Quaternary extinction
The Quaternary period (from 2.588 ± 0.005 million years ago to the present) has seen the extinctions of numerous predominantly megafaunal species, which have resulted in a collapse in faunal density and diversity and the extinction of key ecological strata across the globe. The most prominent event in the Late Pleistocene is differentiated from previous Quaternary pulse extinctions by the widespread absence of ecological succession to replace these extinct species, and the regime shift of previously established faunal relationships and habitats as a consequence. The earliest casualties were incurred at 130,000 BCE (the start of the Late Pleistocene ), in Australia ~ 60,000 years ago, in Americas ~ 15 000 years ago, coinciding in time with the early human migrations. However, the great majority of extinctions in Afro-Eurasia and the Americas occurred during the transition from the Pleistocene to the Holocene epoch (13,000 BCE to 8,000 BCE). This extinction wave did not stop at the end of the Pleistocene, continuing, especially on isolated islands, in human-caused extinctions, although there is debate as to whether these should be considered separate events or parts of the same event. Among the main causes hypothesized by paleontologists are overkill by the  widespread appearance of humans and natural climate change. A notable modern human presence first appeared during the Middle Pleistocene in Africa, and started to establish continuous, permanent populations in Eurasia and Australasia from 100,000 BCE and 63,000 BCE respectively, and the Americas from 22,000 BCE. A variant of the former possibility is the second-order predation hypothesis, which focuses more on the indirect damage caused by overcompetition with nonhuman predators. Recent studies have tended to favor the human-overkill theory. Extinctions by biogeographic realm. Introduction. The Late Pleistocene saw the extinction of many mammals weighing more than 40 kg. The  proportion of megafauna extinctions is progressively larger the further the human migratory distance from Africa, with the highest extinction rates in Australia, and North and South America. Extinctions in the Americas eliminated all mammals larger than 100 kg of South American origin, including those which migrated north in the Great American Interchange. It was only in Australia and the Americas that extinction occurred at family taxonomic levels or higher. This may relate to non-African megafauna and "Homo sapiens" not having evolved as species alongside each other. These continents had no known native species of Hominoidea (apes) at all, so  no species of Hominidae (greater apes) or "Homo". The increased extent of extinction mirrors the migration pattern of modern humans: the further away from Africa, the more recently humans inhabited the area, the less time those environments (including its megafauna) had to become accustomed to humans (and vice versa). There is no evidence of megafaunal extinctions at the height of the Last Glacial Maximum, suggesting that increased cold and glaciation were not factors in the Pleistocene extinction. There are three main hypotheses to explain this extinction: There are some inconsistencies between the current available data and the prehistoric overkill hypothesis . For instance, there are ambiguities around the timing of sudden Australian megafauna extinctions. Evidence supporting the prehistoric overkill hypothesis includes the persistence of megafauna on some islands for millennia past the disappearance of their continental cousins. For instance, Ground sloths survived on the Antilles long after North and South American ground sloths were extinct, woolly mammoths died out on remote Wrangel Island 1,000 years after their extinction on the mainland, while Steller's sea cows persisted off the isolated and uninhabited Commander Islands for thousands of years after they had vanished from the continental shores of the north Pacific. The  later disappearance of these island species correlates with the later colonization of these islands by humans. Alternative hypotheses to the theory of human responsibility include climate change associated with the last glacial period, and the Younger Dryas impact hypothesis as well as Tollmann's hypothesis that extinctions resulted from bolide impacts. Recent research indicates that each species responded differently to environmental changes, and no one factor by itself explains the large variety of extinctions. The causes may involve the interplay of climate change, competition between species, unstable population dynamics, and human predation. Afrotropic and Indomalaya: Africa and southern Asia. The  Afrotropic and Indomalaya biogeographic realms, or Old World tropics, were relatively spared by the Late Pleistocene extinctions. Sub-Saharan Africa and southern Asia are the only regions that have terrestrial mammals weighing over 1000 kg today. However, there are indications of megafaunal extinction events throughout the Pleistocene, particularly in Africa two million years ago, which coincide with key stages of human evolution and climatic trends. The center of human evolution and expansion, Africa and Asia were inhabited by advanced hominids by 2mya, with "Homo habilis" in Africa, and "Homo erectus" on both continents. By the advent and proliferation of "Homo  sapiens" circa 315,000 BCE, dominant species included "Homo heidelbergensis" in Africa, the denisovans and neanderthals (fellow "H. heidelbergensis" descendants) in Eurasia, and "Homo erectus" in Eastern Asia. Ultimately, on both continents, these groups and other populations of Homo were subsumed by successive radiations of "H. sapiens". There is evidence of an early migration event 268,000 BCE and later within neanderthal genetics, however the earliest dating for "H. sapiens" inhabitation is 118,000 BCE in Arabia, China and Israel, and 71,000 BCE in Indonesia. Additionally, not only have these early Asian migrations left a genetic mark on modern Papuan populations, the