The horse was one of the most widespread grazing mammals in North America, yet it mysteriously disappeared about 12-8,000 years ago, at the end of the Pleistocene epoch. The horse, along with other large mammals such as mammoths, camels, lions, short-nosed bears, and giant sloths, disappeared.

There is much disagreement about the cause of this extinction. Some people believe that the abrupt climate change wiped out many large mammals; others think the extinction was caused by early humans that hunted and harassed the herds. Some believe it is a combination of both, and a third theory-- the “surviving horses” theory that hypothesizes that small herds of horses may have survived the extinction and interbred with introduced horses in modern times.

One major proponent of the “prehistoric overkill” theory is Paul Martin, professor of geosciences at the University of Arizona at Tucson. He believes that human migration and predatation is the direct cause of the extinctions in North America. According to his theory, first proposed in 1967, Clovis people preyed upon large mammals 11,000 years ago and caused their extinction. He and other supporters of the overkill hypothesis, including geographer Jared Diamond of UCLA, note that in other regions of the world human migration has corresponded with extinctions. Australia, the West Indies, Bismark Archipelago, Cyprus, Madagascar, Fiji, and New Zealand have all been shown to have lost megafuana as a direct result of human migration. Some people have pointed out that these are mostly islands where large animals could not escape. That is true, but there is no evidence to suggest that the opposite would happen-- that because North America is large, that doesn’t guarantee that animals would survive. Some animals might have needed very specific ranges, humans might have brought diseases or predators or wildfires or any number of things that might have interfered with native animals.

We know humans did at least hunt some animals. Clovis artifacts have been found in Mammoth remains across North America, and in 2001 dig lead by Brian Kooyman resulted in the discovery of spearheads tainted with horse protein, the first evidence that humans hunted horses. The spearheads were found in the St. Mary’s Reservoir in Alberta, Canada. "There has been suggestive evidence at other sites--Lubbock Lake in Texas, for instance--that early peoples were utilizing horses," Kooyman told Hillary Maynell of National Geographic News, "But this discovery raises the very real possibility that overhunting by the Clovis people played a significant role in the extinction."

A 2001 article by Joel Scwartz disagrees with Martin's position and quotes Donald Grayson, an anthropology professor at University of Washington.

Grayson believes that the change in climate during the late Pleistocene epoch (which ended about 10,00 years ago) resulted in the extinction of large mammals. The climate became more similar to today's climate, causing a change in vegetation. "Martin's theory is glitzy, easy to understand and fits with our image of ourselves as all-powerful," says Grayson, "But there is no reason to believe that the early peoples of North America did what Martin's argument says they did." Grayson says that the overkill theory doesn't take into account the fact that there is no evidence that all extinctions occurred during Clovis times-- it is possible that some animals began to die off before humans began hunting them-- and that it ignores the fact that there may have been pre-Clovis humans in North America. There is a site in Monte Verde, Chile with evidence of human occupation 12,500 years ago. If humans entered North America and lived among the mammals prior the extinction, it would be a major thorn in Martin's theory.

I, and many experts, think that a combination of the two theories is probably more accurate-- an already-stressed population could have been driven to the brink by the introduction of a new predator in their territory. Humans also could have brought with them disease or started major wildfires in critical areas. There are many ways humans could have changed life in North America, and most experts agree they probably had some part in the extinction.

Russel Graham, Chief Curator at the Denver Museum of Nature and Science, has been working on Climate models for mammal extinction for almost 30 years. He believes that climate change alone is not enough to trigger the kind of mass extinction seen in North America. He says that the reason the large mammals died off was because they had less range, and animals of a larger body size need wider ranges to support themselves. The article, “Why The Big Animals Went Down In The Pleistocene”, lacks a hypothesis about what caused the shrink in range size-- perhaps it could have been human hunters, a change in climate, or a combination of both.

Evidence shows that horses did decrease in size rapidly before their extinction-- evidence that they were adapting to an environment with less food. If horses were finding it harder to survive, it might have not been because there was actually a lack of vegetation-- it could be because herds were being pushed to harsher climates where they found it harder to survive. In the Pleistocene, the climate and environment became similar to today’s, and today horses have no problem thriving in North America. We would expect that if a change in climate and vegetation had killed off the horses, when reintroduced they should face similar problems surviving. But modern horses thrived, proving that they had no problem living in the current climate and habitat. Martin sums it up nicely in Twilight of the Mammoths when he says, "The explosive spread of free-ranging horses into grasslands of both North and South Amercia following their reintrouction by the Spanish suggests a return of the native." (38)

The rapid change in body size led people in the 19th and 20th centuries to believe that North America was populated by as many as 50 different species of horse during the Pleistocene. Recently, with the use of mitochondria DNA, researchers Jaco Weinstock and Alan Cooper have proposed that there were just two species-- the “caballine” and the “stilt-legged”” horse, both having morphological diversity that caused confusion in the past. The stilt-legged horse (genus Hippidion) was endemic to North America and went extinct. The cabaline, or “true” horse, is a group that includes the modern horse.

Weinstock’s paper states that, “A third significant discovery … is that all caballine horses from western Europe to eastern Beringia--including the domestic horse--are a single Holarctic species…one clade (A) was broadly distributed from central Europe to North America north and south of the ice. The second clade (B), however, seems to have been restricted to North America. If present in the Old World at all, it probably disappeared there before horse domestication took place … all domestic horses…cluster within clade A.”

Dale Guthrie, an ancient American horse researcher, demonstrated the rapid change in size by aligning horse bones from New World Stilt-legged horses in Alaska chronologically, revealing a dramatic shrinkage. He says that the variety in bone size, without information on their relative ages, would have mislead paleontologists into believing that they belonged to different species of horse. "Horses in different environments do respond with changes in body size," explained Guthrie. "I think the (genetic) work really clears up a lot," he says of Weinstock's research.

A third, although far less popular theory proposes that horses never did completely die off in North America-- that they were merely driven nearly to extinction and existed only in small ranges far from humans. These horses could later have interbred with reintroduced Spanish horses. However, there is no solid fossil evidence for this claim, and one would expect Native Americans to have passed down some sort of stories of these wild horses. None seem to exist. Hope Ryden’s book, “America’s Last Wild Horses”, says of the Pyror Mountain herd in Montana, “No one knows for certain where these particular horses came from, or how long they have lived here. Indians from the adjacent Crow Reservation tell of wild horses in the region before the coming of the white man.” Since there are no written accounts left by native Americans, we may never know. If, however, even one small population of horses could be shown to have survived until modern times, the horse would be unquestionably considered a native species.

Most experts acknowledge that early humans probably played a role in the demise of the horse. Humans are a part of nature like all other animals, but we are conscious of our impact on the land and try to minimize the damage we do. When we cause a species to become endangered, we feel that we should react by trying to help the species recover. In the case of the horse-- if it is a native animal-- our reintroduction of the horse was accidental and not a conscious effort. It is a bit ironic that the horse, arguably the most important animal in human history, was hunted to extinction accidentally by early humans then later accidentally reintroduced to its native land thousands of years later by humans.

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