Discoveries at Rancho La Brea and the Debate over Late Pleistocene Extinction

Laci M. Gerhart

Abstract

Since the discovery of the Rancho La Brea tar pits in southern California, the fossils

present in the tar have captivated the minds of scientists and the public alike. Many scientists

spent their careers investigating the cause of the large number of extinctions seen at the end of

the Pleistocene Era, during the Rancholabrean stage. In the scientific community, the debate

focused on climate change and overkill by early humans. Some Native Americans questioned the

foundations of these hypotheses and the role of early indigenous groups in causing extinction.

On all sides, groups fought to discount other theories and develop compelling arguments. As

more fossils were excavated and more advanced methods of study developed, the debate over the

cause of large-mammal extinction became more complicated and more hotly debated. This report

will highlight the changes in the debate throughout the last 200 years, highlighting main theories

and addressing some of the social issues involved, particularly those involving North American

indigenous groups.

1

Rancho La Brea and Early Extinction Theories

The tar seeps in southern California were first documented by Gaspar de Portola in his

diary of the Portola California Expedition of 1769-1770, although the skeletal contents of the pits

would not be known for another hundred years. At this time, the possible extinction of any

organisms on earth was an idea not considered by most scientists. The loss of entire groups of

animals implied imperfection in God’s creation and so the concept was largely ignored.

Compounding the social reluctance to accept extinction were the numerous instances of

discovering species once thought to be extinct. Many of the arguments supporting extinction

were based on fossilized invertebrates, which were frequently later found still living in other

habitats. It was not until 1796 when Georges Cuvier showed the fossil remains of ground sloths

and elephants, which were too large to assume they could be living elsewhere, that the idea of

extinction became a part of scientific understanding. 1

Once extinction events became an accepted part of Earth’s history, numerous theories

were proposed to explain their cause. Foremost during the early 1800s were rapid geological

change, slow and natural changes, and human impact. Rapid geological change was usually put

in the framework of environmental catastrophe. Originally, some scientists argued local or global

flooding events while later arguments included glacial theories. The theory of slow, natural

changes relied more on climatic fluctuations such as changes in temperature, humidity, soil

composition, and floral and faunal assemblages. The theory of human impact quickly met with

skepticism over the time at which humans appeared in North America. Even at this time, the lack

of archaeological sites showing human remains with the remains of extinct animals was a

problem for the human impact theory. Consequently, few earth scientists accepted the theory.

2

After the 1850s, human impact began to gain acceptance simply because other theories seemed

inadequate. Still, by the end of the 19th century, there was little agreement between scientists. 2

By the late 1870’s Rancho La Brea was owned by the Hancock family. Excavation on

the pits had uncovered numerous animal bones, which the family assumed were the remains of

local livestock and wildlife. William Denton, a geologist and paleontologist at Wellesley

University was the first to recognize the bones as ancient specimens in 1875, although 30 years

passed before other scientists corroborated his findings. While some of the animals found in

Rancho La Brea were still living, animals such as the dire wolf, saber-tooth cat, mammoth,

mastodon, and giant ground sloth had clearly gone extinct. 3

Between 1913 and 1915, excavations at Rancho La Brea yielded nearly 1 million

specimens (Fig 1). Scientists and excavators quickly noted an unusual pattern. In living

populations, herbivores usually greatly outnumber carnivores, while in the fossil assemblages in

Rancho La Brea, the opposite was true. Nearly 90% of the mammal fossils excavated were

carnivores and nearly 70% of avian fossils were birds of prey. Scientists explained this

phenomenon using the idea of entrapment; a herbivore would first become mired in the tar and

its cries of distress attracted numerous predators, which also became trapped. 4

Figure 1: taken from Stock 2001, Assistant

vertebrate paleontologist J. W. Lytle in the

“bone room” in the basement of the

Natural history Museum of Los Angeles

County sorting material recovered from the

1913-1915 excavations.