oldest  known pottery in existence was found in China, dated to 18,000 BCE. Particularly during the late Pleistocene, megafaunal diversity was notably reduced from both these continents, often without being replaced by comparable successor fauna. Climate change has been explored as a prominent cause of extinctions in Southeast Asia. Palearctic: Europe and northern Asia. The Palearctic realm spans the entirety of the European continent and stretches into northern Asia, through the Caucasus and central Asia to northern China, Siberia and Beringia. During the Late Pleistocene, this region was noted for its great diversity and dynamism of biomes, including the warm climes  of the Mediterranean basin, open temperate woodlands, arid plains, mountainous heathland and swampy wetlands, all of which were vulnerable to the severe climatic fluctuations of the interchanges between glacial and interglacials periods (stadials). However, it was the expansive mammoth steppe which was the ecosystem which united and defined this region during the Late Pleistocene. One of the key features of Europe's Late Pleistocene climate was the often drastic turnover of conditions and biota between the numerous stadials, which could set within a century. For example, during glacial periods, the entire North Sea was drained of water to form Doggerland . The final major cold spell occurred from 25,000 BCE to 18,000 BCE and is known as the Last Glacial Maximum, when the Fenno-Scandinavian ice sheet covered much of northern Europe, while the Alpine ice sheet occupied significant parts of central-southern Europe. Europe and northern Asia, being far colder and drier than today, was largely hegemonized by the mammoth steppe, an ecosystem dominated by palatable high-productivity grasses, herbs and willow shrubs. This supported an extensive biota of grassland fauna and stretched eastwards from Spain in the Iberian Peninsula to Yukon in modern-day Canada. The area was populated  by many species of grazers which assembled in large herds similar in size to those in Africa today. Populous species which roamed the great grasslands included the woolly mammoth, woolly rhinoceros, "Elasmotherium", steppe bison, Pleistocene horse, muskox, "Cervalces", reindeer, various antelopes (goat-horned antelope, mongolian gazelle, saiga antelope and twisted-horned antelope) and steppe pika. Carnivores included Eurasian cave lion, scimitar cat, cave hyena, grey wolf, dhole and the Arctic fox. At the edges of these large stretches of grassland could be found more shrub-like terrain and dry conifer forest and woodland (akin to forest steppe or taiga). The  browsing collective of megafauna included woolly rhinoceros, giant deer, moose, "Cervalces", tarpan, aurochs, woodland bison, camels and smaller deer (Siberian roe deer, red deer and Siberian musk deer). Brown bears, wolverines, cave bear, wolves, lynx, leopards and red foxes also inhabited this biome. Tigers were at stages also present, from the edges of eastern Europe around the Black Sea to Beringia. The more mountainous terrain, incorporating montane grassland, subalpine conifer forest, alpine tundra and broken, craggy slopes, was occupied by several species of mountain-going animals like argali, chamois, ibex, mouflon, pika, wolves, leopards, "Ursus spp." and lynx, with snow  leopards, Baikal yak and snow sheep in northern Asia. Arctic tundra, which lined the north of the mammoth steppe, reflected modern ecology with species such as the polar bear, wolf, reindeer and muskox. Other biomes, although less noted, were significant in contributing to the diversity of fauna in Late Pleistocene Europe. Warmer grasslands such as temperate steppe and Mediterranean savannah hosted "Stephanorhinus", gazelle, European bison, Asian ostriches, "Leptobos", cheetah and onager. These biomes also contained an assortment of mammoth steppe fauna, such as saiga antelope, lions, scimitar cats, cave hyenas, wolves, Pleistocene horse, steppe bison, twisted-horned antelope, aurochs and  camels. Temperate coniferous, deciduous, mixed broadleaf and Mediterranean forest and open woodland accommodated straight-tusked elephants, "Praemegaceros", "Stephanorhinus", wild boar, bovids such as European bison, tahr and tur, species of "Ursus" such as the Etruscan bear and smaller deer (Roe deer, red deer, fallow deer and Mediterranean deer) with several mammoth steppe species such as lynx, tarpan, wolves, dholes, moose, giant deer, woodland bison, leopards and aurochs. Woolly rhinoceros and mammoth occasionally resided in these temperate biomes, mixing with predominately temperate fauna to escape harsh glacials. In warmer wetlands, European water buffalo and hippopotamus were present. Although these habitats were  restricted to micro refugia and to southern Europe and its fringes, being in Iberia, Italy, the Balkans, Ukraine's Black Sea basin, the Caucasus and western Asia, during inter-glacials these biomes had a far more northernly range. For example, hippopotamus inhabited Great Britain and straight-tusked elephant the Netherlands, as recently as 80,000 BCE and 42,000 BCE respectively. The first possible indications of habitation by hominins are the 7.2 million year old finds of "Graecopithecus", and 5.7 million year old footprints in Crete — however established habitation is noted in Georgia from 1.8 million years ago, proceeded to Germany and  France, by "Homo erectus". Prominent co-current and subsequent species include "Homo antecessor", "Homo cepranensis", "Homo heidelbergensis", neanderthals and denisovans, preceding habitation by Homo sapiens circa 38,000 BCE. Extensive contact between African and Eurasian Homo groups is known at least in part through transfers of stone-tool technology in 500,000 BCE and again at 250,000 BCE. Europe's Late Pleistocene biota went through two phases of extinction. Some fauna became extinct before 13,000 BCE, in staggered intervals, particularly between 50,000 BCE and 30,000 BCE. Species include cave bear, "Elasmotherium", straight-tusked