Quaternary extinction event


The Quaternary period saw the extinctions of numerous predominantly megafaunal species, which 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. However, the great majority of extinctions in Afro-Eurasia and the Americas occurred during the transition from the Pleistocene to the Holocene epoch. 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 part 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 120,000 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

Summary

Introduction

The Late Pleistocene extinction event saw the extinction of many mammals weighing more than 40 kg. The proportional rate of megafauna extinctions is progressively larger the greater the human migratory distance from Africa.
The extinctions in the Americas entailed the elimination of all the larger mammalian species of South American origin, including those that had migrated north in the Great American Interchange. Only in the continents of Australia, North America, and South America did the extinction occur at family taxonomic levels or higher.
The proportional rate of megafauna extinctions being incrementally bigger the larger the migratory distance from Africa might be related to non-African megafauna and Homo sapiens not having evolved as species alongside each other.
For their part, Australia, North America and South America, which respectively had the highest incremental extinction rates, had no known native species of Hominoidea at all, and specifically no species of Hominidae or Homo.
The increased rate of extinction mirrors the sequential pattern of the migration of anatomically modern humans. The further away from Africa, the more recently the area has been inhabited by humans, and the less time the environments had had to become accustomed to humans and vice versa.
There is no evidence of megafaunal extinctions at the height of the Last Glacial Maximum, which lends itself to the hypothesis that increasing cold and glaciation were not factors regarding the Pleistocene extinction.
There are three main hypotheses concerning the Pleistocene 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 extinctions of Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa and South or Southeast Asia, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval.
Evidence supporting the prehistoric overkill hypothesis includes the persistence of certain island megafauna for several millennia past the disappearance of their continental cousins. Ground sloths survived on the Antilles long after North and South American ground sloths were extinct. The later disappearance of the island species correlates with the later colonization of these islands by humans. Similarly, woolly mammoths died out on remote Wrangel Island 1,000 years after their extinction on the mainland. Steller's sea cows also persisted in seas off the isolated and uninhabited Commander Islands for thousands of years after they had vanished from the continental shores of the north Pacific.
Alternative hypotheses to the theory of human responsibility include climate change associated with the last glacial period and the Younger Dryas event, as well as Tollmann's hypothetical bolide, which claim that the extinctions resulted from bolide impacts. Such a scenario has been proposed as a contributing cause of the 1,300-year cold period known as the Younger Dryas stadial. This impact extinction hypothesis is still in debate due to the exacting field techniques required to extract minuscule particles of extraterrestrial impact markers such as iridium at a high resolution from very thin strata in a repeatable fashion, as is necessary to conclusively distinguish the event peak from the local background level of the marker. The debate seems to be exacerbated by infighting between the Uniformitarianism camp and the Catastrophism camp.
Recent research indicates that each single species responded differently to environmental changes, and that one factor by itself cannot explain the large number of extinctions. The causes are complex, and may involve elements of climate change, interspecific competition, 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 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.

Afrotropic and Indomalaya Late Pleistocene & Holocene

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. 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 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. The browsing collective of megafauna included woolly rhinoceros, giant deer, moose, Cervalces, tarpan, aurochs, woodland bison, camels and smaller 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 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-
See also: List of North American animals extinct in the Holocene
reconstruction
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, now restricted to the southern portions of Asia, was present from Iberia to Mexico during the Late Pleistocene
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, Chicago, Illinois, United States
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and megafaunal Californian condor fossil displays at La Brea Tar Pits
skeleton from the La Brea Tar Pits in flight pose
Cuba, with the carcass of a large solenodon
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. 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. Such include the last native North American terror bird, rhinoceros and hyena. 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 or carnivores ) included:
The survivors are in some ways as significant as the losses: bison, grey wolf, lynx, grizzly bear, American black bear, deer , pronghorn, white-lipped peccary, muskox, bighorn sheep, and mountain goat ; 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, jaguar, giant anteater, collared peccary, ocelot and jaguarundi. 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, 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. 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, 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 dying population due to climate change.
Lack of tameable megafauna was perhaps one of the reasons why Amerindian civilizations evolved differently from Old World ones. Critics have disputed this by arguing that llamas, alpacas, and bison were domesticated.

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 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. 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 and Monte Verde 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, 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.
In Sahul, 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 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 compared to other areas such as Australia, the Americas, Madagascar and New Zealand without the earliest humans. A picture arises of the megafauna of Asia and Africa evolving alongside humans, learning to be wary of them, and in other parts of the world the wildlife appearing ecologically naive and easier to hunt. This is particularly true of island fauna, which display a disastrous lack of fear of humans. Of course, it is impossible to demonstrate this naïveté directly in ancient fauna.
of Africa. However, there is evidence of extinction waves, particularly of megafaunal carnivores, coinciding with both cranial and technological developments within ancestral Homo during the Early Pleistocene of Africa. This has suggested a human role in these ecological cascades. H. sapiens skull described from Jebel Irhoud, Morocco, dated to 315,000 BCE.
within the genus Homo.
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. 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. It also suggests that this is independent of climate change.
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 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 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:
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 the Pleistocene ice age might be an explanation for the extinctions.
Critics object that since there were :Image:Five Myr Climate Change.png|multiple glacial :Image:Atmospheric CO2 with glaciers cycles.gif|advances and withdrawals 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 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 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. 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. 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 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 or anthropogenic fire preferentially selects against browse species.

Hyperdisease hypothesis

Theory

The hyperdisease hypothesis 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 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. 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. 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. Unfortunately for such a theory it can not account for several major extinction events, notably Australia and North America. Dogs did not arrive in Australia until approximately 35,000 years after the first humans arrived and approximately 30,000 years after the megafaunal extinction was complete and as such can not be implicated. In contrast numerous species including wolves, mammoths, camelids and horses had emigrated continually between Asia and North America over the past 100,000 years. For the disease hypothesis to be applicable in the case of the Americas it would require that the population remain immunologically naive despite this constant transmission of genetic and pathogenic material.

Arguments against the hyperdisease 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:
This has been supported by a computer model, the Pleistocene extinction model, which, using the same assumptions and values for all variables other than those for hunting of predators. It compares the overkill hypothesis with second-order predation. The findings are that second-order predation is more consistent with extinction than is overkill.
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..
The second-order predation hypothesis is supported by the observation above that there was a massive increase in bison populations.

Second-order predation and other theories

First publicly presented at the Spring 2007 joint assembly of the American Geophysical Union in Acapulco, Mexico, the comet hypothesis suggests that the mass extinction was caused by a swarm of comets 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 comet 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., we would not see the typical impact crater.
A spike in platinum was found in the Greenland ice cores by Petaev et al., 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.