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Author's personal copy EXTINCTIONS OF BIG GAME 1365 EXTINCTIONS OF BIG GAME Todd A Surovell, University of Wyoming, Laramie, WY, USA ã 2008 Elsevier Inc. All rights reserved. Glossary Blitzkrieg A specific form of the overkill hypothesis formulated by Paul Martin with respect to Pleistocene extinctions in the Americas. In blitzkrieg overkill, humans encounter faunas naı̈ve to human predation making them extremely susceptible to extinction by hunting. Rapid human population growth and geographic spread are fueled primarily by hunting of large game, and human colonization and faunal extinctions are complete within a millennium. Hyperdisease A hypothesis developed by Ross MacPhee and Preston Marx to explain the extinction of Quaternary faunas. The hyperdisease hypothesis argues that many prehistoric ‘‘first-contact’’ animal extinctions are explained by the introduction of hyper-virulent and hyper-lethal diseases first introduced to animal populations by colonizing humans and/or their domesticates (e.g., domestic dogs). Megafauna A term generally used to refer to the largest animals present within an ecosystem, most often used with respect to the array of large-bodied species which suffered extinction during the Quaternary. A strict definition refers to animal species weighing more than 44 kg (approx. 100 lbs). Overkill hypothesis The hypothesis that most animal extinctions of the Quaternary can be directly or indirectly attributed to predation by humans. Sitzkrieg A hypothesis developed by Jared Diamond to explain the extinction of Quaternary faunas. The sitzkrieg hypothesis refers to slow, drawn-out extinction events caused by secondary human effects, such as deforestation, anthropogenic burning, and general habitat modification. humans and hominids lived side by side with the woolly mammoth, rhinoceros, cave bear, giant deer, and straight-tusked elephant for thousands of years. They even left painted depictions of some of these animals on cave walls (Figure 1). When humans first arrived in Australia some 50 000 years ago, the island continent was inhabited by a menagerie of marsupial mammalian, avian, and reptilian megafauna. Yet by comparison, the world’s large fauna of the modern era are much reduced in diversity and geographic extent. Surprisingly, though the extinctions of the Quaternary are the most recent ‘mass extinction’ of the fossil record, the issue of cause remains largely unresolved. The idea that humans caused Late Pleistocene extinctions through over-hunting is known as the ‘overkill hypothesis’, but other explanations have been proposed. For example, some argue that Quaternary extinctions may have been caused by climate and ecological changes that have occurred during the last two million years. Others suggest that extinctions may have resulted from the introduction of novel and highly lethal pathogens by humans to populations of animals. Still other researchers have proposed multi-causal explanations that point to both human and natural causes. Of course, extinctions of all taxa in all places and times in the Quaternary need not have the same cause. Different factors may account for animal extinctions over time and space, and dauntingly any or all of these explanations could be correct when viewed at a global scale. Introduction Over the last two million years, humans and our hominid relatives have witnessed a broad wave of animal extinctions deemed by some as the sixth mass extinction to have affected the biological communities of the Earth. Mass extinctions are characterized by dramatic increases in extinction rates in fossil assemblages – large numbers of species disappear from the fossil record over brief windows of geologic time. Although the causes of recent extinctions remain unresolved, some researchers argue that humans were not mere witnesses to but were instead directly responsible for the extinction of most taxa that disappeared from the fossil record within the last two million years. There is little question that prehistoric peoples of the New World coexisted with mammoths, mastodons, camels, horses, and a myriad of other nowextinct animals. In Europe and Asia, Palaeolithic What Happened to the Megafauna? The Overkill Hypothesis The overkill hypothesis states: ‘‘the majority of animal extinctions that have taken place over the Quaternary have resulted directly or indirectly from human hunting.’’ Proponents of overkill do not claim that humans caused all animal extinctions of the Quaternary; nor do they claim that all humancaused extinctions resulted from over-hunting of extinct prey. For example, extinctions of large carnivores might have resulted from competition with humans rather than direct human predation of carnivores. The argument is simply that prehistoric human hunting dramatically modified animal communities, and had humans not colonized the planet, most of the species that suffered extinction during the Pleistocene and Holocene would still exist today. Although Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy 1366 EXTINCTIONS OF BIG GAME the origins of overkill can be traced to the nineteenth century, overkill was championed in the latter half of the twentieth century by Paul Martin, a palaeoecologist at the University of Arizona. Figure 1 Line drawing reproductions of Upper Palaeolithic cave paintings