3

In 1915, G. Allan Hancock donated 23 acres of the family property to Los Angeles

County for continued research. Before the advent of radiocarbon dating, much of the research

published was descriptive in nature. In the 1940s, when radiocarbon dating was developed, the

door was opened for detailed analysis of the timeline of these extinctions. Dating showed the

fossils varied in age from less than 6,000 years old to around 32,000 years old. The currently

extinct large mammals appeared to have disappeared around10-12,000 years ago, at the end of

the Pleistocene Era. 5

The Debate Begins

In 1967, Paul Martin, a professor at the University of Arizona, published Pleistocene

Extinctions, a compilation of essays addressing the extinctions seen at areas like Rancho La

Brea. While human impact had long been thought to cause extinctions, Martin publicized the

“overkill” hypothesis. Extinctions during this time, Martin argued, seemed to target large

mammals and the species that are ecologically dependent on them (such as carnivores and

scavengers) and occurs after early man, called paleoindians, had crossed the Bering Strait land

bridge into North America. Martin argued against climate change as causing any extinction

citing the fact that climatic shifts during the boundary between the Pleistocene and Recent Eras

were no more severe than other shifts that occurred throughout the Pleistocene, which these

animals had clearly survived. In addition, Martin argued, we must afford ancient species the

same adaptability as living organisms – when the environment changed, surely the animals could

have migrated to more favorable areas as modern organisms often do. Consequently, it seemed

clear to Martin that over-hunting of large mammals by paleoindians had caused their extinction

and resulted in cascading extinctions of species also dependent upon the megafauna (Fig 2). 6

4

Figure 2: taken from Martin 2005, left: The Dance of Death by Dr. Wallace Woolfenden, right:

Clovis artifacts, photographed by C. Vance Haynes

In the same book, several other scientists published competing theories on the causes of

large-mammal extinctions during the Rancholabrean event. John Guilday of the Carnegie

Museum argued that differential extinction of large mammals was caused by their greater

requirements for food and space (Fig 3). The end of the Pleistocene marked a remarkable

harshening of the environment during which large changes in temperature and moisture stressed

the animal populations. Competition between species increased and extinction was likely to

occur if a given species was too specialized, competing with a superior species, unable to

migrate to suitable habitat or unable to adapt to rapid environmental change. Paleoindians,

Guilday argued, were small in number and had limited hunting technology. Also, hunting of

mammals as large as mammoths would have been limited to areas without smaller prey.

Consequently, overhunting could have exterminated the last members of a decimated population,

but Guilday determined climate change was the primary cause of extinction. 7

5

Figure 3: taken from Guilday

1984, Differential extinction

model showing increased

susceptibility of large

mammals to extinction due to

increased resource demands

and therefore, increased

competition.

Bob Slaughter of the Shuler Museum of Paleontology agreed with Guilday that climate

change was the root cause, but differed on his ideas of the mechanism by which animals

succumbed to these changes. Slaughter pointed out that it was mostly large mammals that went

extinct, but some smaller mammals, such as the small antelope weighing only 20 pounds, also

disappeared. What did these animals have in common? The mammals that went extinct,

Slaughter argued, all had long gestational periods. The changes in climate during the

Rancholabrean exinction resulted in more extreme winter and summer temperatures. Colder

winters gave animals less time to reproduce in favorable conditions and populations could

quickly be decimated by the loss of young. Species that could not alter their mating habits to the

changes in environment were doomed. While this argument never gained immense support,

similar theories were proposed by R. Dale Guthrie and Richard Kiltie well into the 1980s. 8

6

Martin also published a paper on Pleistocene extinctions for Natural History magazine in

1967. In this publication, Martin highlighted the highly wasteful habits of the paleoindians citing

examples of early man decimating entire herds in order to use only a few animals. In addition,

