"The western U.S. is riddled with fossils of prehistoric "horse-like" species..."
There are horses living out on public lands in the vicinity of Santa Fe. They are seldom seen as they are wary and avoid people. They behave like deer or elk do with regard to people—a long glimpse is mostly what you'd see before they disappear. What are they? Where did they come from?
Since I was trained as a biologist, I was taught to use certain words within very specific definitions. "Native" meant that a creature existed in a place prior to man's arrival, or came to be there without human intervention in anyway. "Wild" meant that it was born there, perhaps had no human contact as an individual, but the species was introduced from elsewhere either intentionally or by accident. And "feral" meant it was an escapee, usually recent, from human husbandry.
With our horses, it's even more complicated. Horses, you see, evolved in America. The western U.S. is riddled with fossils of prehistoric "horse-like" species, some of which survived to become Equus, the genus which includes modern horses, asses and zebras. During the Pliocene Epoch (5 - 2.5 million years ago), Equus and it's cousin Hipparion were widely spread across North and South America. They also migrated across the Bering land bridge and colonized central Asia. During the same epoch, humans crossed from Asia to North America.
At the end of the Pliocene, Epoch North America became drier, warmer and less lush. As the landscape changed, the land became less hospitable to many of the plants and creatures living here. Mammoths, giant bison, prehistoric camels and horses found it harder to survive. The early human inhabitants also hunted all of these for food. By the end of the Pliocene, Epoch none of these species remained in North America except as bones. So there are no living native horses (by the biological definition) here.
In Eurasia and Africa, these animals continued to evolve into a variety of species: horses, asses, and zebras. They were also hunted for food but for whatever reason continued to survive at least some of them. Horses that were once solely food were captured and used as work beasts. They were domesticated by selective breeding for certain traits; the modern horse as we know it is the result of that intense, long-term association with human civilization. Although there are a few ass and zebra species unmodified by man, the only wild, native horse is Przewalski's horse, in Mongolia, which has an extra pair of chromosomes compared with our horses and has never been successfully domesticated.
But back to North America. When Don Juan Onate moved north from Mexico in 1598 to establish his settlement at San Gabriel (now Chamita, New Mexico), he brought 7,000 animals including horses and burros, some of which were breeding stock. Over the following years, horses escaped, were stolen, or abandoned (as during the Pueblo Revolt in 1680).
These escaped horses formed bands and multiplied; natural selection and the starting gene pool gave rise to a tough, small horse called a "mustang." However, it's also well known that horse escapes and abandonment have continued over time and muddy the notion of a "pure" mustang or other horse which can trace its roots solely to the horses of colonial times. There are numerous proponents in New Mexico of "pure" lineage testing, and certainly some validity to their claims, but unfettered horses don't check pedigrees when looking for a mate. If somebody new shows up in the neighborhood, he/she is fair game.
"Wild" horses that carry a brand, saddle scars, are gelded, or routinely hang out with people don't strike me as very wild, either. You'd be surprised how many there are, escaped or abandoned by owners who no longer want them. They might be loose on public land, but they're feral.
Which brings me to this hot-button topic: in this, our third year of drought in landscapes which are arid in good years, can our true, native wildlife compete against wild/feral horses for dwindling food? Even if all owned, claimed domestic stock (read cows) are removed, there are limits to the carrying capacity of a landscape. And horses are very good at making more horses.
I love my equines (I'm currently owned by one) and find the notion of free-range horses romantic. But I guess I love the native wildlife too much to sacrifice them for more horses. What do you think?