of extinct Pleistocene fauna. Irish elk, Lascaux cave a, woolly mammoth, Rouffignac cave b, cave bear c, and two-horned rhinoceros d, Chauvet cave. The keystone to Martin’s argument was the apparent widespread occurrence of ‘first contact’ extinctions across the globe. With the important exceptions of the continents of Africa, Europe, and Asia, waves of animal extinctions immediately followed initial human colonization of many regions of the world (Figure 2). In the 1970s, when considering extinctions in the Americas, Martin developed his most explicit formulation of the overkill hypothesis, a model he called ‘blitzkrieg’. With mathematician James Mosimann, Martin constructed a computer simulation which showed how humans could have colonized the entirety of the unglaciated New World from southern Canada to Patagonia within 1000 years (Figure 3). Rapid population growth and migration would have been fueled by hunting of nowextinct Pleistocene mammals. These animals which had never experienced human hunters would have been naı̈ve and easily killed. Although the overkill hypothesis (or variations thereof) applied to oceanic islands extinctions is generally accepted today, whether overkill explains extinctions on continents remains highly controversial. The most serious obstacle to overkill is that in most regions archaeological evidence for human exploitation of extinct taxa is scarce. In North America, for Continents Islands Colonization by: Archaic Homo Modern Homo 1000 Years before present Major extinctions 10 000 100 000 1 000 000 Australia 2 000 000 3 000 000 Europe South North America America Caribbean Mediterranean New Zealand Madagascar Geographic region Figure 2 The relative timing of human colonization and the major wave of animal extinctions for various continents (left) and islands (right). Solid colors and lines indicate certainty about age. Dashed lines and colors indicate uncertainty about age. Age axis is log-scaled. Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy EXTINCTIONS OF BIG GAME 1367 13 15 13 0B P 400 BP example, although 33 genera of large-bodied mammals suffered extinction during the Late Pleistocene, fewer than five can be shown to have been utilized by humans. If humans caused the extinction of North American species by over-hunting, then they must have killed thousands if not millions of animals, which begs the question, ‘Where is the archaeological evidence?’ Martin has argued that if blitzkrieg-type overkill happened very quickly, little archaeological evidence would be expected. For others, however, this lack of direct evidence has meant that perhaps we should be seeking explanations for extinctions elsewhere, such as in the dramatic swings in global climate that have occurred during the Quaternary. 12 83 0 BP 13 050 BP Climate Change 12 700 BP 12 600 BP 12 500 BP 12 400 BP Figure 3 The Martin/Mosimann ‘blitzkrieg’ model of Pleistocene extinctions. Humans enter North America via the ‘Ice-free corridor’ at approximately 13 400 BP, and rapidly colonize the New World within 1000 years. Human range expansion takes the form of a traveling wave. Along the wave front, naı̈ve Pleistocene fauna are hunted to extinction. Redrafted from Figure 2 from Martin PS (1973) The discovery of America. Science 179, 969–974. Glacial Climate: Variations in the Earth’s orbit over the last two million years have caused climate to oscillate between glacial and interglacial conditions. There were more than 20 instances when continental glaciers expanded and contracted over the Northern Hemisphere (Figure 4). As continental glaciers waxed and waned, oceanic and atmospheric temperatures cooled and warmed, sea levels fell and rose, precipitation regimes were greatly modified, and plant and animal species migrated and reorganized ecological communities. Because swings in climate had major impacts on biological organisms, there is no doubt that animal extinctions could have resulted, but there is considerable argument about how climate change might cause mass extinctions, and whether climate change explains the observed extinctions of the Quaternary. Interglacial Less δ18 O 3 4 5 Volume of glacial ice 2 More 6 0 100 000 200 000 300 000 400 000 500 000 Years before present Figure 4 The last five glacial cycles recorded in stable oxygen isotopes from benthic foraminifera recovered from a core of the Pacific Ocean floor. Oxygen isotopes of marine foraminifera record the isotopic composition of ocean water which is a direct reflection of the volume of glacial ice in terrestrial settings. High values of d18O indicate glacial periods. Low values of d18O, like those observed in modern times, indicate interglacials. Data from Mix AC et al. (1995) Benthic foraminferal stable isotope stratigraphy of site 846: 0–1.8 ma’. In: Pisias NG et al. (eds.) Proceedings of the Ocean Drilling Program. Scientific Results 138, College Station, TX, USA. 839–854. Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy 1368 EXTINCTIONS OF BIG GAME A handful of models have been proposed which attempt to link Quaternary climate change to extinctions. Unlike overkill models, climate change hypotheses are typically constructed with reference to specific geographic regions because extinctions occurred at different times in different parts of the world. The most explicit climatic/ecological extinction models have considered North America. For example, Dale Guthrie, a vertebrate palaeontologist at the University of Alaska, has argued that the transition from glacial to interglacial climate in North America (20 000– 10 000 years ago), resulted in vegetational communities that would have been detrimental to many species of Pleistocene megafauna. Lower levels of seasonality and longer growing seasons in Pleistocene ecosystems produced high-diversity mosaics of vegetation that would have permitted large herbivores to easily meet their nutritional requirements. With the transition to the Holocene, North American biomes were transformed into large-scale low-diversity patches wherein generalist feeders with simple digestive tracts, such as mammoths, mastodons, sloths, and horses would essentially starve, or due to anti-herbivory toxins, be poisoned to death by being forced to eat large quantities of toxic foods, to which these animals had little physiological resistance. According to Guthrie, only large mammals with specialized diets or physiologies, such as bison, pronghorn, and moose, thrived in Holocene ecosystems. Climate-based hypotheses, like overkill, suffer from many problems. Most commonly, it is questioned why it was a particular climate change that caused the extinctions. For example, in the Americas, extinction correlates with the most recent transition from glacial to interglacial climate, but this was just one of many such transitions that occurred during the Pleistocene. Proponents of overkill would argue that this episode of climate change just happens to correlate with the first arrival of human migrants; proponents of climate hypotheses would argue that this period of climate change was uniquely severe. Also, proponents of climate hypotheses must explain why extinctions seemingly always correlate with human colonization. If humans had no causal role in Pleistocene extinctions whatsoever, then the timing of extinction should be independent of human global colonization, and yet human migration and animal extinction seem to be highly correlated (Figure 2). Hyperdisease Due in part to problems with overkill and ecologicalclimatic hypotheses, a third major contending explanation for Pleistocene extinctions has come to the fore – hyperdisease. This idea, proposed by palaeontologist Ross MacPhee and virologist Preston Marx, can explain two aspects of Quaternary extinctions with which overkill and climate change struggle. Climatic models fail to explain the global pattern of ‘first contact’ extinctions, and overkill struggles to explain the paucity of archaeological evidence demonstrating human hunting of extinct fauna. MacPhee and Marx propose that extinctions were caused by the introduction of ‘hypervirulent, hyperlethal diseases’, which they call ‘hyperdiseases’, by humans or domestic dogs to naı̈ve populations of animals, naı̈ve in the sense of having no immunity to such pathogens because they had no prior experience with them. The hyperdisease hypothesis predicts that extinction should follow a first contact pattern and should produce little archaeological evidence of human exploitation of extinct animals. However, the disease model has yet to successfully overcome a range of potential theoretical setbacks: (1) Foremost, it is unclear whether a disease is capable of driving any species to extinction under natural conditions. (2) It is also unknown if a single disease could infect such a broad range of species as those lost during the Quaternary. (3) Finally, this hypothesis is very difficult to test. Efforts are in progress to attempt to recover pathogens directly from remains of extinct animals or from their fecal matter, but even if pathogens are discovered, it will be difficult to determine if in fact the identified pathogen is the sought-after hyperdisease. The Keystone Herbivore Hypothesis South African ecologist Norman Owen-Smith, an expert in ‘mega-herbivores’ (>1000 kg), recognized that the extinction of the largest of the Pleistocene fauna must have had dramatic effects on past vegetation. Extant terrestrial mega-herbivores, the African and Asian elephants and rhinoceroses, through foraging and trampling can dramatically impact ecosystems, turning forests and woodlands into scrubland, savannah, or grassland. The removal of mega-herbivores by hunting or climate change, therefore, could have dramatic effects on the structure of vegetational communities. If mega-herbivores served as keystone species in Pleistocene environments, their removal could have cascading effects causing the extinction of numerous other animal taxa. Owen-Smith calls this idea the ‘keystone herbivore hypothesis’. The Achilles’ heel of the keystone herbivore hypothesis is the prediction that mega-herbivore extinction should have preceded the extinction of other taxa, and to date there is very little evidence to indicate that this actually happened. Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy EXTINCTIONS OF BIG GAME 1369 fauna? When viewed from a global perspective, Quaternary extinctions show a number of patterns that provide clues to cause(s): (1) Extinctions were asynchronous across the globe, but globally appear to correlate to some degree with first human arrival to regions (Figure 2). (2) Quaternary extinctions on continents and large islands disproportionately and severely impacted large-bodied taxa. (Figure 5). (3) Fauna in some regions were more severely impacted than others (Figures 5 and 6). (4) In most regions, archaeological evidence for the interaction of humans and extinct animals is limited. The extinction records in various regions of the world are discussed in the following sections. Sitzkrieg Jared Diamond argues that most Pleistocene extinctions can be attributed to humans, but diverges from traditional overkill models with respect to cause. Using observations of historically recorded extinctions, Diamond notes that many human-induced animal extinctions are not caused by hunting, but have instead resulted from secondary effects. Diamond contrasts the traditional ‘blitzkrieg’ model to what he calls ‘sitzkrieg’. Diamond suggests that extinctions may not have been a lightning-quick predatory assault on Pleistocene fauna by humans, but instead a slow ‘war of attrition’ where extinctions were by-products of secondary impacts, such as felling of forests, anthropogenic burning, and the introduction of non-native competitors. Like the disease model, the ‘sitzkrieg’ model predicts that extinctions should correlate with or postdate human colonization, and that there should be relatively little archaeological evidence for human hunting of extinct fauna. As such, it remains a very difficult model to distinguish from hyperdisease, but some researchers find this idea very compelling for explaining extinctions on oceanic islands. Africa Africa holds a unique position in the Quaternary extinctions because it is the homeland of hominid and modern human evolution (see Modern Humans, Emergence of). In addition, a greater proportion of large-bodied mammalian taxa survived here than in any other region of the world (Figure 6). Proponents of overkill do not see these two facts as independent. Large mammal survival in Africa, they would argue results from the co-evolution of African large mammals with bipedal hominids whose hunting prowess gradually increased over time. Although many genera of large-bodied mammals did suffer extinction in Africa, extinctions do not show clear clustering in time, and many extinct genera were replaced by similar taxa, indicating that many African extinctions likely were caused by interspecific competition rather than anthropogenic factors. The first members of the genus Homo evolved in Africa roughly 2.5 million years ago. However, it The Global Record of Quaternary Extinctions Attempts to test extinction hypotheses typically focus on a handful of key variables. Which species suffered extinction, which survived, and was there a detectable and dramatic increase in extinction rate in the Quaternary? What is the relative timing of initial human colonization, climatic/ecological change, and the major wave of extinctions? Also, what is the archaeological evidence for hunting of extinct Surviving Extinct 100 100 Number of species North America South America 50 50 0 0 0 1 2 3 4 5 6 0 7 1 2 3 4 5 6 7 100 100 Africa Australia 50 50 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 log body mass (g) Figure 5 Body size distributions for extinct and extant Quaternary species in North America, South America, Australia, and Africa. In all regions, extinctions were highly selective, disproportionately affecting large-bodied animals. Redrafted from Figure 1 of Lyons SK, Smith FA, and Brown JH (2004) Of mice, mastodons and men: Human mediated extinctions on four continents. Evolutionary Ecology Research 6: 339–358. Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy 1370 EXTINCTIONS OF BIG GAME Percent megafaunal mammals 100% 75% Extinct Surviving 50% 25% Australia South America North America Eurasia Africa 0% Continent Figure 6 The percentage of extinct and extant megafaunal (>44 kg) mammalian genera by continent. Data from Barnosky AD, Koch PL, Feranec RS, Wing SL, and Shabel AB (2004) Assessing the causes of Late Pleistocene extinctions on continents. Science 306: 70–75. is not until the evolution of Homo erectus roughly 1.8 million years ago or perhaps even hundreds of thousands of years later that hominids had likely developed the ability to effectively hunt large animals. Over this period, there is an absence of clear temporal clustering in animal extinction in Africa, although there are many cases of likely climactically driven local extinctions (species survived in other parts of the continent). The inability to detect waves of extinction in Africa may be attributed either to a true lack of clustering of extinctions in time, or to poor chronological control on extinction events. Among the extinct Quaternary fauna of the African continent are many species of hominids including the entire genus Australopithecus, three genera of proboscideans, and more than 24 genera of ungulates and carnivores. Notably, many extinct genera have been recovered from archaeological contexts, particularly in Early and Middle Pleistocene contexts, but establishing predation of extinction species by pre-modern hominids has proved to be exceptionally difficult. Eurasia Like Africa, Eurasia suffered relatively few losses of large mammals, and hominids and extinct fauna coexisted for an extended period of time. The Eurasian landmass lost two genera of proboscideans including mammoths, at least three species of rhinoceros, hyenas, cave bears, hippopotamus, giant deer, and others. Europe lost proportionately more