New World fauna had not evolved with man as species in Africa and Europe had, and so were

defenseless against this new and aggressive predator. Paleoindians, he argued, were not classic

environmentalists but exploited their environment just as humans do today to the detriment of

countless species. 9

Donald Grayson of the University of Washington followed up Martin’s theories with a

study on bird extinctions at Rancho La Brea. Martin’s theory required that large mammal

extinction rates be higher than general extinction rates at the time. Grayson’s analysis of the

Rancho La Brea fossil assemblage showed that 10 genera of birds went extinct at the end of the

Pleistocene out of 22 genera that were lost during the entire era, or about 45%. For mammals, 32

genera disappeared at the end of the Pleistocene out of 67 that were lost in total, or about 48%.

Martin’s theory only accounts for high extinction rates if the animals in question were directly

hunted by man or were dependent on animals that were hunted. With one exception, the birds in

Grayson’s study were neither predatory on mammals, nor scavengers. Consequently, Grayson

argued that the range of species present in the La Brea tar pits was much too broad to be

explained by human predation alone. 10

Martin countered Grayson’s argument in his 1984 book Quaternary Extinctions saying

“Grayson’s method obscures the fact that virtually all late Pleistocene mammalian extinctions are

of large mammals, which is not the case earlier in the Pleistocene. Furthermore, unlike the large

mammals, most late Pleistocene birds belong to living genera…had birds suffered as much

extinction ad did the large mammals, two thirds of the Wisconsin-age avifauna would be

7

extinct.” (Emphasis in original) Even so, Grayson’s argument was a problem for Martin’s

overkill hypothesis and the two scientists continued to argue their ideas well into the future. 11

Overkill theory gained a new audience with the publication of Ecological Imperialism by

Alfred Crosby in 1986. Crosby reiterated Martin’s hypothesis of the naïveté of New World

animals to the ruthless and efficient paleoindian hunters and described the Pleistocene as “a

multispecies revolution whose aftershocks still rock the biosphere.” This publication was very

effective in promoting the overkill hypothesis and Martin’s work to the general public and the

theory gained more support. 12

The next 20 years saw a distinct change in the arguments proposed against Martin’s

theory. With the promotion of extinction theories in popular media, a wide variety of people

were analyzing the arguments put forth by Martin, Grayson and others. Vine Deloria, Jr.,

published Red Earth, White Lies in 1997 in response to Martin’s theories. While other scientists

attacked the theoretical basis for his findings and the data he analyzed, Deloria also attacked the

social constructs behind Martin’s findings. Deloria began his book with an argument against the

Bering Strait land bridge migration, a crucial part of the overkill hypothesis. Deloria argued that

no one had discussed why early man migrated from Siberia to North America, a dangerous trek

that required crossing numerous mountain ranges. With no knowledge of what was past the

mountains, why would paleoindians have left a home of plentiful food for the unkown? Another

major problem, Deloria stated, is the land bridge theory’s direct contradiction with oral tradition

of numerous tribes. Many tribes’ oral histories include periods of intense cold with excessive ice

and snow – could these tribes have already been in North America during the periodic

progression and retreat of the glaciers?13

8

Similarly, oral traditions in many tribes address the animals in question. This cultural

memory suggests that animals such as giant deer, mammoth, mastodon, giant beaver and saber-

tooth cat may have survived long after the late Pleistocene and potentially still been living when

white men first set foot on the continent. In a publication in 1944, anthropologist Ashley

Montagu wrote that, according to some scholars, “had the first of the Spanish discoverers of

America penetrated into the interior, it is quite possible that they might have met with the living

mammoth.”14

Not only did Martin’s theory ignore the oral histories and traditions of the living tribes of

Indians, it suggested a lack of concern for the Earth among North American tribes living and

dead. Deloria noted that extinctions documented in Europe during the same time period are

frequently attributed to climate change and not the humans inhabiting the area. In support of this

statement is a chapter on Pleistocene mammal extinctions in Europe from Martin’s 1967 book,

