The age of glaciation, the Pleistocene, was marked by long periods of cold temperatures interrupted by smaller, interglacial, periods in which the earth warmed and sea levels rose. At the end of one such interglacial period 30,000 years ago, global temperatures lowered significantly dropping sea levels 300 ft below modern levels and once again locking a vast amount of the world’s water in ice. With so much water tied up in glaciers, landmasses that had previously been submerged were uncovered. One exposed landmass was Beringia, a coastal plain that connected Siberia to Alaska and opened a pathway to the previously isolated continents of the Americas.

Beringia was a pathway for inter-continental travel until around 11,000 years ago when sea levels rose, once again separating the continents. The great span of time when the bridge was open would have been ample enough for animals in and around Siberia to find their way to the pathway and cross it. Among those that found their way into North America were large animals weighing more than 110 pounds* such as the Mammut (American mastodon), Nothrotherium (small ground sloth), Equus (horse)**, Camelops (camel), Symbos (woodland musk ox), Tapirus (tapir), Glyptotherium (glyptodont), Mylohyus (woodland peccary), Tetrameryx (pronghorn) and Mammuthus (mammoth). These animals would have been drawn into the continent by available food and the lack of competition for it. North America flourished with a plethora of land animals until around 11,000 years ago when 35 large mammalian genera vanished. In turn the loss of these large herbivores would have affected the survival of ecologically dependent animals by reducing the number of carnivores as prey species became less abundant.

In 1876 A. L. Wallace wrote in The Geographical Distribution of Animals, “We live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared…it is surely a marvelous fact, and one that has hardly been sufficiently dwelt upon, this sudden dying out of so many large Mammalia.” Since Wallace’s time the search for the cause of the mass extinction in North America has been ongoing. The question then, is what have we learned in the past one hundred years as scientists have searched for, and argued about, the reason so many species died out?

Environmental Cause Theory

The first suspect in the search was the changing environmental conditions occurring in North America, and around the world, about 11,000 years ago. The species that disappeared, the mega fauna, would have been adapted for the harsh climate and icy surroundings the Pleistocene provided. Their large sizes would have been ideal for maintaining body temperature and perhaps even large enough to deter potential predators from attacking them instead of smaller, easier prey. Changes in climate often affect animals the most greatly through changes in the available vegetation. It’s possible then, that, when temperatures began to warm and the environment began to alter, these mega fauna would have found themselves incapable of maintaining environmental cohesion. Plant life would have altered replacing nourishing grasslands with forests in some areas and introducing potentially toxic plants in others. The altering of their nutritionally rich environment to one providing substantially less edibles could have been negative enough to lead to the eventual demise of the large herbivores. This theory sounds viable, however it does not hold against evidence found within the fossil record.

Analysis of Shasta sloth dung found in Rampart Cave, in the Grand Canyon, shows that around 11,000 years ago the giant ground sloth was eating desert plants such as mesquite, Mormon tea, yucca and agave – plants still found in the area today. Clearly in the instance of the ground sloth the changing flora would not have accounted for its sudden disappearance along side the mammoth and horse. Foremost in the argument against this theory is that in the multiple interglacial periods that occurred during the Pleistocene, in which temperatures would have warmed and terrain altered, the mega fauna seem to have survived unharmed around the world. When Darwin journeyed on the Beagle he recorded his observations of not only living species, but also fossils he encountered. These included extinct Pleistocene fauna in stratigraphic layers occurring after the glacial period; proving that, although the changing temperatures and environmental conditions may have been a hindrance to the mega fauna, it was not the overall cause of their collective disappearance.

A curious characteristic of the late Pleistocene extinctions, differing from previous end of era extinctions, is the singling out of one class of organism, the Mammals. There is no evidence of an accelerated extinction in the marine fossil record nor does pollen evidence show a drop in the plant kingdom, which, had dramatic geological events been the cause of the disappearance of land mammals across the globe, should have affected these life forms as well. If it wasn’t the environment that caused the extinction of the large mammals, what else could it have been?

In an effort to discover the impact of environmental stresses on large mammals during the late Pleistocene, researchers are developing techniques to use growth rings in mammoth and mastodon tusks in a similar manner as tree rings are in dendrochronology. Although not fully developed, early analysis has already shown that some male mammoths were reaching sexual maturity at a younger age, something that occurs in modern elephants when there are hunting pressures. Though the research is incomplete it can be used to suggest alternatives theories to that of the environmental cause for Pleistocene extinctions.

