Quantitative analysis of megafaunal extinctions and the tenacity of pleistocene overkill: archeology and the world historian.
A paper published 5 March 2012 in Proceedings of the National Academy of Sciences by five Cambridge scholars determined, through quantitative analysis of data from five global landmasses, that late Quaternary period climate change coincided with the extinctions of most of the northern hemisphere's megafauna that weighed at least 44 kilograms. Furthermore, climate change beginning about 100,000 years ago coincided with the migration of humans in Eurasia, Australia, New Zealand, and later in North America and South America, and by measuring extinction rates of megafauna in these regions the authors determined "that both climatic variables and human arrival were important predictors of extinction rates." (1) Megafaunal extinctions, to name only a few, included the North American and Eurasian mammoth, the North American mastodon and giant sloth, the European woolly rhino, the Australian giant kangaroo and wombat, and the New Zealand moas, a giant flightless bird. The majority of these extinctions occurred in the last 50,000 years.
What distinguishes the Cambridge paper from previous studies is its wide geographical and temporal scope. Climate variability from several hundred thousand years ago was determined through examination of Antarctic ice cores. This data was analyzed in combination with both the earliest and the latest proposed arrival dates for humans on the five landmasses in the study as well as with data regarding extinction dates of megafauna. Generalized linear models were utilized to determine relationships between human presence and climate variation for each landmass. To examine uncertainty between megafaunal extinction and human arrival, the models were subjected to 10,000 combinations of first and last appearances of megafauna for time scales of 700,000 and 100,000 years as well as to 32 combinations of arrival dates for humans, providing a total of 320,000 extinction scenarios.
Regarding climatic variables, a significant relationship was indicated between rates of decrease in temperature and rates of extinction, but rates of temperature increase had a limited effect on extinctions. In most of the predictive models, the combination of climatic variables and human arrival dates coincided with megafaunal extinction rates. However, human arrival provides only "one explanatory variable (compared with four climate variables), which can act in one (700-ky time scale) or two (100-ky time scale) time intervals, whereas climatic variables can act in all of them." (2) Furthermore, the North Greenland Ice Core Project provided data for a 100-ky analysis of climatic variables that compared favorably with the Antarctic Dome C ice core for the same period. Both cores indicated associations between human arrival, the rate of megafaunal extinction, and the maximum rate of temperature decrease among climatic variables.
The quantitative global analysis of climate change in relationship to megafaunal extinction beginning approximately 100,000 years ago further contextualizes this period in human history; moreover, it calls into question the theory of Pleistocene overkill that has been widely accepted since it was first advanced by Paul S. Martin in 1967. (3) Martin's models were, in comparison with the Cambridge paper, only marginally quantitative; indeed, his conclusions were buttressed largely by analogy and by utilizing lack of evidence as evidence, that is, by negative evidence. Although informative, his reliance on range-carrying capacities for mid-twentieth century African game parks and by an isolated historical record "of extraordinary meat consumption and occasional extreme waste among the Plains Indians" (4) are not reliable indicators of either Pleistocene megafaunal population rates or of the manner in which human hunters may have utilized their prey across the range of the entire continent. Furthermore, his human population models assume a reproduction rate that "would unavoidably explode [...] with a force that exceeded ordinary restraint," claiming a rate as high as 3.4 percent annually; this rate is based analogously on an isolated example of the settlement rate for Pitcairn Island as reported by J.B. Birdsell in 1957. (5) Martin also assumes an omnipotent perspective by which Paleolithic pioneers, having crossed the Bering Bridge, "found a productive and unexploited ecosystem of over 107 square miles." (6) He also narrativizes their march across the continent as if the journey was a matter of only a few generations of hunters "who conquered the frozen tundra of eastern Siberia and western Alaska [...] and must have been delighted when they first detected milder climates as their routes turned southward." (7) But most egregious is his near total dismissal of climate variables, an analytical lacuna that clears the way for his hypothesis that humans and humans alone were responsible for the extinction of 35 genera of North American megafauna.