Pleistocene Extinctions, written by Kazimierz Kowalski. In his chapter, Kowalski argues that

“hunting activity of primitive man, even over a very long period, does not necessarily bring the

extinction of his prey” and that “it is very difficult to judge the size of the human population and

the degree to which hunting influenced the population of particular species.” Kowalski noted that

extinction was not a danger for many species until deforestation practices in the Middle Ages

decimated European forests. While the European Pleistocene extinction was not as severe as that

in North America, Kowalski clearly stated “its main cause, in Europe, was the disappearance of

the specific habitat…of the Pleistocene.” So, Deloria inferred, only in North America are Indians

to blame for extinction. This Eurocentric science, he argued, is faulty, unfair and ignores a large

body of evidence present in the traditions of Indian tribes. 15

9

Deloria closes the book with an account of floral extinctions seen in Europe during the

end of the Pleistocene. Evidence suggests Europe lost such species as magnolia, locusts, honey

locusts, catalpa, sassafras, osage orange, hickory, walnut, hemlock, and spruce, among many

others. “It therefore seems painfully obvious” Deloria satirically argues, “that Europeans were

the culprits of this dreadful floricide. With their new-found technology, the single point blade,

they…engaged in a frenzy of destruction in Europe…always cutting more hardwood trees than

their homes needed.”16

Deloria’s argument brought a new dimension to an already complicated debate. In The

Ecological Indian, published in 1999, Shepard Krech III responded to the ideas put forth by

Deloria. The popular image of Indians as quintessential environmentalists is inaccurate, he

argued. Deloria oversimplifies the issue and ignores the resource usage of Indian tribes. Krech

also attacked Martin’s theory challenging his use of the term blitzkrieg as synonymous with

overkill theory. The word blitzkrieg, Krech argued, evokes the aggressive and assaulting imagery

of Nazi Germany in the 1940s and is an unfairly provocative term. Like Deloria, Krech also

challenged Martin’s use of the Bering Strait land bridge theory of human origins in North

America and revisited Grayson’s argument of bird extinctions as problematic for the overkill

theory. Krech went a step further than Deloria, highlighting six other extinction events in the last

10 million years that clearly cannot be correlated with human impact of any kind. While

explanations for these extinctions are also under debate, they are correlated with temperature,

climate and sea-level fluctuations that likely played a role. Ignoring human impact, Krech

argued, is to ignore an important aspect of extinction; however, the same is true for ignoring

climatic variation. 17

10

A few years later, in 2002, Donald Grayson (with David Meltzer of Southern Methodist

University) published two more articles attacking the foundation of Martin’s overkill hypothesis.

In both articles, Grayson highlights four main points of Martin’s theory:

1) Archaeological and paleontological evidence show that human

colonization of islands is followed by vertebrate extinctions.

2) Clovis peoples were the first to enter the Americas, arriving around

11,000 years ago and were the first big-game hunters in North America

3) Extinction of large mammals in North America occurred about 11,000

years ago

4) Clovis people preyed upon a wide variety of now extinct large mammals.

Grayson conceded the first point is certainly true and examples of such extinctions are

numerous. However, extinctions on island ecosystems are due to a variety of anthropogenic

intrusions, not merely hunting. Also, such evidence cannot necessarily be extrapolated to an

entire continent the size of North America. 18

The second premise was thrown into question with the discovery of human artifacts in

Monte Verde, Chile in 1997. These artifacts pre-date the Clovis sites found in North America.

