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Many years ago the world was a different place. It had different environments, different lifeways, and different animals. The Pleistocene geologic epoch, lasting from about 1.6 million to 10,000 years ago (Guthrie, 34) was drastically different in a
ll senses than today. The climate was colder, the wildlife was thriving, and man was just beginning to create the stone tool technology that led us through thousands of years.
The climate of the Ice Age was exactly what one would expect for something with the name "ice." It was colder, sea levels were lower, and there were massive ice sheets covering a significant portion of North America. Due to the lower sea levels, the
edges of continents were more exposed. There was even a connecting piece of land between Alaska and Siberia, known as the Bering Land Bridge or Beringia, where today's Bering Strait exists (Fagan, 71). It was easy for the massive fauna and their smal
ler counterparts to move back and forth between continents without hesitation.
Over thirty-five different genera of mammals became extinct after their migration to North America when the glaciers retreated at the end of the Pleistocene. Twenty-seven vanished from the world completely, and eight of them only in North America (G
rayson, 1987; 8). These mammals are known as megafauna for their massive
size and their great populations. They are also often termed as "big game" as they were hunted once humans followed them into the continent. Donald Grayson describes them as "primarily large herbivores whose adult body weight exceeded one hundred pounds; most of the extinct vertebrates that were not large herbivores may well have been ecologically dependent on
those herbivores (10)." The smaller fauna would feed off the carcasses of the megafauna or would live off their "leftover" plant food.
The extinct fauna include many different mammals. The most well-known being the woolly mammoth (mammuthus sp.). Others include the mastadon, giant ground sloth,
giant beaver, and camels. Large carnivores included the cheetah and the lion and other sabertooth cats. By far the most powerful of these, however, was th
e giant short-faced bear. Even such animals as horses became extinct in North America and then were re-introduced by the Europeans in early historic times (Grayson, 1987;8.) These animals all suffered th
e same dreary fate. The question that plagues scientists today is: What caused these mass extinctions, and why are they so different from other mass extinctions?
There are two main hypothesis as to why these extinctions occurred. Paul Martin's "overkill hypothesis" and the climatic change hypothesis are the most popular explanations, but some have suggested a combination of the two or even different theories
altogether.
In 1859, the Old World's history was revolutionized by excavations at Brixham Cave in England. They showed evidence of humans killing big game, just like amateur archaeologist Boucher de Perthes and his French research had been trying to prove (Grays
on, 1987;8). Paul Martin suggests the same theory in North America. He says the extinctions occurred due to human predation. This theory was the first detailed and precise explanation but it divided the scientific community into two different sides (G
rayson, 1987;10).
Clovis hunters migrated south through an ice-free corridor east of the Canadian Rockies and moved south of the glaciers around 11,500 years ago (Grayson, 1991; 211). He argues that when man entered North America shortly before the extinctions, they
hunted the mammals for food. The animals were killed over a short enough period of time that they were unable to reproduce and repopulate their species fast enough. Martin argues this theory for several reasons. He says, "The only event unique to the L
ate Pleistocene which could conceivably track the regional pulse of extinction outlined above is the deployment of prehistoric people... The continents or island continents that suffered severe extinctions did so during or after the time of initial human
colonization. Africa and Asia, the continents of human origin, were not as severely affected. (12)." The overkill hypothesis can explain the extinctions in North America, as well as those in other parts of the world.
There are several ideas supporting this theory. Clovis man entered what has been described by Archaeologist C. Vance Haynes as "the happiest hunting ground ever known" and by author Bruce Bower as "a hunter's garden of eden (Bower, 284)." It would h
ave been so easy for man to hunt these animals because they were unadapted to human predation. While the human population was soaring due to easy hunting and exposure to few new illnesses, the animals simply couldn't keep up with humans (Bower, 284).
Steven Mithen of the McDonald Institute for Archaeological Research in Cambridge has used a computer model to show that when the human population in North America reached 12 million, the number of mammoths fell significantly (Saul, 14).
Mithen used the assumption that mammoths are similar to contemporary African elephants and used the idea that they bred at the same rate and reacted to climatic changes in the same way. He used a different range of scenarios, setting the mammoth popu
lation at 25 million, 50 million, and 100 million, and introduced an initial human population of 100 people, which increased at various rates. An assumption was also made that each year the number of mammoths killed was proportional to the size of the h
uman population and that each human killed between 0-10 mammoths per year. Mithen ran the simulation until all the mammoths were extinct and the results showed that hunting caused the extinction within 5,500 years of human population in North America. W
hen humans reached the 12 million mark, the mammoth population completed disappeared in the course of a few decades (Saul, 14). Mithen's computer simulations can easily be used to support Martin's overkill theory, but at the same time, there were a lot o
f assumptions made in the simulations.
George Frison of the University of Wyoming conducted similar experiments with freshly killed elephants at the Hwange National Park. He used replica Clovis projectile points and cutting tools made of chert and quartzite and was able to prove that sto
ne tools could inflict crippling or lethal wounds on mammoths. He assumes that the thickness and musculature of mammoths are very similar to those of elephants (Bunney, 35). These experiments may prove that it's possible for mammoths to have been killed
by humans (we have other evidence of this as well), but it does not prove that it was the cause of the extinction.
One of the biggest arguments against the overkill hypothesis is that there are relatively few kill or butcher sites of these animals. There is little association of human artifacts (stone tools) with megafauna remains. This criticism is labeled as t
he "associational critique" (Melzer, 51). Some of the most abundant extinct animals have not yet been found in association with humans. Grayson says, "Mammoth kills are relatively abundant, mastodon kills are rare but known, but horse and camel kills do
not exist, even though the paleontological record shows that horses and camels were abundant on the landscape (1991; 213)."