elephant, "Stephanorhinus", water buffalo, neanderthals, gazelle and scimitar cat . However, the great majority of species were extinguished, extirpated or experienced severe population contractions between 13,000 BCE and 9,000 BCE, ending with the Younger Dryas. At that time there were small ice sheets in Scotland and Scandinavia. The mammoth steppe disappeared from the vast majority of its former range, either due to a permanent shift in climatic conditions, or an absence of ecosystem management due to decimated, fragmented or extinct populations of megaherbivores. This led to a region wide extinction vortex, resulting in cyclically diminishing bio-productivity and defaunation. Insular species on Mediterranean islands such as Sardinia, Sicily, Malta, Cyprus  and Crete, went extinct around the same time as humans colonised those islands. Fauna included dwarf elephants, megacerines and hippopotamuses, and giant avians, otters and rodents. Many species extant today were present in areas either far to the south or west of their contemporary ranges- for example, all the arctic fauna on this list inhabited regions as south as the Iberian Peninsula at various stages of the Late Pleistocene. Recently extinct organisms are noted as †. Species extirpated from significant portions of or all former ranges in Europe and northern Asia during the Quaternary extinction event include- Nearctic:. During the last  60,000 years, including the end of the last glacial period, approximately 51 genera of large mammals have become extinct in North America. Of these, many genera extinctions can be reliably attributed to a brief interval of 11,500 to 10,000 radiocarbon years before present, shortly following the arrival of the Clovis people in North America . Prominent paleontological sites include Mexico and Panama, the crossroads of the American Interchange. Most other extinctions are poorly constrained in time, though some definitely occurred outside of this narrow interval. A genetic study published in 2021 indicates that horses, that were directly related to the modern  horses, were still present in Yukon at least until 5,700 years ago or mid-Holocene. In contrast, only about half a dozen small mammals disappeared during this time. Previous North American extinction pulses had occurred at the end of glaciations, but not with such an ecological imbalance between large mammals and small ones (Moreover, previous extinction pulses were not comparable to the Quaternary extinction event; they involved primarily species replacements within ecological niches, while the latter event resulted in many ecological niches being left unoccupied). Such include the last native North American terror bird ("Titanis"), rhinoceros ("Aphelops") and hyena ("Chasmaporthetes ). Human habitation commenced unequivocally approximately 22,000 BCE north of the glacier, and 13,500 BCE south, however disputed evidence of southern human habitation exists from 130,000 BCE and 17,000 BCE onwards, described from sites in California and Meadowcroft in Pennsylvania. North American extinctions (noted as herbivores (H) or carnivores (C)) included: The survivors are in some ways as significant as the losses: bison (H), grey wolf (C), lynx (C), grizzly bear (C), American black bear (C), deer (e.g. caribou, moose, wapiti (elk), Odocoileus" spp.) (H), pronghorn (H), white-lipped peccary (H), muskox (H), bighorn sheep (H), and mountain goat (H); the  list of survivors also include species which were extirpated during the Quaternary extinction event, but recolonised at least part of their ranges during the mid-holocene from South American relict populations, such as the cougar (C), jaguar (C), giant anteater (C), collared peccary (H), ocelot (C) and jaguarundi (C). All save the pronghorns and giant anteaters were descended from Asian ancestors that had evolved with human predators. Pronghorns are the second-fastest land mammal (after the cheetah), which may have helped them elude hunters. More difficult to explain in the context of overkill is the survival of bison, since these  animals first appeared in North America less than 240,000 years ago and so were geographically removed from human predators for a sizeable period of time. Because ancient bison evolved into living bison, there was no continent-wide extinction of bison at the end of the Pleistocene (although the genus was regionally extirpated in many areas). The survival of bison into the Holocene and recent times is therefore inconsistent with the overkill scenario. By the end of the Pleistocene, when humans first entered North America, these large animals had been geographically separated from intensive human hunting for more than 200,000 years . Given this enormous span of geologic time, bison would almost certainly have been very nearly as naive as native North American large mammals. The culture that has been connected with the wave of extinctions in North America is the paleo-American culture associated with the Clovis people ("q.v."), who were thought to use spear throwers to kill large animals. The chief criticism of the "prehistoric overkill hypothesis" has been that the human population at the time was too small and/or not sufficiently widespread geographically to have been capable of such ecologically significant impacts. This criticism does not mean that climate  change scenarios explaining the extinction are automatically to be preferred by default, however, any more than weaknesses in climate change arguments can be taken as supporting overkill. Some form of a combination of both factors could be plausible, and overkill would be a lot easier to achieve large-scale extinction with an already stressed population due to climate change. Neotropic: South America. The Neotropical realm was affected by the fact that South America had been isolated as an island continent for many millions of years, and had a wide range of fauna found nowhere else, although many of them became  extinct during the Great American Interchange about 3 million years ago, such