly larger brain than the Dinohippus and maintained the facial fossa, one toe and similar teeth. Equus species still retain the gene for growing side toes, however are usually only expressed as splint bones (Hunt, K. 2007).
The species diversified into 12 new species and migrated and evolved to different corners of the globe (Hunt, K. 2007). This included some entering Africa, evolving into zebras, others spreading to Asia and northern Africa, developing into donkeys. The one-toed Equus remained highly adaptable, with horse species surviving in Europe, Asia and Africa. Equus ferus evolved between 630 000 and 320 000 years ago (Critters-2-Go, 2007). They adapted to colder dry climates of northern Asia, and later evolved into the Tarpan and Przewalski’s Horse subspecies.
The Przewalski’s Horse is the closest living wild relative of the domestic horse, Equus caballus. It differs genetically from the domestic horse as it has 66 chromosomes instead of 64 (Draper, J. 1999). They are intractable by nature and cannot be trained for riding; therefore it cannot be domesticated and remains a wild horse. Przewalski’s Horses have a bigger head, stockier build and shorter neck than that of the domestic horse and are smaller than the modern Thoroughbred. They are herbivores and are able to endure harsh environmental conditions (Draper, J. 1999). They once inhabited the plains and grasslands of Mongolia, but as their numbers declined they grouped in the southern part of the Gobi Desert. They were rediscovered in 1879 by a Russian explorer, Nikolai Przhevalsky, after whom they were named and the last true wild Przewalski’s Horse became extinct around 1950. (Taronga and Western Plains Zoo, 2007)
Currently, many external pressures are affecting the Przewalski's Horse’s survival. The causes of the decreasing horse population include climate change, pollution, deforestation, desertification, urban sprawl and even due to human technology. As climate change increases its effect on the earth, animals such as horses will have to adapt. The Przewalski’s Horse may have to migrate north to a climate that is similar to its current habitat, and may become smaller and stronger to survive the warmth. Also as the world increases in population, there is increasing competition for land between animals and humans. The Przewaslki’s Horse will most likely experience behavioural changes due to less land resulting in denser populations. The population of Przewalski’s Horses dwindled during the early 1900s due to increasing competition from livestock for pastureland. Fifteen horses were bred in captivity in the 1950s at zoos around the world. Western Plains Zoo in Dubbo, New South Wales, has been successfully involved in breeding Przewalski’s Horses since 1983 (Taronga and Western Plains Zoo, 2007). In 1995, horses from different zoos were transported to Mongolia and were successfully bred in the wild. There are now between 500 and 1500 currently living in the wild, thought it is still endangered. The release of these horses into the wild will reinforce and improve the likelihood of survival through the genetic viability of the small herd (Taronga and Western Plains Zoo, 2007).
The horse family remains a classic example of evolution, with an extensive line of animals dating back 55 million years ago. The Przewalski’s Horse has had a long and complicated ancestry and is the closest and only wild relative of the domestic horse alive today.
Bibliography
Text
Draper, J. 1999, The Ultimate Enclyopedia of Horse Breeds & Horse Care, Sebastian Kelly/Anness Publishing, Oxford.
‘Horse’, 1990, The World Book Encyclopedia, vol. 8, World Book, Inc. Chicago.
Internet
Critters-2-Go. Prehistoric Horses. [Online] Available 21/08/07
Encyclopædia Britannica Online School Edition. Evolution of the Horse. [Online] Available 21/08/07
Florida Museum of Natural History. Fossil Horse Cybermuseum. [Online] Available 13/08/07
Hunt, K. Horse Evolution. [Online] Available 08/08/07
Taronga and Western Plains Zoo. Przewalski Horse. [Online] Available
Tufts University. Horse Evolution Over 55 Million Years. [Online] Available 24/08/07
Wikipedia. Evolution of the Horse. [Online] Available 22/08/07
Wikipedia. Przewalski’s Horse. [Online] Available 30/08/07
Annotated Bibliography
Judith Draper
Judith Draper compiled an encyclopaedia on horses and their care in 1999. Her information on evolution and the Przewalski’s Horse is brief and under detailed, though provides a starting point or back reference for this topic. The information is somewhat relevant, though not the horse care, is authentic and accurate.
The World Book Encyclopedia
The World Book has been published for many years, though the book used was published 17 years ago. The information provided on the horse and its evolution was basic but essential and also provided the starting point for the topic. It is a very reliable and authentic resource, though its accuracy may be questioned after seventeen years.
Critters-2-Go
Critters-2-Go is a relatively unheard of source that may be unreliable. It has detailed information on the evolution of the horse that is very similar to that of Wikipedia. It appears that either Wikipedia copied off this website or vice versa. The information given is relatively easy to read and is fairly accurate when compared to other resources. The information is relevant to the evolution of the horse.
Encyclopædia Britannica Online School Edition
Encyclopædia Britannica Online School Edition is a website that is reliable for a variety of different topics. However, like many other encyclopaedias, it can be brief and broad and not written by someone who has expertise in the field topic. It appears to be authentic and accurate; otherwise any plagiarism or inaccuracies would be discovered as it reaches such a great global scale. The relevance of the information given is appropriate for this topic as it explains the different stages of evolution, though some passages are unnecessary or basic.
Florida Museum of Natural History
Florida Museum of Natural History is the website of a museum in Florida and is reliable. The information given is similar to that from other sources; however it is also very brief and broad. The data seems to be authentic in that the museum may have conducted research itself or have partnerships of trading information with other museums. The information given is also similar to other sources and is relevant to the evolution of the horse. It also provides interesting diagrams and pictures showing the gradual evolution of the horse.
Kathleen Hunt (Talk Origins)
There is no apparent explanation to who Kathleen Hunt is, or what her link to the evolution of the horse is on this website, but she provides the information. The information like those above is similar to other resources and appears authentic and accurate. The relevance of her information outweighs other sources as it gives are more detailed, understandable and believable evolution of the horse particularly through the past two million years.
Taronga and Western Plains Zoo
Taronga and Western Plains Zoo are two connected zoos of New South Wales, Australia that conduct research and breeding on horses and in particular the Przewalski’s Horse. Western Plains Zoo was one of the zoos in the world that participated in the breeding in captivity of Przewalski’s Horses and introducing these horses back into the wild in Mongolia. Their information seems first hand and easy to read, and the information given is relevant for a description of the Przewalski’s horse and for its survival.
Tufts University
Tufts University is a private research university in Boston, USA. It provides a brief but accurate portrayal of a timeline of the evolution of the horse. Also provided are bones of each stage of the evolution. A university website would seem reliable and accurate, and it is similar to other information given on other sources.
Wikipedia
Wikipedia is the renowned encyclopaedia for having information tampered with by amateurs and corrected by staff. Its information is fairly and should only be used to give a basic idea of the topic, and other resources should be used to build upon or clarify this information. The information seemed to be correct when compared to other sources but is usually added by unqualified people who have copied information from other websites. Importantly, it had much of the horse anatomy incorrect or inaccurately named.