The Evolution of the Horse
The very first horses evolved on the North American continent over 55 million years ago. Over millions of years they roamed the grasslands slowly extending their range to most of the continents on earth. Then horses migrated across the Bering land bridge from North America into what is now Siberia. From there, they spread across Asia into Europe and south to the Middle East and Northern Africa.
Our understanding of the evolution of the horse has evolved as well (see timeline below). Palentologist Kathleen Hunt has done a good job of outlining this evolution on her "Talks Origin" site. In the 1870's, the paleontologist O.C. Marsh published a description of newly discovered North American horse fossils. At the time, very few transitional fossils were known. The sequence of horse fossils that Marsh described (and that T.H. Huxley popularized) was a striking example of evolution taking place in a single lineage. Here, through a series of clear intermediates, one could see the fossil species Eohippus transforming into an almost totally different-looking (and very familiar) descendent, Equus. Biologists and interested laypeople alike were justifiably excited.
But horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and occasionally reversed "direction." Also, horse species did not always come into being by gradual transformation (anagenesis) of their ancestors; instead, sometimes new species split off from ancestors (cladogenesis) and then coexisted with those ancestors for some time. Some species arose gradually, others suddenly. Overall, the horse family demonstrates the diversity of evolutionary mechanisms. The most modern equids (descendants of Parahippus) are called equines. Strictly speaking, only the very modern genus Equus contains "horses." Kathleen Hunt has produced an Equus evolutionary tree that graphically shows the development of horses.
Then, about 8,000 B.C. -- succumbing to climate change and human hunters -- the horses vanished from North America completely. Click here to view the migration map, a visual representation of the migration patterns of the horse and their eventual reintroduction to North American soils.
Meanwhile, across the sea, horses were becoming a fixture of many ancient civilizations, and establishing their place in human history. In 1,000 B.C. the first horses were domesticated and used for transportation of both humans and cargo. 500 years later, Persian officials began using mounted couriers for message relaying. Horses had become a part of human life.
It wasn't until 1500 A.D. that the horse was reintroduced to North America when the Spanish came to conquer the New World in the 16th Century. Their small, sturdy mounts spread quickly, once again, throughout America.
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(Note, this will take you to RealNetworks. PBS is not responsible for advertising content on external pages.)To learn more about fossil horses on this continent view the video interview with Dr. Michael Voorhies. Dr. Voorhies is Curator of Vertebrate Paleontology at the University of Nebraska Museum and is an equine specialist and in charge of the fossil bone collection at the state museum. His research interests are paleontology and geologic history of the state of Nebraska and the Great Plains, especially the history of warm blooded mammals who roamed the area. The following is an excerpt of an interview with Voorhies about the origin of the horse. In this video segment he talks about the evolution of the horse and one spectacular archeological site. To find out more about the Ashfall
archeological site click below.
• Ashfalls article by Mike VoorheisClick on the timeline text below to read more information about the evolutionary stages of the early horse.
For more information and additional links on this topic visit The Talk Origins Archive.