as the "Sparassodonta" family. Those that survived the interchange included the ground sloths, glyptodonts, litopterns, pampatheres, phorusrhacids (terror birds) and notoungulates; all managed to extend their range to North America. In the Pleistocene, South America remained largely unglaciated except for increased mountain glaciation in the Andes, which had a two-fold effect- there was a faunal divide between the Andes, and the colder, arid interior resulted in the advance of temperate lowland woodland, tropical savanna and desert at the expense of rainforest. Within these open environments, megafauna diversity  was extremely dense, with over 40 genera recorded from the Guerrero member of Luján Formation alone. Ultimately, by the mid-Holocene, all the preeminent genera of megafauna became extinct- the last specimens of "Doedicurus" and "Toxodon" have been dated to 4,555 BCE and 3,000 BCE respectively. Their smaller relatives remain, including anteaters, tree sloths, armadillos; New World marsupials: opossums, shrew opossums, and the monito del monte (actually more related to Australian marsupials). Intense human habitation was established circa 11,000 BCE, however partly disputed evidence of pre-clovis habitation occurs since 46,000 BCE and 20,000 BCE, such as at the Serra  da Capivara National Park (Brazil) and Monte Verde (Chile) sites. Today the largest land mammals remaining in South America are the wild camels of the "Lamini" group, such as the guanacos and vicuñas, and the genus "Tapirus", of which Baird's tapir can reach up to 400 kg. Other notable surviving large fauna are peccaries, marsh deer ("Capreolinae"), giant anteaters, spectacled bears, maned wolves, pumas, ocelots, jaguars, rheas, emerald tree boas, boa constrictors, anacondas, American crocodiles, caimans, and giant rodents such as capybaras. The Pacific (Australasia and Oceania). One thing that makes the extinction of the Australian megafauna particularly special  is that prior to and into the Quaternary period the vast oceans prevented homo sapiens and other animals from reaching the outer world: islands like Australia and Madagascar.  As a result, organisms in these places evolved in isolation for millions of years taking on quite different structures from their Afro-Asian relatives.  One key characteristic of the Australian animals is that they were marsupials.  Marsupials are animals that give birth to tiny, helpless offspring and then nurture them with breast milk in their abdominal pouches.  While many of the animals of Australia were marsupial these types of animals were almost  unknown in Africa and Asia.  Secondly many of the animals in Australia were megafauna, megafauna are animals that weight 100 pounds or more.  During the quaternary period homo sapiens acquired the technology to sail the oceans allowing them to settle the outer world.  Evidence shows that homo sapiens first arrived in Australia about 70,000 year ago and colonizing the landscape in about 30,000 years.  Within this 30,000-year period 90% of megafauna species inhabiting Australia went extinct. Scientists hold three main theories behind the extinction of the Australian megafauna.  One is that it was caused by overkill after the widespread  appearance of homo sapiens, another is that they went extinct due to natural climate change, and the final theory is that extinction was caused by a combination of overkill by humans and natural climate change.   Beginning with the idea of a human caused extinction event from the book "" by Yuval Noah Harari, there exists three pieces of evidence that support this hypothesis.  First, despite Australia’s climate changing at the time it is hard to say natural climate change alone could have caused such a massive extinction event.  For example, the giant diprotodon inhabited Australia for more than 1.5 million  years before humans showed up and the species is known to have survived 10 previous ice ages.  So, if the giant diproton had survived 10 previous ice ages why did it along with 90% of the other megafauna die out around 45,000 years ago around when homo sapiens colonized Australia.  Secondly, typically when climate change is the cause of mass extinctions, sea organisms usually experience greater consequences than organisms living on land.  Yet there is no evidence of any significant extinction of oceanic fauna 45,000 years ago in Australia.  This points to homo sapiens being the cause of the extinction  of the Australian megafauna because at the time homo sapiens still overwhelmingly lived off the land as opposed to the sea.  Thirdly the extinction of megafauna following the arrival of humans has been seen in multiple outer world locations over the last millennia.  Examples include New Zealand only 800 years ago and Wrangel, an Arctic island, 4000 years ago.  Therefore, it makes sense that this pattern would also hold true in the case of Australian megafauna. Given that 90% of Australian megafauna went extinct within 30,000 years of the arrival of human beings one must ask themselves why these animals  were not fit to survive against humans.  One of these reasons is that megafauna are exceptionally large animals.  As a result, these animals breed slowly, have long pregnancies periods, and produce few offspring per pregnancy.  In addition, there is usually a long break in between pregnancies.  Secondly many of these animals were marsupials so there existed a long development period from birth to maturity and offspring were dependent on their parents for breastmilk for quite some time.  These two characteristics combined contributed to the extinction of Australian megafauna because hunting is more detrimental to these types of populations than others .  For example, if human beings killed one diprotodon every few months it would be enough for yearly diprotodon deaths to outnumber diprotodon births causing population decline, and within a thousand years this could lead to extinction. Another aspect making Australian megafauna less fit to survive against humans is that homo sapiens evolved separately from the Australian megafauna, so the megafauna had no predeveloped fear towards humans making hunting them easier. The second hypothesis held by scientists is that the Australian megafauna went extinct due to natural climate change.  