Martin's negative evidence includes the lack of kill sites as proof of the rapid rate of killing: "Extinction would have occurred before there was opportunity for the burial of much evidence by normal geological processes. Poor paleontological visibility would be inevitable." (8) Two more pieces of negative evidence that Martin uses to claim sudden overkill are "the absence of cave paintings of extinct animals in the New World and the lack of ivory carvings such as those found in the mammoth hunter camps of the Don Basin. The big game was wiped out before there was an opportunity to portray the extinct species." (9)
David J. Meltzer provides a succinct categorical critique of Martin's theory in his 2010 First Peoples in a New World: Colonizing Ice Age America. First, Meltzer points to the geographic and environmental challenges faced by people as they colonized the vast stretches of North and South America. He also questions the ability of humans to make the necessary adaptations to live in the wide variety of regions between Alaska and Tierra del Fuego in as little as 1000 years, the time scale proposed by Martin, the brevity of which is essential to accommodate his blitzkrieg liquidation scenario of 35 genera of megafauna within 350 years, and a time scale that posits a colonization front that advances in an unstoppable human wave with the steadiness and determination of an organized army. Second, the reproductive rate of 3.4% that Martin proposes for Pleistocene humans does not correlate to the reproductive rate of approximately 1.5% for most of the world's hunter-gathers. Martin's rate is based largely on isolated and atypical rates such as the Pitcairn Islands following the arrival of the Bounty mutineers. Third, each species possesses its own unique behavior, so the tracking and killing strategies of their hunters must vary accordingly; therefore, one might question the ability to amass the continent-wide knowledge base that would be required by Clovis-age peoples to hunt nearly three dozen genera of megafauna to extinction within 350 years. Fourth, foragers tend to move to new areas when resource returns begin to decline, unless groups are prevented from migrating by other bordering peoples, a circumstance unlikely in the thinly populated Pleistocene, so with the opportunity to move to areas promising more lucrative resource returns, it is, in Meltzer's words, "highly unlikely Clovis hunters behaved like Sherman's army in its scorched-earth march across Georgia." Fifth, the naivete of Pleistocene megafauna as imagined by Martin does not corroborate with evidence regarding the rapid speed at which animals process information regarding new predators. Also, before the arrival of humans, all megafauna were certainly familiar with their predators, and human presence added merely one more to the list. In short, the window of opportunity was likely opened only briefly for Pleistocene hunters to take advantage of their prey's unfamiliarity with new dangers. Sixth, Martin's posited megafaunal naivete as instrumental to overkill can operate only if humans did not inhabit North America before the advent of Clovis technology, but North American human presence (and potential megafaunal predation) dates to thousands of years before Clovis as indicated by the archeological record. (10)
In 2003, David J. Meltzer and Donald Grayson published "Requiem for North American Overkill," (11) which met with an acerbic refutation from Stuart Fiedel and Gary Haynes.12 Echoing Martin's defense of overkill, they cited negative evidence as evidence and argued that the scarcity of kill sites is to be expected, since the period of megafaunal predation lasted only approximately three centuries, coinciding with the overlap of Clovis technology and megafaunal prey. (13) Furthermore, the role of climate change is summarily dismissed:
Grayson and Meltzer advocate a vague theory of climate change in place of overkill, while candidly admitting that for now, "none (of the climate change hypotheses) connect particular climate variables with particular organisms in powerful ways." Climate change has always been the main theoretical alternative to human predation. Grayson and Meltzer offer no new refinements to the climate model, and seem unaware of recent developments in the study of latest Pleistocene climates. (14)
The recent developments referred to by Fiedel and Haynes concern the Younger Dryas, and these studies led them to conclude that "the Younger Dryas episode is unique in its faunal consequences only because its onset coincided with the arrival of human hunters." (15) Ironically, their zero-sum adherence to the theory of Pleistocene Overkill demanded that climate change could be only coincidental, a mere backdrop to the monocausal megafaunal blitzkrieg, which occurred independent of environmental context. Of course, Grayson and Meltzer were fully aware of the Younger Dryas. Indeed, Meltzer agreed with the conclusion regarding the uniqueness of the era: "the Younger Dryas was unique in the annals of glacial history. It has traditionally been thought the events ending this last glacial cycle were no different than those ending previous glacial cycles." But he went on to explain, "That's now doubtful, but that's good news: it might help explain why many of the Pleistocene mammals, which so successfully had survived previous glacial-interglacial cycles, succumbed to this one." (16)
Martin's own last words on the topic are in his 2005 Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. Here he continues to dispel the role of climate change as a factor in megafaunal extinctions and continues to argue "that virtually all extinctions of wild animals in the last 50,000 years are anthropogenic, that is, caused by humans." (17) His reaction to the role of climate in Pleistocene extinctions deserves to be quoted at length.
