The Horse's "Tale"
Most scientists agree that the dog-sized, multitoed early horse (Eohippus, the "Dawn Horse") first appeared in North America more than 55 million years ago. Eventually, various horse species migrated into Asia via the Bering land bridge. About 10 million years ago, the horse family (Equidae) reached both its height of diversity and its maximum numbers: The old and new worlds were crowded with coexisting equid species that browsed forests and grazed plains.
|Over the course of more than 50 million years, the horse evolved from a dog-sized, three-toed forest dweller to a grazing animal with one large toe. Hooves protecting the horse's foot are particularly well suited for running.|
© JOEL HANSON, WILDHORSEPHOTOS. COM
Between 8,000 and 12,000 years ago, at the end of the last major Ice Age, all horse species died out in the Americas as part of a mass extinction that also eliminated such mammals as the woolly mammoth and the sabertooth cat. While scientists are not certain what caused these disappearances, climate change and overhunting by humans seem to be the most likely explanations.
Horse fossils uncovered by 19th-century paleontologists appeared to provide evidence of direct lineage from the Dawn Horse, through transitional species, to the very different- looking modern horse genus, Equus (which includes horses and burros as well as zebras). However, modern scientists now know that horse evolution did not proceed in a straight line. In fact, there are so many side branches in the horse's "family tree" that it might more aptly be called a "family bush." Traits evolved at different rates and not always together. In some cases, multiple species split off from one ancestor and then coexisted with that ancestor and with one another. Certain species arose gradually, others emerged suddenly.
Horses, along with rhinoceroses and tapirs, belong to the order Perissodactyla—hoofed animals with an odd number of toes. While most early horses had three toes, modern horses have only one large toe on a streamlined foot, which permits great running speed. The size and shape of horse teeth also changed considerably over the millennia as the animals began to specialize in eating grasses and other abrasive vegetation. Modern horse teeth feature high crowns and efficient grinding ridges and are able to grow gradually out of the jaw as their tops wear down.