Approximately 11,000 years ago a variety of animals went extinct across North America.
Mostly mammals larger than approximately 44 kg (about 100 pounds).
Before this extinction the diversity of large mammals in North America was similar to that of modern Africa. As a result of the extinction, relatively few large mammals are now found in North America.
Why did these animals go extinct?
Scientists do not know for sure. This is an active area of research.
Scientists who study the extinction have identified two major mechanisms that may have caused the extinction.
Almost all scientists working on this problem agree that the extinction was caused by one of these two mechanisms or by some combination of them.
Human Hunting
The human hunting or so-called "Pleistocene Overkill" hypothesis is coincident with the appearance of Clovis Paleoindian peoples.
Just after 12,000 years ago Clovis people from Asia entered the New World (or if you agree with Stanford, they possibly evolved from local populations). They may have been the first people to set foot in North America. Their sites and artifacts, including distinctive projectile points, are found over much of North America.
The Case for pre-15,000 BP Human Settlement
There is is relatively little North American evidence for the pre-15k date
-- much more comes from South America, from northeast Brazil and Northern Chile, and now
the tropical rainforests
Most widely discussed
Toca do Boqueirão da Pedra Furada, a large rockshelter in Brazil, with putative crude
artifacts of quartzite and quartz that are heard to identify as human-made
--charcoal dates to 40,000 BP, but question is why occupants would use the difficult to
use stone when fine-grained materials available nearby (video)
Monte Verde in Northern Chile has several huts and other materials solidly dated from 12,000 BP, but an earlier layer at 30,000 BP
A very recent find by Anna Roosevelt in a rainforest rockshelter, Caverna da Pedra Pintada in Belem, Brazil has 69 dates in the 10,000-12,000 years BP range that are not Clovis, but may have been part of a cultural complex that gave rise to early SA cultures.
In North America the best known is the Meadowcroft Rockshelter in southwestern
Pennsylvania
--James Adavasio's excavation have well documented radiocarbon date suite of 70
dates solidly in the 10,950-12,950 BP range.
--Early C-14 dates at13,320-19,600 BP, but Haynes disputes them
Others:
The issues are as defined by Dennis Stanford:
But we do know that with arrival/appearance of Clovis, things started happening.
These people hunted and gathered wild animals and plants. The animals they hunted included many that went extinct. However, they also hunted numerous animals that survived.
Many scientists think that these people caused the extinction in North America at the end of the Pleistocene. Researchers who support this view generally favor one of two explanations. The first is that human over-hunting directly caused the extinction. The second is that over-hunting eliminated a "keystone species" (usually the mammoths or mastodon) and this led to environmental collapse and a more general extinction.
Environmental Causes
Between about 15,000 and 10,000 years ago the climate and environments of North America were changing rapidly. Temperatures were warming. Rainfall patterns were changing. The glaciers were melting. The seasonal difference in temperatures was increasing.
The first proposed solution - temperature change
- is based on the observation that the extinctions coincided with the wasting of the
last glaciers. It posits that global warming was the cause of the large mammal
extinctions. However, since it was proposed, it has become evident that today's mean
annual temperature is no warmer than that of previous interglacials. Thus warmer
temperature alone is not a sufficient explanation for the extinctions since the large
mammals survived similar temperatures in previous interglacials.
"The dismissal of the temperature change prompted the question: What makes this interglacial different than previous interglacials? One answer is the entrance of Homo sapiens to the New World. This led to the overkill hypothesis which pins it on those found at the scene of the crime."
These climate changes were causing fundamental changes in the ecosystems of North America. Plants and animals were moving out of areas they had lived in and into new areas. Communities were coming apart and reorganizing.
Many scientists think that these climatic and ecosystem changes caused the extinction at the end of the Pleistocene. The environmental changes might have caused extinction by eliminating food sources, disrupting birth schedules, or exposing animals to climatic conditions to which they were not adapted.
Is North America the only place where Late Pleistocene extinctions occurred?
North America is not the only continent which experienced an extinction of this kind near the end of the Pleistocene.
In South America most of the species of medium to large mammals also went extinct approximately 11,000 years ago.
In Australia a major extinction also occurred. The timing of this extinction is much more poorly known; however, it appears to date to between 40,000 and 20,000 years ago.
Europe, Asia, and Africa also experienced some extinction toward the end of the Pleistocene. However, on all of these continents the extinction was less severe (fewer species involved).
