Wild Horses: Evolution
Origins of the Horse
The very first horses evolved on the North American grasslands over 55 million years ago. Then, they deserted North America and migrated across the Bering land bridge into what is now Siberia. From there, they spread west across Asia into Europe and south to the Middle East and Northern Africa.
In the 1870s, 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. He thought that through a series of clear intermediates, one could see the fossil species, Eohippus, transform into an almost totally different-looking (and very familiar) descendent, Equus.
But horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and even 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 what we know as "horses". Kathleen Hunt has produced an Equus evolutionary tree that graphically shows the development of horses.
Then, about 8000 BCE, succumbing to climate change and human hunters, horses completely vanished from North America. This migration map is 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. In 1000 BCE, the first horses were domesticated and used for transportation for both humans and cargo. 500 years later, Persian officials began using mounted couriers for message-relaying. Horses had become an integral part of human life.
It wasn't until the 16th century, when the Spanish came to conquer the New World, that the horse was reintroduced to North America. These small, sturdy mounts once again spread quickly throughout the Americas.
Watch this video interview with Dr. Michael Voorhies about the evolution of the horse. Dr. Voorhies, an equine specialist, is Curator of Vertebrate Paleontology at the University of Nebraska Museum and is in charge of their fossil bone collection. His research interests are paleontology and geologic history of Nebraska and the Great Plains, especially the history of warm-blooded mammals who roamed the area.
ORIGINS OF THE HORSE