The main reason this theory exists is that there is evidence of  megafauna surviving up until 40,000 years ago, a full 30,000 years after homo sapiens first landed in Australia.  Implying that there was a significant period of homo sapiens and megafauna coexistence. Evidence of these animals existing at this time come from fossils records and ocean sediment.  To begin with, sediment core drilled in the Indian Ocean off the coast of the southwest Australia indicate the existence of a fungus called Sporormiella which survived off the dung of plant eating mammals.  The abundance of these spores in the sediment prior to 45,000 years ago indicates a lot of large mammals existed  on the southwest Australian landscape up until that point.  The sediment data also indicated that the megafauna population collapsed within a few thousand years around the 45,000 years ago suggesting a rapid extinction event.  In addition, fossils found at South Walker Creek, which is the youngest megafauna site in northern Australia, indicate that at least 16 species of megafauna survived there up until 40,000 years ago.  Furthermore, there is no firm evidence of homo sapiens beings at South Walker Creek 40,000 years ago, therefore no human cause can be attributed to the extinction of these megafauna.  However, there is evidence  of major environmental deterioration of South Water Creek 40,000 years ago which the extinction can be attributed to.  These changes include increased fire, reduction in grasslands, and the loss of freshwater.  The same environmental deterioration is seen across Australia at the time further strengthening the climate change argument.  Australia’s climate at the time could best be described as an overall drying of the landscape due to less mean annual precipitation causing less freshwater availability and more drought conditions across the landscape. Overall, this led to changes in vegetation, increased fires, overall reduction in grasslands, and a greater competition for  already scarce amount of freshwater.  In turn all these environmental changes proved to me too much for the Australian megafauna to cope with causing 90% of megafauna species to go extinct. The third hypothesis shared by some scientists is that human impacts and natural climate changes led to the extinction of Australian megafauna. To begin with it is important to note that approximately 75% of Australia is semi-arid or arid landscape, therefore it makes sense that megafauna species utilized the same freshwater resources as humans.  As a result, this could have increased the amount of megafauna hunted due to  the competition for freshwater as the drought conditions persisted.  On top of the already dry conditions and diminishing grasslands, homo sapiens used fire agriculture to burn impassable land.  This further diminished the already disappearing grassland which contained plants that were key dietary component of herbivorous megafauna.  While there is no scientific consensus on the true cause of the extinction of Australian megafauna it is plausible that homo sapiens and natural climate change both had an impact because they were both in Australia at the time.  Overall, there is an immense amount of evidence pointing to humans being the culprit but  by ruling out climate change completely as a cause of the Australian megafauna extinction we are not getting the whole picture.  The climate change that occurred in Australia 45,000 years ago destabilized the ecosystem making it particularly vulnerable to hunting and fire agriculture by humans; this is probably what led to the extinction of the Australian megafauna.    In Sahul (a former continent composed of Australia and New Guinea), the sudden and extensive spate of extinctions occurred earlier than in the rest of the world. Most evidence points to a 20,000 year period after human arrival circa 63,000 BCE, but scientific  argument continues as to the exact date range. In the rest of the Pacific (other Australasian islands such as New Caledonia, and Oceania) although in some respects far later, endemic fauna also usually perished quickly upon the arrival of humans in the late Pleistocene and early Holocene. This section does only include extinctions that took place prior to European discovery of the respective islands. The extinctions in the Pacific included: Some extinct megafauna, such as the bunyip-like "Diprotodon", may remain in folk memory or be the sources of cryptozoological legends. Relationship to later extinctions. There is no general agreement  on where the Holocene, or anthropogenic, extinction begins, and the Quaternary extinction event ends, or if they should be considered separate events at all. Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 100,000 and 200,000 years ago, which is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar, in a similar way that any large, adaptable predator moving into a new ecosystem would. In many cases, it is suggested even minimal hunting pressure was enough to wipe out large fauna , particularly on geographically isolated islands. Only during the most recent parts of the extinction have plants also suffered large losses. Overall, the Holocene extinction can be characterised by the human impact on the environment. The Holocene extinction continues into the 21st century, with overfishing, ocean acidification and the amphibian crisis being a few broader examples of an almost universal, cosmopolitan decline of biodiversity. Hunting hypothesis. The hunting hypothesis suggests that humans hunted megaherbivores to extinction, which in turn caused the extinction of carnivores and scavengers which had preyed upon those animals. Therefore, this hypothesis holds Pleistocene humans responsible for the  megafaunal