A closer look at extinction from overuse
Late Pleistocene extinctions. In late Pleistocene, during the last 50,000 years, there were mass extinction events in many different parts of the world, involving at least 200 genera. But this was different from previous episodes of mass extinction:
1. It was much more selective, involving mainly the megafauna: the large herbivores (mammoths, mastodons, huge ground sloths, cave bears, woolly rhinoceros, other rhinoceroses, etc.) and the carnivores that fed on them, the dire wolves and sabre-tooth cats. There was no accelerated extinction of smaller terrestrial species, plants, or marine organisms.
The following disappeared from America, Europe and Australia:
2. It occurred at different times on different land masses:
Time of start of major extinction episodes (years before present)
This excludes any global catastrophe or climatic change as an explanation.
In all of these cases except Africa, the extinctions occurred shortly after the first arrival of prehistoric man.
The first humans were faced with animals that had evolved in man's absence, and the animals were probably easily overcome.
Therefore, the most plausible explanation is that these extinctions were caused by overexploitation by human hunters.
In Africa, massive extinction does not coincide with the arrival of man. He had been evolving there for millions of years without causing mass extinctions (possibly was not as carnivorous as his descendants in other parts of the world) but it does coincide with the maximum development of advanced early Stone Age hunting cultures.
Many authors have remarked that to see what the Pleistocene was like, you should go to Africa. Africa still has more large herbivores (including elephants, hippos, rhinos, etc.) than any other place on earth. But, even in Africa, the big game we see today is only about 70% of the genera that were present in mid-Pleistocene. About 50 genera disappeared about 40,000 years ago.
It is paradoxical that the region where man has existed the longest (Africa) retained a
wide variety of big game whereas the areas where he arrived more recently have suffered a
more complete loss.
--Perhaps the African big game had time to evolve defensive behavior, whereas species
elsewhere were caught defenseless and naive by a newly arrived advanced hunting culture.
Australia was first colonized by humans (already Homo sapiens) around 50,000 years ago and subsequently (about 13,000 yrs b.p.) lost all of its very large mammals (including giant wombats as big as grizzly bears and giant kangaroos; in fact all except some kangaroos), its giant snakes and reptiles and half of its large flightless birds.
North America.
The data are clearest for North America, where 70 species = 95% of the megafauna disappeared about 11,000 years ago. This is exactly the time when North America was colonized by man, and his arrival and skill as a hunter at that time is documented by the appearance of artifacts.
In some cases accurate dating methods have shown that certain species became extinct at exactly the times that humans arrived. Giant ground sloths and mountain goats in the Grand Canyon both went extinct 11,100 years ago which is the time that the human hunters arrived (within the accuracy of dating methods, which is +200 years).
The disappearing mammals in North America included all of the following:
*Some of these fossils are directly associated with human artifacts in archaeological sites.
The carnivores on the list were probably not hunted directly, but were dependent on the large herbivores for food, so soon followed them to extinction.
There is also direct evidence for killing by humans. The human archeological sites of
11,000 b.p. have stone projectile points which were presumably used in hunting the large
mammals.
--One mammoth skeleton has eight stone spearpoints among its ribs. Some of the large
mammals were trapped in pits, some were cornered using fire.
The most recent analysis of late Pleistocene extinctions in North America
suggests that they happened over just a few hundred years.
--Paleoindian blitzkrieg model
This explains why there is so little archaeological evidence for mammoth-hunting in the New World. The total number of mammoths from archeological sites in North America is 38; in Asia, where mammoths were hunted on a sustainable basis for many thousands of years, there are many more mammoth remains -e.g. remains of 1000 mammoths at just one site in Czechoslovakia and of 100,000 horses at another site.
Paul Martin has suggested that the human population quickly expanded south from the Bering land bridge to the position of present-day Edmonton, then south, and exterminated the big game as they went.
South America was also colonized by humans about eleven thousand years ago, and since that time it has lost 80% of its genera of large mammals, including ground sloths, horses, and mastodons.
The overkill hypothesis suggests that as humans crossed into the New World they hunted the herbivores to extinction. The carnivores which depended upon those herbivores went extinct because their prey was gone. This is based on the observation that the pattern of extinctions follows the march of humankind:
This is based on the reasoning that in Africa animals learned and evolved as H. sapiens developed new hunting techniques. The animals of new territories were naive about hunting and were more vulnerable - there was a greater gap between the ability of the hunter to kill and the ability of the animal to evade being killed. This predicts that the later H. sapiens entered a territory the more animals will be killed.