Assuming the Bering Strait land bridge theory, paleoindians must have been in the Rancho La

Brea area long before they were leaving artifacts in South America. And if Clovis hunters were

not the first paleoindians in the area, the naïveté of the animals to humans certainly no longer

applies. Perhaps it was not the humans themselves, Grayson suggested, but the new technology

of spear points that migrated across an already existing Clovis population. Regardless, such

evidence posed a severe problem for overkill theory.19

11

The third point, Grayson noted, may be accurate, although at this time, only 15 of the 35

extinct genera could be shown to have survived past 12,000 years ago. This data allowed for the

possibility of some extinctions pre-dating Clovis arrival in North America, although it was

certainly not evidence for such a theory. 20

The fourth point, Grayson argued, is certainly inaccurate. There are noticeably few kill

sites in North America compared to areas in Europe and these sites only showed predation on

mammoth and mastodon (and bison – which was not driven to extinction). Grayson’s arguments,

specifically those regarding arrival dates of humans and the lack of kill-sites, gained strength and

were a hard blow to the overkill theory. 21

Grayson noted the lack of evidence supporting overkill theory and questioned why such a

theory could become so popular. Maybe, he hypothesized, it was all political. Martin’s first book

on the subject, Pleistocene Extinctions, was published in 1967, the same year the Environmental

Defense Fund was launched and five years after Rachel Carson’s famous book, Silent Spring.

The following year Paul Ehlerlich would publish The Population Bomb and within 3 years, the

U.S. National Environmental Policy would be passed and the first Earth Day created. “We

suggest,” he wrote, “that the overkill argument captured the popular imagination during a time of

intense concern over our species’ destructive behavior toward life on earth. It retains that grasp

today.” 22

Still, even Grayson noted that the climate change argument had a long way to go to prove

itself. Numerous hypotheses had been forwarded but none had yet gained widespread support.

This was mainly due to the difficulty of linking a particular climatic variable to a particular

organism in any powerful way. To do so is extremely painstaking since each species must be

treated individually, while overkill theory can lump them all together as “prey”. Still, progress

12

had been made on this approach in other parts of the world and Grayson was hopeful that

increased research would prove successful for species at Rancho La Brea as well. 23

Stuart Fiedel and Gary Haynes, proponents of overkill theory, quickly countered

Grayson’s Requiem article. These two researchers argued a new point that had surfaced

explaining why so few Clovis kill sites existed. The rapid kill rates postulated by overkill (and

leading to the controversial name blitzkrieg) could have decimated the animal populations before

any appreciable evidence of the kills could be buried and preserved. This aspect of the theory

promotes invasion of a highly efficient predator over simple cultural evolution resulting in better

hunting technology. Grayson and Metzler published a follow-up response again highlighting the

political aspects of overkill theory and the lack of archeological and peleontological

understanding present in overkill arguments. 24

The following year, four Australian scientists led by Stephen Wroe published an article

discussing overkill hypothesis. Understandably, much of the article centered around extinctions

on the Australian continent, but the scientists discussed several aspects of Martin’s theory that

also applied to North America. While Martin and his colleagues had proposed a seemingly

plausible explanation for the lack of large mammal kill sites, the Australian scientists questioned

why kill sites showed biased preservation of taxa. If paleoindians were slaughtering 35 genera of

large mammals, why were only mammoth and mastodon represented in Clovis kill sites? The

authors also questioned the naïveté of the large mammals to human hunters. While such behavior

is observed on island faunas, the authors argued that this was due to the lack of any terrestrial

predators of any species. Since continental species often have defensive behavior towards other

predators (in this case the dire wolf, saber-tooth cat, etc) these behaviors could quickly be

applied to human predators as well. Wroe and his colleagues also point out that the blitzkrieg

13

theory assumes no regulatory behavior in the paleoindians. Many “primitive peoples,” they

noted, have property rights systems that keep resource usage under control and we cannot

assume that paleoindians were any different.25

Wroe and his colleagues also pointed out a faulty assumption in Martin’s ideas on climate

change. Martin assumes, they argued, that if climate change were the cause, extinctions should

have been simultaneous across the globe. The fact that extinctions were not simultaneous proves

to Martin that climate is not the cause. Martin’s argument, however, requires that climate change

be manifested in nearly identical ways in all continents and that all flora and fauna responded to

these changes in identical ways. Clearly, these are questionable assumptions since local

conditions can vary widely even when global changes are well-established and different species

of animals and plants respond in widely varying ways to changes in their environment.