Overkill Theory

There is evidence of human activity in North America around 13,500 years ago cal, long before the mega fauna vanished. Stone points that have been bifacially flaked, and have the distinguishing feature of a flute cut down the center, have become the defining characteristic for a tool technology known as Clovis. Named for the location of the type-site in New Mexico, the discovery of the Clovis tool technology was important in dating the arrival of man in North America, something people have been wondering about since Columbus encountered well established and thriving cultures in 1492. Although theories on how humans migrated to the Americas are still being bandied about, the most widely accepted theory is that man arrived in a combination of ways rather than through one entry point or at one time. One way would have been by a coastal route along the Pacific Rim. This theory relies on the fact that even at the height of glaciation the oceans never froze, providing an active eco system with warmer temperatures and readily available food sources. The second route that seems most likely is over the Beringia land bridge. This theory maintains that man would have followed his food source, the mega fauna, across the harsh terrain provided by the land bridge and entered North America in Alaska. Skeptics believe that the environment of Beringia would have been too harsh for man to travel, but others believe it would have been thriving with its own eco system providing ample food sources for the journey.

No matter which venue early humans took to enter North America, the fact that they were here during the late Pleistocene has been established. It is likely that the first humans to enter the Americas were hunters because at that time there would have been no other food source available during a majority of the year. The earliest piece of evidence confirming the presence of humans in Alaska was found in 1978 in the form of fluted spear points alongside charcoal from fire hearths, which have been carbon dated to be about 13,600 years old. The mesa upon which these artifacts were found provided a 360º view of the surrounding area, and most likely was used as a lookout for a hunting party. Since 1978 numerous human sites have been found, some associated with specific big game kills. It is the mammoth that is associated with the earliest North American hunters due to the focus that seems to have been concentrated on this specific big game animal. In Blackwater, New Mexico evidence of eight mammoths were found associated with Clovis projectile points carbon dated between 11,630 - 11,040 years ago. In Burning Tree, Ohio the butcher-marked bones of a mastodont were located and dated to 11,660 years ago. In Lehner, Arizona evidence of thirteen mammoths associated with Clovis projectile points and dating to 10,900 years ago were found. The list of mammoth kill sites associated with early Paleoindian culture and dated from 11,700 to as early as 10,670 is long. It’s not hard to imagine that the initial arrival of human hunters invoked no instinctive fear from these giant animals, who had existed for such a long period without the presence of an aggressive predator species, something that may well have aided in their eventual extinction.

The large number of sites, as well as the large number of mammoth kills at some of these sites, seem to indicate a focus on these large game animals for Paleoindians. This is not to suggest they only ate mammoth meat, for evidence of other species has been located. Bone evidence of the hunting of camel, pronghorn, bison, bear, tapir and jackrabbit have also been found, although in far fewer numbers than mammoth. For many years it was thought that the horse died out long before humans came to North America. However researchers in Alberta, Canada discovered the remains of the extinct horse (Equus conversidens) dating from 11,000 to 11,350 BP. Associated with the remains were various projectile points, among them two Clovis points that tested positive for protein residue belonging to the species Equus. Analysis of the projectile points showed impact burnination and fractures indicating they were indeed used as hunting implements and proving that the Clovis and the extinct horse did come in contact.

Investigations into the fossil record and carbon dating techniques have shown that 80% of the North American animal population disappeared within 1000 years of the arrival of man. In modern times the extinction of animal species has been closely associated with the arrival of man in its territory. Humans have been known to encroach on animal territories as populations enlarge; to hunt animals to extinction, or near enough to it, for sport as well as subsistence; and, to introduce toxins to the environment that have interrupted the normal life cycles of some birds and smaller life forms. It has only been in recent years that humans have begun taking responsibility for the creatures inhabiting the planet along side them. As such, the recognition for the destruction often caused by enlarging and developing human populations has been widely acknowledged. To draw a line connecting the extinction of animal populations in historic times to the extinction of animal populations in prehistoric times, both of which coincided with the arrival of man in one form or another, is not hard to see happening. So it is hardly surprising that this connection was made shortly after the environmental cause theory was disqualified.

