The horse has been invaluable to humans since it was first domesticated in Central Asia some 6,000 years ago. Its speed and strength was harnessed to help us hunt prey, fight wars, work fields, and generally broaden our horizons. Without the horse, the course of human history might well look very different today.
Less well known is the important role played by horses in shaping our understanding of a much deeper history—long-term evolution in animals.
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Writing this week in the journal Science, paleontologist Bruce J. MacFadden said the evolution of horses involved many more twists and turns than previously imagined.
Modern steeds did not follow a relatively smooth transition from the diminutive, foxlike forest browsers that were their earliest ancestors to those impressive, open-plains athletes we know today. Rather, horses fluctuated considerably in form and size over time.
MacFadden, who is the vertebrate-paleontology curator at the Florida Museum of Natural History in Gainesville, said horses have proved especially popular with evolutionary scientists.
"There is a long, continuous fossil sequence of horses extending 55 million years in North America, providing the tangible evidence to trace individual steps or changes in evolution over a prolonged period," he said.
In 1876 Thomas Huxley, a distinguished British biologist and a close friend of Charles Darwin, was introduced to U.S. paleontologist O.C. Marsh and his large collection of horse fossils.
The fossils came from fossil-hunting expeditions in the western United States. While horses went extinct in North America some 10,000 years ago, horses originated there before dispersing around the globe. They were reintroduced to North America by Spanish explorers in the 1600s.
With Huxley's publicity, Marsh's collection soon came to be seen as a classic example of how an animal's evolution could be traced back through a single line.
This sequence, from the earliest forest dwellers to modern-day savanna zebras, has since been reproduced in countless textbooks and natural history museum exhibits.
Family Tree
Later fossil discoveries in the 20th century suggested the evolution of modern-day Equus (the genus that includes domesticated horses, zebras, donkeys, and asses) was far more complex. Its ancestral tree sprouted numerous branches. Many of these led to species that no longer exist.
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