Overkill theory for the demise of the mega-fauna of the Pleistocene

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.