At the forefront of the argument for the overkill theory is Paul Martin. Martin’s theory is that humans were directly responsible for the mass extinction in the Late Pleistocene due to over hunting. He argues that the presence of a culture in which mammoth bones were used as building material in the construction of habitations represents a wave of hunting people sweeping southward across the continent. These people would have targeted the large game animals, and other animals as well, and migrated in the chase of these creatures. When the game was hunted out they would have followed a herd, leaving the trail of mammoth bones behind them for archaeologists to uncover thousands of years later. In the case of predators such as the saber tooth tiger, it is plausible that man caused their extinction by depleting their food source in their travels. Others would argue that there wasn’t enough time for man to hunt so many creatures to extinction. Computer simulations have been carried out to test the growth of human population in conjunction with the rate of animal disappearance since Martin’s overkill theory first made its appearance.

In 2001, John Alroy set out to test this theory using computers once again, this time using better programs and newer technology. The parameters he set included “slow human population growth rates”, “random hunting and low maximum hunting effort.” The results seem to show that the carrying capacity of each inhabited region of North America may have been over extended by the growth of human populations. As such the accelerated extinction of big game animals, the focus of Paleoindian hunters, would have been inevitable. The tests Alroy performed resulted in varying distributions of extinction times for the late Pleistocene, however the most likely scenario involved three waves. The first wave of extinction would have occurred within 1000 years of human arrival, the second in the next few hundred years and the third would have involved hunting the few groups remaining. Varying factors such as “human induced habitat change,” “the introduction of pandemic diseases to native herbivore species by humans” and “selective human hunting of individual prey species” were missing from the model Alroy presented. He theorized that a model with these factors added in would result in faster extinction rates with slower population growth necessary. His results were discounted by Donald Grayson, however, who found that although Alroy’s model closely matches the data used, he assumes that all of the extinctions fall during the period after the arrival of the Clovis and uses over hunting to explain their demise despite the lack of evidence of hunting of certain species.

Recently another computer simulation was performed. This time the model included the body masses for a range of 198 extinct and 433 surviving species from around the world, as well as parameters for “maximal replacement rate and equilibrium density of mega faunal prey populations, density of human populations, maximal rate of off-take by human hunters, and relative naivety of prey.” The model showed that the larger the species became the slower their reproduction rates would be, in turn dwindling their numbers and making them less able to cope with rapidly changing circumstances and social disruptions. All of these factors would explain the swift extinction rate of the large mammals as human hunters moved across the continent. The populations would have taken more time to rebuild than the large quantity of mammoth bones at sites like Blackwater suggests it took to kill them in. With a few sites of 15, 18 and 13 kills each it’s easy to picture how animals such as the mammoth could have been wiped out in 1000 years assuming they were slow to reproduce. In addition the offspring would have been easy prey to carnivores such as the saber tooth tiger, leopard and the flat faced bear, who may have become more aggressive killers in competition with man.

Even without the use of computer technology researchers have been using ethnographic evidence to support the overkill theory. In the early 70s anthropologist W.T. Vickers documented the hunting patterns of the Siona-Secoya Indians on the Aguarico River in Ecuador. A hunter-gatherer group, information about which animals the Siona-Secoya Indians hunted, and their success rates over five years provided a first hand look at the overkill theory in process. After only five years Indians began having difficulty hunting larger mammals and birds and had to rely on smaller creatures for subsistence. The migration of the Siona-Secoya Indians to this previously un-utilized habitat resulted in the depletion of the larger mammals, exactly what is thought to have happened when the Clovis migrated into the Americas.

Challengers of Martin’s overkill theory often use one of three reasons it could not have happened that way. The first is that there would not have been enough time for man to hunt the vast quantity of animals that once roamed the North American continent. Despite the computer simulations and the ethnographic observations, they feel that the continent was too large and the population of humans would have been much too small for 80% of the animals to die out solely from hunting. They also refer to the Monte Verde site as support against extinction through overkill. In Monte Verde the skins of mastodons had been utilized as a covering for homes for over a thousand years before the extinctions of their kind in North America. Martin himself does not comment on the Monte Verde data, tending to dismiss all discussion concerning the search for possible pre-Clovis sites while speaking generally enough in his works to cover the eventual wide acceptance of data confirming such a site. (Grayson, 2001) And finally there is the lack of evidence that Clovis hunters were utilizing the entirety of the now extinct animal kingdom. Largely evidence of big game kills has been focused on the mammoth with projectile points found embedded in bone and other definitive evidence. The lack of evidence is what keeps many archaeologists and vertebrate paleontologists from going along with the overkill theory. It’s only recently that data proving that horses were hunted by Clovis has come into light, and other than a few animals aside there are still many creatures un-represented at Clovis kill sites. However, just as the horse data was only recently retrieved, it is possible that in time more sites will be discovered containing the protein residue or bone fragments of various other extinct animals.

