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Pleistocene Horses of North America

The Pleistocene epoch occurred 1.8 million to 10,000 years ago.  It is often called the "Ice Age" because several different glaciations occurred during its time, each separated by warmer "interglacial periods."  The last glaciation ended about 10,000 years ago and is known as the Wisconsinian Glaciation.

By the beginning of the Pleistocene, there was only one genus of horses, Equus, still remaining in North America.  Although horse generic diversity was low, horses were still very abundant animals and continued to numerically dominate ungulate communities in North America (Guthrie 2003).

Fossil deposits from the mid- and late-Pleistocene of North America usually contain remains of two horses: a caballine horse and a stilt-legged equine.  Both forms belonged to the genus Equus, but were from genetically distinct lineages (Weinstock et al. 2005). 

The now-extinct stilt-legged horses had long, graceful legs similar to the extant Asiatic wild asses (e.g. onager).  The resemblance, however, was apparently the result of parallel evolution, caused by adaptation to similar arid environments.  DNA studies show that the stilt-legged lineage was genetically separate from Asiatic wild asses and was endemic to North America (Weinstock et al. 2005).

Some Pleistocene fossils from North America have been assigned to various species of extinct and extant Asiatic wild asses (Winans 1989; Eisenmann 1992; Azzaroli 1998).  However Weinstock et al. (2005) suggest that some or all of these fossils are really stilt-legged equines that have been misidentified.  Clearly more study is needed to determine if Asiatic wild asses ever inhabited North America.

Another important question is how many Equus species lived in North America during the Pleistocene.  During the nineteenth and twentieth centuries, over 50 different species were described by paleontologists, based mainly on variation in the sizes of  fossil bones.  However, modern-day caballine horses are extremely variable in size, so there is a consensus among scientists today that the number of Pleistocene species should be reduced (Winans 1989; Azzaroli 1998).  Weinstock et al. (2005) have even gone so far as to suggest that there were really only two horse species present in North America during the mid and late Pleistocene: a caballine and a stilt-legged equine.  Both species, they suggest, showed great geographic variation in size because of adaptation to different environments.

A recent study of caballine fossils in the northern hemisphere reveals that those of late Pleistocene times belonged to two clades: (1) an endemic North American group, and (2) a Holarctic group found in both North America and Eurasia (Vilß et al. 2001).  The familiar domestic horse of today comes from the second clade (see below).

Extinction of Horses in North America

After over 55 million years of evolution and residence in North America, horses became extinct there.  This extinction occurred either in the late Pleistocene or early Holocene.  (The Holocene is the period of time we live in now.  It began after the Wisconsonian glaciers melted, roughly 10,000 years ago.)

The extinction of North America's horses occurred during a time period when many other large mammals throughout the world also became extinct.

It is hard to find agreement in the literature about terminal dates.  KurtÚn and Anderson (1980) reported a dating of 8,000 years ago for horse fossils from Alberta, Canada, but MacFadden (2005) writes that North American horses became extinct roughly 10,000 years ago.

In Alaska, stilt-legged horses became extinct about 31,000 years ago, while caballine horses became extinct about 12,500 years ago (Guthrie 2003).  Interestingly, Alaskan caballines showed a precipitous decline in body size before extinction, and vanished 1,300 years before woolly mammoths (Mammuthus primigenius) became extinct in the same area (Guthrie 2003).

Various theories have been proposed to explain the extinction of horses and other large mammals from North America.  The most popular hypotheses are climate change, overhunting by humans for food (overkill hypothesis), or a combination of the two.

Because climate change often causes alterations in the abundance of many other organisms, such as food plants, disease vectors, predators and competitors, extinction scenarios involving climate change can be diverse and involve many different mechanisms.

Although still unproven, the overkill hypothesis is a plausible explanation and should be given serious consideration. However, the reader needs to be wary of the political agenda of some of its supporters, and the dubious conclusions that they derive from it.

For example, the overkill hypothesis is sometimes cited as "evidence" that American Indians living in historical times, thousands of years after the extinctions, did not believe in preserving the environment.  No thinking person would argue that modern-day Scandinavians have all the beliefs and values of their Viking ancestors one-thousand years ago, yet overkill proponents routinely make this dubious argument about American Indians for even longer periods of time (7,000-10,000 years).

The overkill hypothesis is also used by extremists to argue that America's wilderness parks should be opened up for mining and other commercial development, on the grounds that Indians allegedly destroyed the pristine character of these areas thousands of years ago by killing off their horses and other large mammals.  This line of reasoning is similar to the unethical argument that once a woman has been raped its ok to do anything else to her because she is no longer "unspoiled."

Some researchers propose that North American caballine horses did not become extinct, and instead persisted until historical times (Clutton-Brock 1981).  This hypothesis, while intriguing, is not generally accepted because:  (1) No horse bones from the late pre-Columbian era have been found to support the idea, and (2) no indisputable images of horses have been found in late pre-Colombian American Indian "art."  Furthermore, when the Spanish arrived with their horses to Mexico in the 16th century, the Aztecs and other educated peoples of that region did not initially understand what horses were.  All horses found today in North America are thus believed to be descended from horses brought to the New World from the Old World after the year 1492.