Consequently, they argued, extinction is likely not due to one cause or the other, but rather

“technological and cultural change against a backdrop of climate fluctuation and invasion by

other species.”26

In his 2005 book, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of

America, Martin addressed some of these attacks. In this book, he discussed “deadly

syncopation” or the idea that extinctions must coincide with some other event. He argued that

extinctions are clearly correlated with the spread of humans across the globe. If climate change

were the cause, it must have followed the same timeline as human migrations. “Despite

accumulating evidence that humans caused the megafaunal extinctions,” he wrote “some

members of the climate-change school are in deep denial.”27

Martin explained the lack of kill sites by again highlighting the rapid extinction rates. The

rate at which an event occurs determines its visibility in the archaeological record. The blitzkrieg

14

of hunting by paleoindians would not have left much of a fossil trace. Survivors such as the

bison, he argued, escaped extinction by retreating to uninhabited (or uninhabitable) areas. 28

For climate change to be a viable theory, Martin argued, it would need to prove three

points:

1) There was significant climatic change around the time of extinction and

in the places that extinction occurred

2) These changes were unique to the end of the Pleistocene

3) These changes were powerful enough to kill of large mammals while

sparing most small mammals as well as plants and marine life.

Since no climate change argument had yet met these requirements, Martin discounted the theory

and ridiculed those who still argued its validity. “Whatever exceptions those of us supporting the

overkill model may be willing to make based on the field data, defenders of the climate school

seem to feel they must warn the good people of Hamlin that Pied Pipers are dancing off with

their intellectual children.”29

Evolution of an Argument

As can be seen in the publications previously discussed, the tone of the argument

changed drastically throughout the half-century of debate. In the 1960s and 1970s, both climate

change and overkill proponents argued their sides using the paucity of data available. While both

sides strongly disagreed, the arguments were primarily limited to differences in interpretation.

Into the 1980s and 1990s the debate turned simply to disproving the other side. Both schools

appeared to believe that proving their theory was based entirely on disproving the other one. The

publications reflect this shift as many articles attack the assumptions and conclusions of the other

15

side instead of citing evidence supporting the authors’ own theory. Following the turn of the

century, the arguments became more personal, with both groups attacking the intelligence and

integrity of the other side.

For example, Martin compared searching for pre-Clovis human artifacts to “something

less than serious science, akin to the ever-popular search for Big Foot or the Loch Ness Monster”

while Grayson stated the absence of evidence supporting overkill “removes the hypothesis from

the realm of science and places it squarely in the realm of faith. One may or may not believe in

the overkill position, but one should not confuse it with a scientific hypothesis.” In addition,

Grayson attacks Martin’s explanation for the lack of kill sites, saying “it is a rare hypothesis that

predicts a lack of supporting evidence, but we have one here, and we have it only because

evidence for it is, indeed, lacking.” 30

The personal attacks were not limited to the scientific field. Vine Deloria, Jr. accused

science and the media of intentionally insulting his people. “As I saw rednecks and conservative

newspaper columnists rant and rave over the supposed destruction of these large animals, I saw a

determined effort to smear American Indians as being worse than our present industrialists.” He

also chastised scientists for excluding the cultural history of North American Indian tribes,

saying “western science today is akin to a world history which discusses only the Mediterranean

peoples.” Martin responded to Deloria acknowledging that some people may be uncomfortable

with an anthropogenic explanation because of such associations. “Being uncertain of the

details… social scientists may simply decide that environmental explanations must be the answer

to the mystery of megafaunal extinctions.” Martin states that he is not trying to “argue the

relative merits of social scientific and scientific approaches to paleontology” but adds that he

16

“do(es) not find Deloria’s approach of any help in unraveling the mysteries of megafaunal

extinction.” 31

This change in attitudes highlights how personal this debate has become, specifically for

the American Indian tribes. In many ways, it is no longer just a debate about large-mammal

extinctions, but also a debate about the origins of human life in North America and the

ecological responsibility of early humans.