[S]ome members of the climate-change school are in deep denial (Grayson and Meltzer 2002, 2003). A cadre of archeologists, especially those who claim or prefer to believe that people were in the New World before the extinctions began, agrees with them. In addition, many vertebrate paleontologists of my generation, born in the first third of the twentieth century, support the climatic paradigm. (18)
To his credit, the 1984 Quaternary Extinctions: A Prehistoric Revolution that Martin edited with Richard G. Klein includes a section on geologic-climatic models. (19) In the introduction, he generously offers the possibility that "if climatic models ultimately prevail in accounting for Pleistocene extinction, they are likely to be derived from examples offered in this section." (20) Seven articles are included, two of which examine extinctions across the long range: S. David Webb identifies five major extinction episodes over the last ten million years, and his data suggests their potential relationship to terminal stages of glaciation; (21) Philip D. Gingerich also takes the long view of extinctions with the framing question, "Viewed in the context of the entire Cenozoic era, are Pleistocene extinctions unusual?" (22) Russell W. Graham and Ernest L. Lundelius, Jr. examine the coevolutionary disequilibrium between plants and animals that may have occurred in the late Pleistocene and consequently triggered megafaunal extinctions. (23) The remaining four articles of the section examine Paleolithic climatology and geology along lines similar to these three articles. (24)
Paul Martin was known for his generosity as a scholar, and he welcomed exchange with those who opposed Pleistocene Overkill, particularly because such exchanges would sharpen research into his own theory. Furthermore, Martin's credentials as an extraordinarily meticulous and gifted archeologist were joined with a famously charismatic personality, certainly an overall winning combination that undoubtedly left a highly influential mark on his students. He never absolutely ruled out the possible role of climate change in Pleistocene extinctions, but he remained a loud, if increasingly isolated, proponent of overkill.
Martin did go as far as to say that "climate change can reduce, increase, or shift species' ranges, reduce or increase the availability of nutritional quality of forage; change the length of seasons; and otherwise regulate populations." But as soon as he opened the door, he slammed it shut again by proclaiming "the climate-change proponents seem to me to assume their conclusions rather than to prove it," (25) a statement that paralleled his remark earlier in the text that "climatic change is always of interest but not crucial in formulating explanations." (26) Martin's rejection of climate change theory resulted in a highly polarized scholarly atmosphere: in one corner is the overkill party; in the other corner is the overchill party.
Paul Martin died in late 2010. Perhaps, had he lived to see the Cambridge study, which was published less than two years after his death, he would have reconsidered his harsh dismissal of climate change as a factor in Quaternary extinctions. Though such speculation is pointless, Martin left no few disciples who have far too many years invested in parroting the overkill theory and, understandably, resist changing their fierce adherence to his dated conclusions. Stuart Fiedel and Gary Haynes are only two examples. Yet, the Cambridge study is by no means a zero-sum analysis and its conclusions do not reject the anthropogenic factor in late Quaternary megafaunal extinctions. Again, as underscored in the study, "both climatic variables and human arrival were important predictors of extinction rates." (27)
Among world historians, a survey of ten current major world history textbooks indicates a widely shared understanding that late Quaternary extinctions were the result of both anthropogenic and climatic actions. Eight of the ten texts provide clear statements regarding the dual roles, (28) one mentions extinctions but does not address a cause for it, (29) and one does not cover the subject of megafaunal extinctions. (30) Though a textbook survey provides only a single metric by which to gauge the current state of understanding among world historians regarding megafaunal extinctions, it is no small measure, and the broad consensus indicates the world historical approach at work, which is to understand events in broad historical context and to maintain a healthy suspicion for monocausal theories.
Of course, when it comes to narrativizing human history in the Pleistocene, world historians must rely completely on the archeological record. It is for this reason that the PNAS paper on megafaunal extinctions is of vital importance to us. Although its conclusions are perhaps not surprising, they are definitive to a degree unmatched by previous studies. The paper further indicates the obsolescence of Martin's term, "Pleistocene Overkill," because the term--and of course the theory to which the term points--is too narrow and does not allow room for the environmental factor in the complex equation concerning late Quaternary megafaunal extinctions.
Paul Jentz, North Hennepin Community College
(1) Graham W. Prescott et al., "Quantitative Global Analysis of the Role of Climate and People in Explaining Late Quaternary Megafaunal Extinctions," Proceedings of the National Academy of Sciences of the United States of America 109, no. 12 (2012): 4527.
