There is a widely held but mistaken notion that the 30,000 wild horses roaming the American West are descendents of Spanish horses brought to the New World in the 1500's. With a few notable exceptions, the truth is less glamorous.

Today's "mustangs" (from mestango-the stray) are mostly feral horses, a mixture of breeds whose gene pools have been enhanced over time by occasional input from released draft, thoroughbred, and Arabian stock. Most of these deliberate releases occurred during the Great Depression. Genuine mustangs, direct descendents of the Spanish horse, are found only in a few herds of northern Wyoming. Nevertheless, nearly 500 years of natural selection has given us a wild horse that can survive some of the toughest conditions anywhere.

In the mid 1800's around 40 million cattle were dumped onto the vast empty of western United States. No surprise, the semiarid steppe of the region soon became overgrazed. Even less of a surprise, local ranchers blamed the damage on the few thousand "exotic" horses that had lived there nicely for 300 years.

Of course, the ranchers were wrong on both accounts. Their cattle stripped the range bare and horses are not exotic to the Americas at all. The species originated in North America some 55 million years ago and migrated thence to the rest of the world. Although the Pleistocene temporarily vanquished the horse from its native land, we can thank the Spanish Conquistadors for correcting that mistake.