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Origin and Evolution


If you retrace the lineage of the modern horse back to its pre-Equus beginning, you will arrive at:

Oehippus – 55-50 mya (million years ago) – Wyoming
Orohippus – 53-45 mya – Wyoming, Oregon
Mesohippis – 37-32 mya – Colorado, Great Plains, Canada
Miohippus – 32-25 mya – Great Plains, western US, Florida
Parahippus – 24-17 mya – Great Plains, Florida
Merychippus – 17-11 mya – in most of US
Pliohippus – 12-6 mya – Colorado, Great Plains, Canada
Dinohippus – 13-5 mya – throughout N. America 
Plesippus – 3.5 million years ago – N. America and Eurasia
And finally,
Equus – 5 mya – worldwide except Australia & Antarctica

This means that the horse species originated in North America, and eventually spread into Asia and then Europe, including Spain. Unlike their brethren that stayed behind in N. America, these equine migrants to Asia and Europe survived. Those that settled in the Mongolian steppes became the Mongolian Pony, and those in Spain eventually became the Spanish Barb. Of course the two are just different breeds of the same species.

Unfortunately indeed, at the end of the last ice age (about 14,000-10,000 years ago), those wild horses that had remained in N. America were driven to extinction along with the other Megafauna (Woolly Mammoth, Saber-Tooth Tiger, Giant Ground-Sloth, Cave Bear…), presumably by humans.

This begs the question whether these native American horses and the distant Spanish horses had evolved into different species or not. I doubt that they did, but it doesn’t matter in terms of determining whether or not the Mustang is an invasive species.

In the 15th Century, the Spanish Conquistadors invaded the Americas, bringing their Barb horses with them, and their cattle. Some of these Barbs escaped or were set free, became feral, adapted to the land, and eventually became the truly wild Mustang.

Some may yet argue that the Spanish horse could be a direct descendant of Plesippus, not of Equus, but the wild horses in N. America, of Asia, and of Europe, and therefore Spain, were all Equus, and have all evolved from a common equine ancestor that originated in N. America. The Mustang, a descendant of the Spanish Barb, is therefore a direct descendant of the original and unquestionably native N. American Equus. The Mustang line merely took a round-the-world tour and has come back home to roost.

On the other hand, The modern American cattle did not originate in N. America at all, but in Asia and Europe, in which light then it is the real invasive species, and should be removed like any other, at least from public land – rightful home of the Mustang, and other native species such as deer, pronghorn antelope and bison, but not of the modern American cattle.

Since Columbus

The first horses from Spain first arrived in the Americas at the end of the 15th Century via the second voyage of Columbus in 1493. The native people incorporated horses into their cultures as of the 17th Century. By the 18th Century, most plains native tribes had horses. Concurrently, there were European horses infused from the east, comprising “French”, “Norman” and “Canadian” varieties. Some of these drifted west and were distributed farther by the native tribes via their horse-raiding and horse-trading activities. And of course some of these also escaped and became intermixed in the gene pool. At some point, even Thoroughbred blood was introduced. All in all, the American Mustang is genetically not homogeneous, which is expressed by a range of phenotypes, even including draft horse types in some herds, whereas other herds do exhibit the original Spanish horse characteristics.

Population-wise, it exploded some time in the 18th Century. Of special significance is the year 1769, when the Spaniards, who had made major inroads into California with large numbers of horses, established missions, ranches and permanent settlements. As of that point, the horse population in California rose drastically. By 1800, 24,000 horse were reported to exist in California. By 1805, the killing of unwanted horses to prevent overpopulation had become commonplace. In all, this was the second ever culling operation for population control in US history. Since California was somewhat isolated by the Sierra Nevada from the rest of the country, the Californian development did not much affect population developments elsewhere.

The first culling operation took place in Texas around 1787. Texas had been receiving horses from Mexico as of the 16th Century, and had its horse population boosted in 1686 when Alonso De Lyon’s expedition brought 700 horses, followed in subsequent years by thousand from various other sources. Some deliberately left horses and cattle on the range to feed themselves, while other horses strayed. By 1787, these animals had become overpopulated for the resources available. A round-up gathered nearly 8,000 “free-roaming mustangs and cattle.” By the late 1700s, the largest numbers of wild horses were found in what today are the states of Texas, New Mexico and Oklahoma.

By all accounts, which seldom fully agree, the wild horse’s peak population occurred around the cusp of the 18th and 19th Centuries, when guestimates ranged from 2 million and 5 million in America, with Texas topping the list at up to 1 million, and the rest shared by the other ten wild horse states

In the early 20th Century, the wild horse numbers declined drastically due to large numbers of them being captured for use in the Spanish-American War and WW1. In the 1930s, the total population left is estimated to be 50,000-150,000, meaning that the land could support a lot more. In 1946, the General Land Office (GLO) and the United States Grazing Service (USGS) combined to form the Bureau of Land Management (BLM) to manage the public land. The BLM, at least its ex-US-Grazing-Service component, along with the Forest Service (FS), began promptly to remove wild horses from the land they administered. Public capturing of Mustang became rampant. By the 1950s, the Mustang population had dropped to no more than 25,000.

Between 1946 and 1959, no law existed to protect the wild horses and burros. Capture methods by government and public included use of motor vehicles and airplanes, and poisoning of water holes.

In 1959 the first federal free-roaming horse protection law came into effect, popularly known as the “Wild Horse Annie Act”, prohibiting the use of motor vehicles and aircraft for capturing free-roaming horses and burros. Still, capturing or killing of Mustangs were not ruled out.

In 1971, the definitive no-kill [Wild and Free-Roaming Horses and Burros Act]  (WFRHABA) was passed, which essentially prohibited the direct killing of horses and burros by the BLM and FS, and the selling of them without restriction. At that point there were estimated to be approximately 17,300 horses and 8,000 burro (25,300 in total) on the BLM-administered lands and 2,039 horse and burros on the FS-administered lan, totaling 26,000 bureauwide. The BLM established Herd Management Areas (HMAs) and Appropriate Management Levels (AML – meaning Carrying Capacity) for each HMA, coincidentally (?) setting the total AML at 26,000.

In 2017, the population had risen to 72,000; in 2018, up to 82,000, and in 2019 or 2020, topping 100,000, meaning that there are some 70,000 in excess of the AML of 26,000 set by the BLM. Meanwhile, the expensive short-term corral-type holding facilities, capacity ~45,000, and the less expensive longer-term holding pastures, capacity ~30,000, have been by and large filled, meaning that any more captures and the captives will have nowhere to go but the slaughterhouse.

The rest will be future history.

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