Are wild horses truly “wild,” as an indigenous species in North America, or are they “feral weeds”—barnyard escapees, far removed genetically from their prehistoric ancestors? The question at hand is, therefore, whether or not modern horses, Equus caballus, should be considered native wildlife.
The question is legitimate, and the answer important. In North America, the wild horse is often labeled as a non-native, or even an exotic species, by most federal or state agencies dealing with wildlife management, such as the National Park Service, US Fish and Wildlife Service, and the Bureau of Land Management. The legal mandate for many of these agencies is to protect native wildlife and prevent non-native species from causing harmful effects on the general ecology of the land. Thus, management is often directed at total eradication, or at least minimal numbers. If the idea that wild horses were, indeed, native wildlife, a great many current management approaches might be compromised. Thus, the rationale for examining this proposition, that the horse is a native or non-native species, is significant.
The genus Equus, which includes modern horses, zebras, and asses, is the only surviving genus in a once diverse family of horses that included 27 genera. The precise date of origin for the genus Equus is unknown, but evidence documents the dispersal of Equus from North America to Eurasia approximately 2–3 million years ago and a possible origin at about 3.4–3.9 million years ago. Following this original emigration, several extinctions occurred in North America, with additional migrations to Asia (presumably across the Bering Land Bridge), and return migrations back to North America, over time. The last North American extinction probably occurred between 13,000 and 11,000 years ago (Fazio 1995), although more recent extinctions for horses have been suggested. Dr. Ross MacPhee, Curator of Mammalogy at the American Museum of Natural History, and colleagues, have dated the existence of woolly mammoths and horses in North America to as recent as 7,600 years ago. Had it not been for previous westward migration, over the 2 Bering Land Bridge, into northwestern Russia (Siberia) and Asia, the horse would have faced complete extinction. However, Equus survived and spread to all continents of the globe, except Australia and Antarctica.
In 1493, on Columbus’ second voyage to the Americas, Spanish horses, representing E. caballus, were brought back to North America, first in the Virgin Islands, and, in 1519, they were reintroduced on the continent, in modern-day Mexico, from where they radiated throughout the American Great Plains, after escape from their owners or by pilfering (Fazio 1995).
Critics of the idea that the North American wild horse is a native animal, using only selected paleontological data, assert that the species, E. caballus (or the caballoid horse), which was introduced in 1519, was a different species from that which disappeared between 13,000–11,000 years before. Herein lies the crux of the debate. However, neither paleontological opinion nor modern molecular genetics support the contention that the modern horse in North America is non-native.
Equus, a monophyletic taxon, is first represented in the North American fossil record about four million years ago by E. simplicidens, and this species is directly ancestral to later Blancan species about three million years ago (Azaroli and Voorhies 1990). Azzaroli (1992) believed, again on the basis of fossil records, that E. simplicidens gave rise to the late Pliocene E. Idahoensis, and that species, in turn, gave rise to the first caballoid horses two million years ago in North America. Some migrated to Asia about one million years ago, while others, such as E. niobrarensis, remained in North America.