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.
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.
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.
In other words, horse evolution was never as straightforward as Marsh's smooth fossil sequence suggested.
Kathleen Hunt, a biologist at the University of Washington in Seattle, said the modern-day horse is "merely one twig on a once flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived."
MacFadden, the study author, agrees that horse evolution was, in fact, a pretty messy affair—a jumble of evolutionary processes such as random genetic variation and natural selection.
"Any changes in morphology [physical form and function], such as in tooth or limb evolution, can be explained within this framework," he said. "Equine mammals are adaptable critters whose size, diet, and range depended on geography and climate."
Horse teeth, which preserve well as fossils, provide evidence to support this. For instance, when grassland habitats became far more extensive around 20 million years ago, horses with teeth that were adapted to browsing forest vegetation declined. But horses with high-crowned, or tall, teeth for grazing on grasslands flourished.
"Grasses are highly abrasive and tend to increase tooth wear," MacFadden said. "A higher-crowned tooth is better adapted to feeding on grasses." Horses living today have such teeth.
Chemical analysis by MacFadden to determine animal diet and the shape of later fossil teeth, however, indicates that some species went back to eating leaves as well as grasses.
Likewise, ideas about horse body size have also changed. The popular notion that horses started off the size of small dogs and grew progressively bigger is now shown to be false.
From the tooth-fossil evidence, MacFadden found that during an explosion in horse diversity some 20 million years ago, many species got smaller as well as larger. He was able to estimate their body size because teeth are proportional to the sizes of the horses the teeth once belonged to.
These findings contradict what is known as Cope's Law, an idea based on the work of 19th-century paleontologist Edward Drinker Cope. The law states that within any group of animals there is a tendency for descendants to grow progressively larger.
"There are so many exceptions where you go from small to large and back to small again that you have to ask how many exceptions to the rule you can accept before the central concept is no longer correct," MacFadden said.
John Flynn, mammal-fossil curator at New York City's American Museum of Natural History, said MacFadden's findings are important because horses have been one of the mainstays of evolutionary studies.
And MacFadden himself says there is so much more to learn from these textbook examples of evolution.