extinction. One variant, known as "blitzkrieg", portrays this process as relatively quick. Some of the direct evidence for this includes: fossils of some megafauna found in conjunction with human remains, embedded arrows and tool cut marks found in megafaunal bones, and European cave paintings that depict such hunting. Biogeographical evidence is also suggestive: the areas of the world where humans evolved currently have more of their Pleistocene megafaunal diversity (the elephants and rhinos of Asia and Africa) compared to other areas such as Australia, the Americas, Madagascar and New Zealand without the earliest humans. Circumstantially, the close correlation in  time between the appearance of humans in an area and extinction there provides weight for this scenario. The megafaunal extinctions covered a vast period of time and highly variable climatic situations. The earliest extinctions in Australia were complete approximately 50,000 BP, well before the last glacial maximum and before rises in temperature. The most recent extinction in New Zealand was complete no earlier than 500 BP and during a period of cooling. In between these extremes megafaunal extinctions have occurred progressively in such places as North America, South America and Madagascar with no climatic commonality. The only common factor that  can be ascertained is the arrival of humans. This phenomenon appears even within regions. The mammal extinction wave in Australia about 50,000 years ago coincides not with known climatic changes, but with the arrival of humans. In addition, large mammal species like the giant kangaroo "Protemnodon" appear to have succumbed sooner on the Australian mainland than on Tasmania, which was colonised by humans a few thousand years later. Worldwide, extinctions seem to follow the migration of humans and to be most severe where humans arrived most recently and least severe where humans originated — in Africa (see figure "March of Man  below). This suggests that prey animals and human hunting ability evolved together, so the animals evolved avoidance techniques. As humans migrated throughout the world and became more and more proficient at hunting, they encountered animals that had evolved without the presence of humans. Lacking the fear of humans that African animals had developed, animals outside of Africa were easy prey for human hunting techniques. Extinction through human hunting has been supported by archaeological finds of mammoths with projectile points embedded in their skeletons, by observations of modern naïve animals allowing hunters to approach easily and by computer models by Mosimann 	Quaternary extinction
19312565	and Martin, and Whittington and Dyke, and most recently by Alroy. A study published in 2015 supported the hypothesis further by running several thousand scenarios that correlated the time windows in which each species is known to have become extinct with the arrival of humans on different continents or islands. This was compared against climate reconstructions for the last 90,000 years. The researchers found correlations of human spread and species extinction indicating that the human impact was the main cause of the extinction, while climate change exacerbated the frequency of extinctions. The study, however, found an apparently low extinction rate 	Quaternary extinction
19312566	in the fossil record of mainland Asia. Overkill hypothesis. The overkill hypothesis, a variant of the hunting hypothesis, was proposed in 1966 by Paul S. Martin, Professor of Geosciences Emeritus at the Desert Laboratory of the University of Arizona. Objections to the hunting hypothesis. The major objections to the theory are as follows: Climate change hypothesis. At the end of the 19th and beginning of the 20th centuries, when scientists first realized that there had been glacial and interglacial ages, and that they were somehow associated with the prevalence or disappearance of certain animals, they surmised that the termination of 	Quaternary extinction
19312567	the Pleistocene ice age might be an explanation for the extinctions. Critics object that since there were glacial in the evolutionary history of many of the megafauna, it is rather implausible that only after the last glacial maximum would there be such extinctions. However, this criticism is rejected by a recent study indicating that terminal Pleistocene megafaunal community composition may have differed markedly from faunas present during earlier interglacials, particularly with respect to the great abundance and geographic extent of Pleistocene Bison" at the end of the epoch. This suggests that the survival of megafaunal populations during earlier interglacials is  essentially irrelevant to the terminal Pleistocene extinction event, because bison were not present in similar abundance during any of the earlier interglacials. Some evidence weighs against climate change as a valid hypothesis as applied to Australia. It has been shown that the prevailing climate at the time of extinction (40,000–50,000 BP) was similar to that of today, and that the extinct animals were strongly adapted to an arid climate. The evidence indicates that all of the extinctions took place in the same short time period, which was the time when humans entered the landscape. The main mechanism for extinction was  probably fire (started by humans) in a then much less fire-adapted landscape. Isotopic evidence shows sudden changes in the diet of surviving species, which could correspond to the stress they experienced before extinction. Evidence in Southeast Asia, in contrast to Europe, Australia, and the Americas, suggests that climate change and an increasing sea level were significant factors in the extinction of several herbivorous species. Alterations in vegetation growth and new access routes for early humans and mammals to previously isolated, localized ecosystems were detrimental to select groups of fauna. Some evidence obtained from analysis of the tusks of mastodons  from the American Great Lakes region