large mammals than Asia where mega-herbivores, like Asian elephants and rhinoceroses, survived to the present in tropical Southeast Asia. Unlike Africa, which sustained a hominid presence across most of the continent throughout the Pleistocene, large portions of high-latitude Eurasia remained uncolonized by Homo until the Late Pleistocene, providing refuge for many now-extinct taxa until the end of the last Ice Age. During the Pleistocene, continental glaciers expanded and contracted over most of northern Europe many times causing dramatic ecological shifts (see Paleoenvironmental Reconstruction, Methods). Hominids and animals underwent repeated range shifts in response to glacial oscillations. The genus Homo first migrated out of Africa and into the southerly latitudes of Eurasia approximately 1.8 million years ago, or slightly earlier. It is unclear whether hominids maintained a consistent presence in Eurasia during the Early Pleistocene (1.8–0.78 million years ago), but by the start of the Middle Pleistocene c. 0.78 million years ago, Homo was likely to stay in Eurasia. By 50 000–45 000 years ago, modern Homo sapiens had spread through most of Eurasia. Precise extinction dates for many Eurasian species are poorly known, especially for Asian species and those species which suffered extinction prior to 50 000 years ago, but numerous now-extinct animals coexisted with hominids for hundreds of thousands of years before suffering extinction. Although numerous mammalian extinctions occurred throughout the Quaternary in Europe, extinction rates among large mammals increased substantially in the Late Pleistocene between c. 50 000 and 10 000 years ago with extinctions possibly occurring in two pulses. Straight-tusked elephant and hippopotamus, species common in temperate fossil assemblages, were the first to suffer extinction, around 50 000–40 000 years ago. At this time, not only were continental glaciers Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy EXTINCTIONS OF BIG GAME 1371 expanding pushing these species southward, but modern humans made their first incursions into Europe. Arctic species like woolly rhinoceros and mammoth, well adapted to cold glacial conditions survived through the Last Glacial Maximum (c. 20 000 years ago) only to suffer extinction as the climate warmed c. 10 000 years ago. It is also after the Last Glacial Maximum that major human influxes into arctic regions occurred. A case can be made of overkill and/or climate change as the extinction causes for Europe since extinctions seem to occur at times when both humans and climate are on the move. Nowextinct mammal species did survive into the Holocene in isolated geographic pockets. For example, dwarfed mammoths survived until 4000 years ago on Wrangell Island in high arctic northeast Asia, and the giant deer, Megaloceros survived into the Early Holocene in western Siberia and the Ural mountains. Although human associations with extinct fauna, like rhinoceros, elephant, and mammoth, are fairly common in Lower and Middle Palaeolithic assemblages, direct associations are considerably less common in the Upper Palaeolithic when the majority of extinctions occurred. There are more than a dozen archaeological sites showing subsistence use of proboscideans (Elephas and Mammuthus) throughout Eurasia spanning more than 600 000 years of prehistory. These sites gradually increase in latitude with age, possibly reflecting slow human northward range expansion with concomitant proboscidean range contraction. Nonetheless, the causes of Eurasian mammalian extinctions remain as unresolved as on any continent. Australia During glacial phases of climate, sea levels dropped, and the map of Australia was dramatically altered because the island continent became connected to the adjacent islands of Papua New Guinea and Tasmania, forming the Ice Age continent of Sahul (see Oceania: Australia). Prior to human colonization, Australia was home to 28 genera and 55 species of vertebrate fauna that are now extinct (Figure 7). Australian extinctions were among the most severe of any continent, having lost 88% of its megafaunal mammalian genera (Figure 6). The largest known marsupials, the diprotodonts, large browers weighing perhaps up to 2000 kg, suffered extinction during the Late Pleistocene, as did 24 species of macropods (a family of marsupials which includes the kangaroos, tree kangaroos, wallabies, and padmelons). Also lost were a handful of carnivores including Megalania prisca, a huge meat-eating lizard. The flightless Genyornis newtoni was the largest of the five or six birds to have suffered extinction. Genyornis left Figure 7 Silhouettes of the extinct Pleistocene fauna of Australia drawn to scale. Reproduced from Murray in P. Martin and R. G. Klein (1984) (eds.) Quaternary Extinction: A Prehistoric Revolution Tucson: University of Arizona Press. abundant eggshells in Australia’s fossil record that can be readily dated and analyzed to determine the animal’s diet. Studies of Genyornis eggshells have produced critical data about timing of extinction and ecological change in Australia. Gifford Miller and colleagues dated more than 700 Genyornis eggshell fragments by the radiocarbon and amino acid racemization methods and found that the giant flightless bird went extinct between 55 000 and 45 000 years ago. The extinction date for Genyornis is