Hyperdisease Theory

The most recent development in the search for a cause has been that of the hyperdisease theory. In historic times humans have wiped out vast populations of each other during ‘first contact’ scenarios when diseases were accidentally transmitted. Having not been exposed to these diseases before the new population had no formidable defenses and succumbed quickly to illness and death. In accordance with these documented ‘first contact’ scenarios, the hyperdisease theory suggests that when humans came into North America they brought with them a plague that, once transmitted to the mega fauna, wiped them out with a swiftness their bodies didn’t have time to adapt to. The basis for this theory does not stand on a discovered pathogen so much as a rejection of the environmental cause and the overkill theories. In the search for a third option scientists have emerged with this hyperdisease hypothesis based on the known effect of disease introduced to human populations and the belief that pathogens were carried in through parasites, dogs, rats and other creatures following humans across the continent.

In order for the hyperdisease theory to work humans would have had to have carried with them a disease so potent it would spread quickly, kill many and yet not affect humans, and would have had to have been able to sustain itself when there were no hosts available to infect. This seems illogical, and the inability to locate a modern disease that meets these criteria has long been the stumbling block to hyperdisease theory. However, recently the spread of West Nile Virus has allowed a look into such a disease. When introduced to North America, West Nile Virus entered an environment where no immunal defenses had been built against it. Through hosts such as blue jays, house finches and crows the West Nile Virus spread quickly, infecting 33% of the birds tested in New York in the year 2000. The problem presented by the study of the West Nile Virus is that it seems to infect and kill indiscriminately of size, something that would not support the mass extinction of only large mammals in the Pleistocene. This theory is still new and being developed, but unless direct evidence of a pathogen is found in animal remains or a modern disease is located that operates efficiently with a size bias, there is little likelihood that it will remain as viable a possibility as either the environmental cause or overkill theories present.


The mostly likely cause of the mass extinctions in North America at the end of the Pleistocene is a combination of both environmental cause and overkill. The timing of the arrival and quick spread of Clovis hunters and the geological affects of global warming at the end of the Pleistocene may have been instrumental in the disappearance of so many species in such a short span of time. The role of environmental cause would have been represented by a change in habitat, food source and the animals themselves. It was the ice age that led to the physical enlargement of the mega fauna in the first place. This in turn resulted in slower birth rates, which would have kept them from maintaining population numbers when a new predator, man, was introduced. The altering of habitat and vegetation would have affected some, but not all, of the mega fauna as shown in the case of the Shasta ground sloth. The support for the overkill theory relies on indisputable evidence of the hunting of various species including but not secluded to the mammoth; the timing of human arrival and mass extinction is too close to be disregarded; the evidence of widespread Clovis technology throughout North America indicating population growth and fast migration speeds; and, that previous climate changes similar to those at the end of the Pleistocene were marked by no mass extinctions of such a narrow representation of animal species, namely the singling out of mammals, which renders environmental cause ineffective on its own. Where either of these theories seems to fail on its own the other theory seems to be able to explain, so together they create a extremely plausible theory for the cause of the mass extinction of the mega fauna in North America.

* It’s important to note that in many cases the weight of those creatures termed ‘mega fauna’ has been an indiscriminant factor in being categorized as ‘mega fauna.’ (Brook, 2004) As such, the weight in many instances can vary to as little as 90 pounds, and for the purposes of this paper Martin’s (1967) table of extinct Pleistocene mammals was utilized to determine the large mammals from the small mammals.

** Some sources say that the horse evolved on the American continents and actually crossed over into Asia over the landbridge. These sources say the horse thrived in Eurasia and declined in the Americas for the reasons mentioned in this post.

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