Evolution: Characteristics Dependent on Geography, Climate

While conventional wisdom suggests that horses simply became bigger throughout history and changed from diminutive shrub nibblers to the masterful grass-eating specimens we have today, current evidence seems to discount that theory. Bruce MacFadden, MPh, PhD, an equine forensic paleontologist at the University of Florida, has been working with horse evolution since the 1970s, and he is considered a world authority on equine evolution. He penned an article for the March issue of Science that suggested time wasn't the only factor in the evolution of equids. His fossil horse study suggested horses were adaptable creatures whose size, diet, and distance traveled also depended on such factors as geography and climate.

"Modern-day horses, zebras, asses, and their kin are just a small part (of the equid family)," said MacFadden. "They are the tip of the iceberg in terms of us understanding what horses have been like (throughout history). The family tree of horses is much more complicated than is portrayed in textbooks and museum exhibits today."

MacFadden's research on this topic has been going on for more than 12 years, and it focuses on the chemical makeup of fossilized equine teeth. By analyzing the carbon content of teeth, MacFadden can determine a horse's diet and migration, because some foods were readily available and some weren't. The size of the horse can also be determined, since body size and tooth size are proportional.

MacFadden says scientists once thought the more primitive horses, which lived 55-20 million years ago, were primarily browsers that gave way to grazing animals in the late Miocene period. But this linear evolution might not be as straight as once thought. MacFadden said there are quite a few "branches" to the equine family tree, because some horses became mixed feeders (eating both leaves and grasses) during this period. Also at this time, some horses grew larger while others became smaller or remained the same size.

"Those were probably a combination of horses that were initially adapted to being forest dwellers, then there is a whole new group of animals that became grazers during this time," said MacFadden. "The primary diversity that is seen during this adaptive radiation (the spread of a group of organisms into new habitats) is among the grazers. The grazing group gave rise to modern-day horses."

MacFadden also said this is not yet a widely accepted notion. "I would say, by specialists in the field, people accept this theory," said MacFadden. "For people who are writing textbooks or people interested in general science, it's not widely accepted. The more we can let people know what the true story of horse evolution is, the better."

About the Author

John V. Wood

John V. Wood is an Emmy Award-winning journalist, and now teaches his craft to high school students in North Carolina. Wood has been published in numerous national magazines/newspapersover his career, and published his first book in June 2010. Wood currently lives in Willow Spring, NC.

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