unique because it is one of only very few extinct taxa whose extinction date is well established. Determining the precise timing of Australian extinctions has proved problematic. In some critical localities, it appears that extinct fauna were geologically redeposited into younger contexts creating the appearance of survival into time periods long after which the animals suffered extinction. Also, it is increasingly apparent that most Australian extinctions occurred near the temporal limit of radiocarbon dating (c. 50 000 years ago). One rigorous study of extinction dates in Australia by Richard Roberts and colleagues found that at least six genera of megafauna, in addition to Genyornis, suffered extinction at approximately 46 000 years ago. Therefore, the time period between 55 000 and 45 000 years ago appears to be the key to solving the riddle of Australian extinctions. Although the precise date of human colonization of Australia remains controversial, well-dated contexts at Lake Mungo and Devil’s Lair both suggest a colonization date between 50 000 and 45 000 years ago. At other more controversial sites, colonization Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy 1372 EXTINCTIONS OF BIG GAME dates have been proposed in excess of 60 000 and even 100 000 years ago. Bearing in mind the possibility for older sites, as Australian researchers have done more work and improved dating of critical localities, dates for extinctions and human arrival seem to be converging at a 10 000 year period between 55 000 and 45 000 years ago. To many researchers, this is strong albeit circumstantial evidence for human involvement in animal extinctions. Of course, temporal correlation is not necessarily causation, and it remains possible that climate change could have played a major or minor role in Australian extinctions. However, the climate change argument does not fare well in Australia because this critical time period, 55 000 to 45 000 years ago, was not a period of dramatic climatic shift in Australia. However, a recent study of the prehistoric diets of Genyornis, emus, and wombats found that when Genyornis went extinct, the diets of surviving emus and wombats shifted suddenly and permanently in multiple regions of Australia. To Gifford Miller and colleagues, this dietary shift indicates a major reorganization of Australian ecosystems right at the time of extinction, and they argue that this ecological change cannot be explained by climatic forcing. Instead, they hypothesize that burning of Australian environments by humans caused this environmental change, thus driving Genyornis and other species to extinction. Others have suggested that maybe it was not the ecological change that caused the extinction, but it was the extinction that caused the ecological change. Australia lost many species of large browsing marsupials, and their removal could have had dramatic ecological consequences as Owen-Smith has proposed for the keystone herbivore hypothesis. Though the precise cause remains unclear, many researchers are convinced of a human involvement in Australian Pleistocene extinctions. Not helping the overkill case in Australia, there are no kill sites of extinct Pleistocene fauna, and direct evidence of human use of megafauna is extremely rare. The Americas Extinctions in North and South America were similar to those in Australia, except they occurred 30 000 years later. In North America, 33 genera of megafauna were lost including, mammoths, mastodons, horses, camels, ground sloths, lions, cheetahs, dire wolves, giant shortfaced bears, and others. In South America, extinctions were more severe, where 50 genera of large mammals disappeared in the Late Pleistocene. These include four genera of proboscideans, three genera of horses, the great majority of large ungulates, and many genera of ground sloths and giant armadillos. Extinctions in North and South America disproportionately impacted the largest mammals, and a clear spike in extinction rates occurred in the latest Pleistocene. Though New World Pleistocene extinctions are better dated than those in Australia, the precise timing of extinctions continues to be a matter of contention. Chronological control on North American extinctions is better than that for South America, but extinctions on both continents appear to have been more or less simultaneous occurring in a narrow time interval between c. 13 300 and 12 500 years ago. Extinctions in North and South America correlate in time with both initial human colonization and a period of dramatic climate change, the most recent transition from glacial to interglacial climate. The best-dated New World extinctions correlate well with the start of Younger Dryas stadial (c. 12 900–11 500 years ago), a 1300–1400 year long abrupt reversal to glacial conditions where global temperatures cooled and glaciers readvanced (Figure 8). Humans entered the New World from northeast Asia across the Bering land bridge in the millennia preceding the Younger Dryas. For more than six decades, it was believed that the initial colonization of the New World was represented by the Clovis complex characterized by the production of lanceolate basally intended, fluted projectile points. Correlating very well New World extinctions, the Clovis complex dates to a very narrow time interval from c. 13 400 to 12 800 years ago. Recently, however, a