(2) Ibid., 4528.
(3) Paul S. Martin, "Prehistoric Overkill," Pleistocene Extinctions: The Search for a Cause, eds. Paul S. Martin and H.E. Wright (New Haven: Yale University Press, 1967), 75.
(4) Paul S. Martin, "The Discovery of America," Science, New Series 179, no. 4077 (1973): 972.
(5) Ibid., 970.
(6) Ibid., 969.
(7) Ibid., 970.
(8) Ibid., 969.
(9) Ibid., 972.
(10) David J. Meltzer, First Peoples in a New World: Colonizing Ice Age America (Berkeley: University of California Press, 2010),256-258.
(11) Donald K. Grayson and David J. Meltzer, "A Requiem for North American Overkill," Journal of Archeological Science 30 (2003).
(12) Stuart Fiedel and Gary Haynes, "A Premature burial: Comments on Grayson and Meltzer's 'Requiem for Overkill,'" Journal of Archeological Science 31 (2004).
(13) Ibid., 122-123.
(14) Ibid., 124.
(15) Ibid 125-126.
(16) David J. Meltzer, 60.
(17) Paul S. Martin, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America (Berkeley: University of California Press, 2005), 3.
(18) Ibid., 165.
(19) Paul S. Martin and Richard G. Klein, eds., Quaternary Extinctions: A Prehistoric Revolution (Tucson: The University of Arizona Press, 1984).
(20) Ibid., 187.
(21) S. David Webb, "Ten Million Years of Mammal Extinctions in North America" in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 189-210.
(22) Philip D. Gingritch, "Plesitocene Extinctions in the Context of Origination-Extinction Equilibria in Conzoic Mammals," in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 211-222.
(23) Russell W. Graham and Ernest L. Lundelius., Jr., Coevolutionary Disequilibrium and Plesitocene Extinction," in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 223-249.
(24) John E. Guilday, "Pleistocene Extinction and Environmental Change: Case Study of the Appalachians, in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 250-258; R. Dale Guthrie, "Mosaics, Allelochemics and Nutrients: An ecological Theory of Late Plesitocene Megafaunal Extinctions,: in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 259-298; Richard A. Kiltie, "Seasonality, Gestation Time, and Large Mammal Extinctions," in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 299-314; and James E. King and Jeffrey J. Saunders, "Environmental Insularity and the Extinction of the American Mastodont, in Quaternary Extinctions: A Prehistoric Revolution, ed. Paul S. Martin and Richard G. Klein (Tucson: The University of Arizona Press, 1984), 315-339.
(25) Paul S. Martin, 2005, 167.
(26) Ibid., 48.
(27) Graham W. Prescott et al., 4527.
(28) See Jerry H. Bentley and Herbert F. Ziegler, Traditions and Encounters: A Global Perspective on the Past, 5th ed. (New York: McGraw Hill, 2011), 108-109; Jerry H. Bentley, Herbert F. Ziegler, and Heather E. Streets-Salter. Traditions and Encounters: A Brief Global History, 3rd ed. (New York: McGraw Hill, 2014), 64; Felipe Fernandez-Armesto, The World: A History, 2nd ed. (Upper Saddle River: Prentice Hall, 2010), 24; Edward H. Judge and John W. Langdon, Connections: A World History, 2nd ed. (Boston: Pearson, 2012), 97; John P. McKay et al., Understanding World Societies: A Brief History, Volume 1 (Boston: Bedford, 2013), 10; Peter von Sivers, Charles A. Desnoyers, and George B. Stow, Patterns of World History: Brief Edition (New York: Oxford University Press, 2013), 22; Peter N. Stearns et al., World Civilizations: The Global Experience, 6th ed. (Boston: Longman, 2011), 167; and Robert W. Strayer, Ways of the World: A Brief Global History with Sources, 2nd ed. (Boston: Bedford, 2013), 18, 22.
(29) Albert M. Craig et al., The Heritage of World Civilizations, 8th ed. (Upper Saddle River: Pearson, 2009), 41.
(30) Howard Spodek, The World's History, 4th ed. (Boston: Prentice Hall, 2010).
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|Title Annotation:||Special Section: Global Environmental History|
|Publication:||World History Bulletin|
|Date:||Sep 22, 2013|
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