Conclusion

The fossil evidence of Pleistocene extinctions at Rancho La Brea continues to be

controversial and hotly debated. Even in recent years, there is little consensus among scientists.

For example, articles in Science Daily throughout the past several years have included titles such

as “Humans Hunted Mammals to Extinction in North America,” “Blame North American

Megafauna Extinction on Climate Change, Not Human Ancestors,” “Evidence Acquits Clovis

People of Ancient Killings, Archaeologists Say,” and “Climate Change and Human Hunting

Combine to Drive Wooly Mammoth Extinct.” Scientists such as Martin and Grayson continue to

publish new questions and interpretations of the fossils found in Rancho La Brea and

anthropologists like Deloria and Krech continue to question the foundations and implications of

such arguments. 32

17

Figure 4: taken from Martin and Wright 1984, favored paradigms for extinctions as concluded by

chapter authors. Parentheses indicate secondary or inferred paradigms

While some scientists strongly support either climate change or paleoindian overkill as

the sole cause of extinctions during the Pleistocene, many scientists cite a combination of the

two, along with other anthropogenic ecological interference, as the most likely hypothesis. This

trend can be seen in Martin and Wright’s 1984 book Quaternary Extinctions in which they chart

the “favored paradigms” of each chapter author included in the book (Fig 4). Many of the

authors cite numerous potential factors as acting in concert to produce the extinctions seen

during the Rancholabrean event. While this debate will likely never be finalized, continued

research on the Rancho La Brea fossils and the habitat surrounding them perhaps will one day

illuminate the true cause of the late Pleistocene extinction. 33

18

1 Chester Stock (rev by John Harris) 2001. Donald Grayson In: Martin 1984 2 Donald Grayson In: Martin 1984 3 Chester Stock (rev by John Harris) 2001 4 John Harris (ed) 1992 5 Chester Stock 2001. Leslie F Marcus and Rainer Berger In: Martin and Klein (eds) 1984 6 Paul S. Martin In: Martin and Wright (eds) 1967 7 John Guilday In: Martin and Wright (eds) 1967 8 Bob H. Slaughter In: Martin and Wright (eds) 1967 9 Paul Martin “Pleistocene Overkill” In: Ternes (ed) 1975 10 Donald Grayson 1977 11 David W. Steadman and Paul S. Martin, In: Martin 1984 474 12Alfred Crosby 1986 13 Vine Deloria Jr.1997 14 Ibid. 105, 130-136 15 Ibid 98. Kazimierz Kowalski In: Martin and Wright 1967 16 Vine Deloria, Jr. 1997, 233-234 17 Shepard Krech III 1999 18 Donald Grayson and David Meltzer 2002. Donald Grayson and David Meltzer 2003 19 Ibid 20 Ibid 21 Ibid 22 Donald Grayson and David Meltzer 2003 23 Donald Grayson and David Meltzer 2003 24 Stuart Fiedel and Gary Haynes 2004. Donald Grayson and David Meltzer 2004 25 Stephen Wroe, Judith Field, Richard Fullagar and Lars S. Jermin 2004 26 Ibid 300-301, 320 27 Paul Martin 2005 28 Ibid 29Ibid 167, 176 30 Donald Grayson and David Meltzer 2002, 346. Donald Grayson and David Meltzer 2003, 588 31 Vine Deloria 1997 xiv, 211. Paul Martin 2005 145-147 32 Science Daily articles 33 Wroe et al 2004. Arthur Jelinek In: Martin and Wright 1967. Peter Mehringer In: Martin and Wright 1967. Larry Marshall In: Martin and Wright 1984