handful of sites that appear to predate Clovis have been excavated, most notably Monte Verde, Childe, dating to 14 800 years ago, more than 1000 years before Clovis. The significance of pre-Clovis sites for the extinctions debate is twofold. First, if these sites truly predate Clovis, Martin’s blitzkrieg simulation, which assumes colonization by Clovis, cannot be correct. Second, humans and Pleistocene fauna coexisted in the New World for a slightly longer period than previously believed. Whether the existence of preClovis sites is problematic or beneficial to the general overkill hypothesis, however, is not clear because it has important implications for the archaeology of overkill. The thorn in the side of the New World overkill hypothesis has been the paucity of archaeological evidence demonstrating the exploitation of extinct fauna. In North America, there is relatively abundant evidence for hunting of mammoth during Clovis times, and there is minimal evidence for hunting of mastodon, horse, and camel. But considering that 33 genera of large mammals suffered extinction in the Late Pleistocene of North America, the overkill hypothesis is weakened by this scarcity of evidence. However, if humans were in the New World 1000–2000 years prior to Clovis and hunted Pleistocene fauna to extinction, the Clovis period Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy EXTINCTIONS OF BIG GAME 1373 −32 δ18 O −36 Temperature Younger Dryas Warmer −34 −38 Colder −40 −42 20 000 15 000 10 000 5000 0 Years before present Figure 8 The Younger Dryas stadial (highlighted in blue) recorded in stable oxygen isotopes of glacial ice in the GISP2 core from Greenland. Oxygen isotopes of glacial ice record the isotopic composition of precipitation which is strongly related to air temperature. High values of d18O indicate warm temperatures. Low values of d18O indicate cold temperatures. The Younger Dryas was a 1400 year reversal to glacial-like conditions that punctuated the warming trend that occurred between 20 000 and 10 000 years ago. New World extinctions appear to correlate with the onset of the Younger Dryas. Data from Grootes PM and Stuiver M (1997) Oxygen 18/16 variability in Greenland snow and ice with 103 and 105 year time resolution. Journal of Geophysical Research 102: 26455–26470. correlates only with the tail end of the extinction event where many populations of extinct fauna would have been highly depressed, and the majority of hunting of extinct fauna might have occurred prior to Clovis, a portion of the archaeological record of which we have a tiny sample. Therefore, the archaeological evidence for overkill might be in the millennium just prior to Clovis, a portion of the record which has remained virtually invisible. Despite claims that the overkill hypothesis for North America has itself perished, it lives on because it remains as likely an explanation for Quaternary extinctions as any other model. Island Extinctions Vertebrate extinctions on oceanic islands have figured prominently in discussions of Quaternary extinctions, and unlike continental extinctions virtually all researchers agree that human impacts were responsible for the great majority of island extinctions, although precise mechanisms remain unresolved. Blitzkrieg-type overkill is advocated by some researchers, while others argue island extinctions were likely much more akin to Diamond’s sitzkrieg scenario. For overkill advocates, islands serve as important controls in the extinction debate. Many oceanic islands were not discovered or colonized by humans until well into the Holocene, and they show the survival of many genera after the extinction of their continental counterparts with extinction occurring only after human arrival. For researchers who advocate climatic/ecological hypotheses, island fauna survived because islands are insolated from climatic change, or island extinctions are irrelevant to continental extinctions because island faunas are much more vulnerable to human effects. Island animals live in small populations, within confined geographic ranges, without access to a pool of conspecifics from which to recruit new individuals. A repeated pattern is seen on oceanic islands; with human colonization, a large range of species suffer extinction (Figure 2). Island regions that have figured prominently in extinction discussions are the West Indies, multiple Pacific archipelagos, the Galapagos, the islands of the Mediterranean, Madagascar, and New Zealand. Unlike continents, island extinctions typically impacted a broad array of vertebrate taxa including small-bodied birds, reptiles, and mammals. Among the first island extinctions were those of the Mediterranean. Early Holocene human colonization resulted in the decimation of the mammalian fauna of Mediterranean islands where only two relict Pleistocene mammalian species remain. Prior to human arrival, Mediterranean islands were home to dwarf varieties of elephants, hippos, and red deer which survived long after their nearby continental relatives; also lost were other species of ungulates, rodents, insectivores, birds, and reptiles. In the Caribbean, ground dwelling sloths, large rodents, and a wide array of small mammals, lizards, and birds suffered extinction with human colonization in the Middle Holocene. Like Mediterranean hippos Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374 