appears inconsistent with the climate change hypothesis. Over a span of several thousand years prior to their extinction in the area, the mastodons show a trend of declining age at maturation. This is the opposite of what one would expect if they were experiencing stresses from deteriorating environmental conditions, but is consistent with a reduction in intraspecific competition that would result from a population being reduced by human hunting. Increased temperature. The most obvious change associated with the termination of an ice age is the increase in temperature. Between 15,000 BP and 10,000 BP , a 6 °C increase in global mean annual temperatures occurred. This was generally thought to be the cause of the extinctions. According to this hypothesis, a temperature increase sufficient to melt the Wisconsin ice sheet could have placed enough thermal stress on cold-adapted mammals to cause them to die. Their heavy fur, which helps conserve body heat in the glacial cold, might have prevented the dumping of excess heat, causing the mammals to die of heat exhaustion. Large mammals, with their reduced surface area-to-volume ratio, would have fared worse than small mammals. A study covering the  past 56,000 years indicates that rapid warming events with temperature changes of up to had an important impact on the extinction of megafauna. Ancient DNA and radiocarbon data indicates that local genetic populations were replaced by others within the same species or by others within the same genus. Survival of populations was dependent on the existence of refugia and long distance dispersals, which may have been disrupted by human hunters. Arguments against the temperature hypothesis. Studies propose that the annual mean temperature of the current interglacial that we have seen for the last 10,000 years is no higher than that  of previous interglacials, yet some of the same large mammals survived similar temperature increases. Therefore, warmer temperatures alone may not be a sufficient explanation. In addition, numerous species such as mammoths on Wrangel Island and St. Paul Island survived in human-free refugia despite changes in climate. This would not be expected if climate change were responsible (unless their maritime climates offered some protection against climate change not afforded to coastal populations on the mainland). Under normal ecological assumptions island populations should be more vulnerable to extinction due to climate change because of small populations and an inability to migrate  to more favorable climes. Increased continentality affects vegetation in time or space. Other scientists have proposed that increasingly extreme weather—hotter summers and colder winters—referred to as "continentality", or related changes in rainfall caused the extinctions. The various hypotheses are outlined below. Vegetation changes: geographic. It has been shown that vegetation changed from mixed woodland-parkland to separate prairie and woodland. This may have affected the kinds of food available. Shorter growing seasons may have caused the extinction of large herbivores and the dwarfing of many others. In this case, as observed, bison and other large ruminants would have  fared better than horses, elephants and other monogastrics, because ruminants are able to extract more nutrition from limited quantities of high-fiber food and better able to deal with anti-herbivory toxins. So, in general, when vegetation becomes more specialized, herbivores with less diet flexibility may be less able to find the mix of vegetation they need to sustain life and reproduce, within a given area. Rainfall changes: time. Increased continentality resulted in reduced and less predictable rainfall limiting the availability of plants necessary for energy and nutrition. Axelrod and Slaughter have suggested that this change in rainfall restricted the  amount of time favorable for reproduction. This could disproportionately harm large animals, since they have longer, more inflexible mating periods, and so may have produced young at unfavorable seasons (i.e., when sufficient food, water, or shelter was unavailable because of shifts in the growing season). In contrast, small mammals, with their shorter life cycles, shorter reproductive cycles, and shorter gestation periods, could have adjusted to the increased unpredictability of the climate, both as individuals and as species which allowed them to synchronize their reproductive efforts with conditions favorable for offspring survival. If so, smaller mammals would have lost fewer offspring  and would have been better able to repeat the reproductive effort when circumstances once more favored offspring survival. In 2017 a study looked at the environmental conditions across Europe, Siberia and the Americas from 25,000–10,000 YBP. The study found that prolonged warming events leading to deglaciation and maximum rainfall occurred just prior to the transformation of the rangelands that supported megaherbivores into widespread wetlands that supported herbivore-resistant plants. The study proposes that moisture-driven environmental change led to the megafaunal extinctions and that Africa's trans-equatorial position allowed rangeland to continue to exist between the deserts and the  central forests, therefore fewer megafauna species became extinct there. Arguments against the continentality hypotheses. Critics have identified a number of problems with the continentality hypotheses. The extinction of the megafauna could have caused the disappearance of the mammoth steppe. Alaska now has low nutrient soil unable to support bison, mammoths, and horses. R. Dale Guthrie has claimed this as a cause of the extinction of the megafauna there; however, he may be interpreting it backwards. The loss of large herbivores to break up the permafrost allows the cold soils that are unable to support large herbivores today. Today, in the  arctic, where trucks have broken the permafrost grasses and diverse flora and fauna can be supported. In addition, Chapin (Chapin 1980) showed that simply adding fertilizer to the soil in Alaska could make grasses grow again like they did in the era of the mammoth steppe. Possibly, the extinction of the megafauna and the corresponding loss of dung is what led to low nutrient levels in modern-day soil and therefore is why the landscape can no longer support megafauna. Arguments against both climate change and overkill. It may be observed that neither the overkill nor the climate change hypotheses  can fully explain events: browsers, mixed feeders and non-ruminant grazer species suffered most, while relatively more ruminant grazers survived. However, a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by either Second Order Predation (see below) or anthropogenic fire preferentially selects against browse species. Hyperdisease hypothesis. Theory. The hyperdisease hypothesis, as advanced by Ross D. E. MacFee and Preston A. Marx, attributes the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans. The hyperdisease hypothesis proposes that humans or animals traveling with them (e.g., chickens  or domestic dogs) introduced one or more highly virulent diseases into vulnerable populations of native mammals, eventually causing extinctions. The extinction was biased toward larger-sized species because smaller species have greater resilience because of their life history traits (e.g., shorter gestation time, greater population sizes, etc.). Humans are thought to be the cause because other earlier immigrations of mammals into North America from Eurasia did not cause extinctions. Diseases imported by people have been responsible for extinctions in the recent past; for example, bringing avian malaria to Hawaii has had a major impact on the isolated birds of the  island. If a disease was indeed responsible for the end-Pleistocene extinctions, then there are several criteria it must satisfy (see Table 7.3 in MacPhee & Marx 1997). First, the pathogen must have a stable carrier state in a reservoir species. That is, it must be able to sustain itself in the environment when there are no susceptible hosts available to infect. Second, the pathogen must have a high infection rate, such that it is able to infect virtually all individuals of all ages and sexes encountered. Third, it must be extremely lethal, with a mortality rate of c. 50–75%. Finally , it must have the ability to infect multiple host species without posing a serious threat to humans. Humans may be infected, but the disease must not be highly lethal or able to cause an epidemic. One suggestion is that pathogens were transmitted by the expanding humans via the domesticated dogs they brought with them, though this does not fit the timeline of extinctions in the Americas and Australia in particular. Second-order predation hypothesis. Scenario. The Second-Order Predation Hypothesis says that as humans entered the New World they continued their policy of killing predators, which had been successful in  the Old World but because they were more efficient and because the fauna, both herbivores and carnivores, were more naive, they killed off enough carnivores to upset the ecological balance of the continent, causing overpopulation, environmental exhaustion, and environmental collapse. The hypothesis accounts for changes in animal, plant, and human populations. The scenario is as follows: Support. This has been supported by a computer model, the Pleistocene extinction model (PEM), which, using the same assumptions and values for all variables (herbivore population, herbivore recruitment rates, food needed per human, herbivore hunting rates, etc.) other than those for hunting of predators . It compares the overkill hypothesis (predator hunting = 0) with second-order predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that second-order predation is more consistent with extinction than is overkill (results graph at left). The Pleistocene extinction model is the only test of multiple hypotheses and is the only model to specifically test combination hypotheses by artificially introducing sufficient climate change to cause extinction. When overkill and climate change are combined they balance each other out. Climate change reduces the number of plants, overkill removes animals, therefore fewer plants are eaten. Second -order predation combined with climate change exacerbates the effect of climate change. (results graph at right). The second-order predation hypothesis is supported by the observation above that there was a massive increase in bison populations. Younger Dryas impact hypothesis. First publicly presented at the Spring 2007 joint assembly of the American Geophysical Union in Acapulco, Mexico, the Younger Dryas impact hypothesis suggests that the mass extinction was caused by fragments of a disintegrating asteroid or comet 12,900 years ago. Using photomicrograph analysis, research published in January 2009 has found evidence of nanodiamonds in the soil from six sites across  North America including Arizona, Minnesota, Oklahoma, South Carolina and two Canadian sites. Similar research found nanodiamonds in the Greenland ice sheet. Arguments against/for the impact hypothesis. Debate around this hypothesis has included, among other things, the lack of an impact crater, relatively small increased level of iridium in the soil, and the relative probability of such an event. That said, it took 10 years after publication of the Alvarez theory before scientists found the Chicxulub crater. If the bolide struck the Laurentide ice sheet as hypothesized by Firestone et al. (2007), a typical impact crater would not be visible . A spike in platinum was found in the Greenland ice cores by Petaev et al. (2013), which they view as a global signal. Confirmation came in 2017 with the report that the Pt spike had been found at "11 widely separated archaeological bulk sedimentary sequences." Wolbach et al. reported in 2018 that "YDB peaks in Pt were observed at 28 sites" in total, including the 11 reported earlier and the one from Greenland.