The overkill hypothesis has been supported by the evidence of projectile points found imbedded in the bones of extinct animals, by bones of extinct animals found in association with archaeological sites and by a computer simulation by Whittington & Dyke based on the relative recruitment rates of H. sapiens and the various animals.
Some are dissatisfied with this solution. Martin is suggesting that humans found naive fauna which were easy to hunt and that they swept across the two continents of the New World in 1000 years, killing as they went, but there's something about it that just doesn't seem plausible.
It runs counter to our sense of how the predator/prey relationship works.
In formal models it is generally assumed that predators and prey are linked in a mutually causal loop . Thus, as prey population increase predators increase and as prey populations decrease so do predator populations. This makes it difficult if not impossible for predators to kill off all their prey.
In complex ecosystems as a given prey species become scarce predators start hunting other prey which are more plentiful. This makes it even less likely that the prey will go extinct.
If our notion of predator/prey relationships is correct, then as herbivores were over- hunted, human populations would have contracted. This would allow prey populations to increase. The overkill hypothesis suggests that despite contracting herbivore populations people continued to over-hunt. They continued to herd animals off cliffs and into arroyos, they didn't change their prey or their life style. This makes us feel dissatisfied because it suggests that humans don't behave like other predators, that they are somehow different. That goes against the principle of uniformitarianism - that the dynamics of today are applicable to past times."
Does this allow us to dismiss the argument?
Certainly indicates that there is more work to be done. The correlation of the introduction of humans and the extinctions is hard to dismiss. There must be more evidence which will allow us to verify, dismiss, or, perhaps, alter the model.
In the last 15 years it has become more and more evident that we cannot use current environments as a simple analogy for the environments of the Pleistocene (before 10,000 BP) because the pattern of where animals and plants lived is so different . The literature from many different fields is full of references to "defunct species associations," "communities without modern or extant counterparts," and "disharmonious species associations." These all indicate that not only were there were more species but species which today have disjunct ranges (are allopatric) had overlapping ranges (were sympatric) during the Pleistocene.
Pleistocene Survivors
In North America, the only survivors of the big herbivores of the megafauna are the elk, moose and buffalo. The American Buffalo (also called American Bison; taxonomically correct name = American Plains Buffalo) was the symbol of the wild west, and subject of much frontier and Indian lore, but it was brought almost to extinction by overzealous hunters.
The prairie population of buffalo was enormous, estimated at 30 to 40 million individuals. In 1839, a trader rode through a single herd that was 45 miles long. Possibly the high population was a result of the elimination of other large herbivores that competed with the buffalo for food and space.
Source Materials
If you want to understand what the Plains and Midwest were like during the last glacial period, be sure to go to the fascinating and colorful web exhibit assembled by the Illinois State Museum on The Midwestern US, 16,000 Years Ago. In that exhibit, you will find out all kinds of things about the climate, land and animals that had an impact on the First People.
Starting about 18,000 years ago, the glaciers began to retreat. For a fine demonstration of how this happened, take a look at an animated image of the glacial retreat. As the glaciers retreated, the climate began to grow more mild. The tundras near the glaciers were replaced by forest and grasslands. The animals such as the mammoth that lived on the tundras also moved north, but they started to die out. Some scientists have suggested that they could not adjust to the changing climate. Others have hypothesized that they were no longer able to resist certain diseases. The most controversial idea is that they were overhunted by the Paleoindians. If you would like to know what some scientists think about this, look at some of the following web sites. Much of this lecture comes from these sources:
The Late
Pleistocene Extinctions
The Illinois State Museum exhibit does a good job of explaining the extinctions from both
the viewpoint of environmental causes and the human overhunting hypothesis.
Extinction and Depletion
from Over-Exploitation
A Biological Conservation course lecture outlines the issues well.
The Pleistocene Holocene Transition -
The Case of the Arboricidal Megaherbivores
Elin Whitney-Smith presents the extinctions as a sort of detective story with Holmes and
Watson, making excellent deductions along the way. It is a bit complicated for the
non-specialist, but you can learn a lot by going through it.
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larry-zimmerman@uiowa.edu
University of Iowa Anthropology
08.20.98