Author's personal copy 1374 EXTINCTIONS OF BIG GAME and elephants, Caribbean sloths survived thousands of years after the extinction of large North and South American sloths. The islands of New Zealand and Madagascar also show waves of extinction at the time of human colonization. Beginning around 2000 years ago, Madagascar lost many species including seven genera of giant lemurs, at least six genera of large flightless birds, a pygmy hippo, an aardvark, a large viverrine carnivore, and giant tortoises. Limited but clear archaeological evidence of human butchery of extinct giant lemurs has been recovered from Madagascar, as have a few specimens of human-modified bones of other extinct fauna, such as a tibiotarsus from the 3 m tall, 500 kg elephant bird. The islands of New Zealand were first colonized by humans only about 900–1000 years ago, but within a matter of centuries, the large flightless moas and other species were decimated. New Zealand is particularly important in the extinctions debate because clear and abundant archaeological evidence for human predation of moa species has been recovered there. More than 100 archaeological sites show evidence of human subsistence use of moas. That humans hunted and caused moa extinction is not disputed, but the relative importance of hunting, anthropogenic burning, and the introduction of non-native rats and domestic dogs in moa extinction is a matter of debate. Where Are We Now? In recent decades, the amount of information available to researchers studying Pleistocene extinctions has grown substantially. With the exception of the development of a general consensus that human impacts, whether hunting or otherwise, regularly caused island extinctions, this dramatic increase in data has not led to a dramatic increase in conclusions about the cause (s) of Quaternary extinctions. The fundamental problem is that extinction hypotheses are very difficult to test directly, and they tend to be incredibly flexible, capable of accommodating virtually any evidence brought forth. For example, does a scarcity of archaeological evidence support or refute the overkill hypothesis? Unfortunately, there is no simple answer to that question. What has become increasingly clear is that a strong circumstantial case can be made for a significant human role in Quaternary extinctions, not only on islands but also on continents. With the exception of Europe, Asia, and Africa, one could determine the timing of human colonization of virtually the entire world by studying only the palaeontological record. A wave of extinctions occurring within the last 50 000 years would indicate that humans had arrived. To many researchers, this evidence alone indicates clear human agency in Quaternary extinctions. If, however, islands are eliminated from this scenario, this temporal correlation only occurs on three large landmasses, North America, South America, and Australia, and perhaps it becomes more feasible to argue that these correlations are just coincidence. But extinctions on continents show consistent trends with respect to body size, disproportionately affecting the largest species, the preferred prey of hunter-gatherers. Yet if humans caused the continental extinctions of dozens of genera of Pleistocene fauna, must it have been the perfect crime? To kill these many animals and leave so little evidence, to some researchers seems like an impossible feat. To others, little evidence is expected. Such disagreements highlight why it has been so difficult to determine the causes of big game extinctions in the Quaternary. See also: Archaeozoology; Butchery and Kill Sites; Migrations: Australia; Pacific; Modern Humans, Emergence of; New World, Peopling of; Oceania: Australia; Paleoenvironmental Reconstruction, Methods; Siberia, Peopling of. Further Reading Barnosky AD, Koch PL, Feranec RS, Wing SL, and Shabel AB (2004) Assessing the causes of Late Pleistocene extinctions on continents. Science 306: 70–75. Diamond JM (1989) Quaternary megafaunal extinctions: Variations on a theme by Paganini. Journal of Archaeological Science 16: 167–185. Grayson DK (2001) The archaeological record of human impacts on animal populations. Journal of World Prehistory 15: 1–67. MacPhee RDE (1999) Extinctions in Near Time: Causes, Contexts, and Consequences. New York: Kluver /Plenum. MacPhee RDE and Marx PA (1997) The 40 000-year plague: Humans, hyperdisease, and first-contact extinctions. In: Goodman SM and Patterson BD (eds.) Natural Change and Human Impact in Madagascar, pp. 169–216. Washington, DC: Smithsonian Institution Press. Martin PS and Klein RG (eds.) (1984) Quaternary Extinctions: A Prehistoric Revolution. Tucson: University of Arizona Press. Miller GH, Fogel ML, Magee JW, Gagan MK, Clarke SJ, and Johnson BJ (2005) Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction. Science 309: 287–290. Owen-Smith N (1987) Pleistocene extinctions: The pivotal role of megaherbivores. Paleobiology 13: 351–362. Stuart AJ, Kosintsev PA, Higham TFG, and Lister AM (2004) Pleistocene to Holocene extinction dynamics in giant deer and woolly mammoths. Nature 43: 684–689. Surovell TA, Waguespack NM, and Brantingham PJ (2005) Global archaeological evidence for proboscidean overkill. Proceedings of the National Academy of Sciences 102: 6231–6236. Encyclopedia of Archaeology (2008), vol. 2, pp. 1365-1374