Martin almost immediately answered this critique by saying that because the extinctions happened over such a short time the chances of archaeological remains diminished. Other parts of the world, however, have an abundance of kill sites when extinctions
occur due to overkill. New Zealand, for example, lost their large flightless birds with the arrival of people 1,000 years ago. There are many kill sites there that undoubtedly prove they are extinct due to overkill. Martin defends his claims by saying
either scarcity or abundance of kill sites can support overkill (Grayson, 1987;12).
Arguments against the overkill hypothesis include many arguments about dating. Because there are no trustworthy carbon-14 dates older than 15,000 years ago, it is difficult to tell if the extinction of these mammals occurred earlier than that, indica
ting a gradual extinction rather than all the species becoming extinct at the same time (Grayson, 1987;8). It is generally assumed by Martin and others that the time frame of the extinctions was between 12,000 and 10,000 years ago (Martin, 11). Remains
of the Barrington mountain goat and Shasta ground sloth in the Grand Canyon show and indisputable date of 11,000 years ago. As Grayson says, "The timing of Ice Age extinctions is really very poorly understood. Radiocarbon chronologies are bad in North A
merica and worse in Europe (Bower, 285)."
Another major argument against the overkill theory is presented by Ernest L. Lundelius, who says that humans and megafauna have co-existed on other continents for long periods of time without any sort of problem (Lundelius, 13). Martin defends his ar
guments by stressing that other places, such as Australia and New Zealand and Madagascar seem to have the same pattern of extinctions when humans arrive (Martin, 11). Lundelius refutes that by saying, "In Australia, there is some evidence that humans pr
obably arrived 15,000 years before the major extinctions...For humans to have been the causative agent of extinction in these areas, one must invoke large population increases or significant advances in hunting technology. This model also fails to explai
n the loss of non-prey animals (13)."
Grayson furthers Lundelius' claims by trying to explain the loss of several species of birds at the end of the Pleistocene. He has discovered 10 extinct classes of North American birds from the Ice Age. Martin explains this by claiming the birds wer
e scavengers that fed on carnivore leftovers. He says the same for other non-prey animals (Bower, 285). Grayson counters with evidence that only one of the extinct birds seemed likely to be dependent on mammals for their existence (Grayson, 1987;12). T
his indicates a climatic change. Norman Owen-Smith of the University of Witwatersand in South Africa makes a similar claim by saying that the extinctions of the megaherbivores would have resulted in the changed vegetation and geographic patterns of the s
maller herbivores (Bower, 285).
Scientists who disagree with the overkill hypothesis generally support the climatic change hypothesis. This theory has been around since the extinct animals were first recognized in 1800 (Grayson, 1987;8). Basically, the theory is that the end of th
e Ice Age changed the climate so much that many animals couldn't adapt and either died off or changed habitats. The evidence for this is seen in the major number of animals that did change their habitats at the end of the Pleistocene. For example, the y
ellow-cheeked vole shared a habitat with the eastern pack rate in the Ice Age, whereas today they are over 1,200 miles apart (Bower, 285). The vole is today found in Alaska, but during the Pleistocene lived in Tennessee (Grayson, 1987;12). Grayson says
"Some North American mammals retained their geographic ranges unaltered, the distribution of others changed drastically, while still other became extinct. Those whose ranges changed were primarily small, while those that became extince were primarily larg
e (1987;12)."
Lundelius fully supports the climatic change theory because the changes at the end of the Pleistocene were by far the most rapid and amazing in the earth's history. Reduction of the size of glaciers caused rising sea levels, change in temperature an
d rainfall, and reduction of lake sizes. He believes that habitat destruction caused the mass extinctions (Lundelius, 13). He also notes the "disappearance of the disharmonious floral and faunal assemblages" at the time of the extinctions (Lundelius, 14
).
Other climatic changes, as noted by Steven M. Stanely, were "changes in the salinity, or salt content, of the oceans; reduction of the level of dissolved oxygen in the oceans; and lowering of sea level, which reduces the area of habitats on shallow
sea floors (Stanley, 14)." Paul Martin contests this by saying there was no oceanic extinctions at the end of the Pleistocene (Martin, 12).
Martin also counters the climatic change hypothesis by stating that climatic change by itself is not enough to explain different intensities of extinctions on different continents-- why was extinction heavy in America and Australia, but light in Asia
and Africa (Martin, 12)? Steven Mithen agrees by saying that because the animals had survived thousands of years of climatic change that there must be some other cause (Saul, 14).
Perhaps a good way to distinguish which theory is correct is to compare the Pleistocene extinctions to other mass extinctions in the world's history. About 650 million years ago, an extinction attacked three-lobed anthropods. There is little evidenc
e of this, much less an explanation of it (Stanley, 15). Major
extinctions also occurred at the end of the Permian, 245 million years ago. These extinctions were catastrophic (occurring in less than 1000 years) as well, but there is still little exp
lanation (Martin, 11). Lundelius claims, however, that the extinctions that took place at 5, 1.8, and .6 million years ago all coincided with glacial retreats (Ludelius, 14).
Probably the most famous of extinctions, the dinosaurs, can possibly be explained by climatic changes as well. This took place on land and in the sea in the Mesozoic era. Stanely says, "We have evidence that, although a sudden event at the very end
of the ear may have administered the final coup, the ecosystem was already deriorating-- both on land and in the sea. (Stanely, 18)." These extinctions generally support the climatic change theory, as humans didn't exist at the time they occurred.
Basically, the climatic change theory seems to have more supporting evidence, whereas the overkill hypothesis has been tested more. More likely, it is a combination of the two different theories to accurately explain the mass extinctions. Whatever
happened, it was a tragic loss of fauna, but their deaths will still remain a mystery.
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