Domestication of Caballine Horses

When horses became extinct in the New World, some species of Equus still survived in the Old World (e.g. zebras, wild asses and caballines).  Their ancestors had dispersed there years earlier via the Bering Land Bridge, which connected Alaska to Siberia during periods when sea levels were lower.  Many of these horse species are still extant, however most surviving species are now endangered.  They were summarized at the beginning of this review.

Throughout the Holocene, wild caballine horses continued to range across the grasslands of Europe and Asia.  Approximately 5,000 years ago, wild caballines were captured at numerous locations in this vast geographic area and domesticated by diverse peoples, as the knowledge and technology for capturing, taming and riding horses spread (VilÓ et al. 2001; Bendrey 2012).

Thus, the domestic horse of today originated not from one local population of wild horses, but from numerous populations spread across Eurasia (VilÓ et al. 2001; Bendrey 2012).  Only one of these original wild caballine populations still exists. It is known as Przewalski's Horse (Photo 8, Photo 9).  (Przewalski was a famous Russian explorer-biologist of the 19th century).

Many different species of animals have been domesticated by humans, so it is important to remember that there are varying degrees of animal domestication.  Caballine horses today are among the least domesticated of all farmyard animals.  Unlike other domestics, most horses are in practice wild, fully capable of living and surviving without any aid from people (Zimov 2005).  Yet this remarkable animal has profoundly influenced human history (VilÓ et al. 2001; Bendrey 2012).

North America's Place in Horse History

As we have seen, North America was the birthplace of the horse family and the theater in which almost all its evolution took place. During the 56 million years of horse evolution, a few horse taxa dispersed from North America to other continents, and these emigrations resulted in small radiations.

However, virtually all horse genera, including Equus, evolved in North America and no other continent ever produced such a great diversity of horses as North America did which, during the peak of horse evolution in the Miocene, comprised at least a dozen genera that numerically dominated ungulate communities.

Today, North America is once again inhabited by a diversity of horses.  Where various prehistoric horses once lived, the latest versions of Equus now graze, including appaloosas, morgans, quarters, thoroughbreds and many other breeds.  Some of these horses have even established wild herds, reverting to the lifestyle of their ancestors (Photo 12).

Sticklers insist on calling these latest free-ranging North American horses "feral" rather than "wild," because their ancestors were bred by humans.  However, because the adjective "feral" implies gene selection by humans rather than the environment, many would argue that it is inaccurate since it tells only a small part of the story.

The ancestors of modern-day horses evolved for over 50 million years in North America, and there acquired all of their horse-like characters long before they ever met humans.  Because their most important characteristics are not the result of human-supervised breeding, but rather natural selection by the North American wilderness and sexual selection by other wild horses, these horses are far more "wild" than "feral".  In addition, as mentioned above, the horse is actually one of the least domesticated of all farmyard animals.

Should North America's modern wild horses be considered invasive, non-native species?  Here again the answer is not clear because we don't know exactly what caused the extinction of horses in North America thousands of years ago.  If humans did extirpate these earlier horses, then the presence of wild horses in modern-day North America could be seen as a wholesome restoration of one endemic element that was lost long ago by human carelessness.

Whichever way we decide to view North America's modern wild horses, it is essential that they be managed in an ecological way.  It is unwise and unethical to restore any wild herbivore species to an ecosystem without also restoring the predators that can limit its density and prevent overgrazing and excessive destruction to vegetation and soils.  Overpopulation is also bad for the horses themselves because it leads to starvation, stunting and the spread of diseases.

Where it is politically impossible to re-introduce predators, other mechanisms to control wild horse density need to be put in place.  The annual Pony Penning Roundup and auction of foals on Chincoteague Island, Virginia, and the use of contraceptive vaccines on nearby Assateague Island are good examples of alternative solutions (Henry 1947; Turner and Kirkpatrick 2002).

Closing Remarks

The modern-day people of North America, like humans everywhere, are passionate horse lovers.  The fossil record's revelation to them that their continent is the ancient motherland of horses, has deepened the already strong emotions that they feel toward horses, and strengthened the bonds that they have with these extraordinary animals.

One example is the recent interest many North Americans have shown for saving various species of Old World horses from extinction, on the grounds that such species either once occurred in North America or are closely-related to horses that did.  Some Americans have even suggested introducing these species back into North America, a controversial proposal that is being studied more closely and debated (Donlan 2005; Oliviera-Santos and Fernandez 2010).

In the wild horse, with its untamed character and independent spirit, many Americans see a soul mate, a natural symbol of their own love for freedom, with deep and ancient roots in their own continent.  At the same time, the fossil record's sobering revelation of the horse's extinction in North America eight thousand years ago, reminds Americans today of the vulnerability of all nature and the need to make environmental protection a high priority.


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Information about this Review

The author is:  Dr. Paul D. Haemig (PhD in Animal Ecology)

Photograph:  Icelandic horses by Hanna Dalberg (Sweden).  The Icelandic breed is especially popular in Sweden because it is adapted to living in the cold climate of the far north.

The proper citation for this review is:

Haemig PD  2012  Evolution of Horses.  ECOLOGY.INFO 33

If you are aware of any important scientific publications about the evolution of horses that were omitted from this review, or have other suggestions for improving it, please contact the author at the following e-mail address: 

director {at} ecology.info

ę Copyright 2005-2012 Ecology Online Sweden.  All rights reserved.

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