Search This Blog

Wednesday, May 30, 2018

Quaternary extinction event

From Wikipedia, the free encyclopedia

Late Pleistocene landscape of northern Spain, by Mauricio Antón (left to right: Equus ferus, Mammuthus primigenius, Rangifer tarandus, Panthera leo spelaea, Coelodonta antiquitatis)

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 (the start of the Late Pleistocene), 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). However, this extinction wave did not stop at the end of the Pleistocene, but continued, 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.[1] Among the main causes hypothesized by paleontologists are overkill by the widespread appearance of humans and natural climate change.[2] A notable modern human presence first appeared during the Middle Pleistocene in Africa,[3] and started to establish continuous, permanent populations in Eurasia and Australasia from 120,000BCE and 63,000 BCE respectively,[4][5] and the Americas from 22,000 BCE.[6][7][8][9] 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.[10][11][12]

Pleistocene or Ice Age extinction event

The Late Pleistocene extinction event saw the extinction of many mammals weighing more than 40 kg. The proportional rate of megafauna extinctions is consecutively larger the greater the migratory distance from Africa.
The extinctions in the Americas entailed the elimination of all the larger (over 1000 kg) 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 sapiens (anatomically modern humans) not having evolved as species alongside each other.

For their part specifically, Australia, North America and South America, which respectively had the highest incremental extinction rates, had no known native species of Hominoidea (apes) at all, much less species of Hominidae (greater apes), and especially not native species of the Homo subgroup (the genus Homo comprises the species Homo sapiens, which includes modern humans,[clarification needed] as well as several extinct species classified as ancestral to or closely related to modern humans; all indigenous human groups are ultimately descendants of anatomically modern humans recently migrated out of Africa in anthropological time scale).

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 (including its megafauna) had had to accustomize to human arrival and vice versa.

There is no evidence of megafaunal extinctions at the height of the Last Glacial Maximum, indicating that increasing cold and glaciation were not factors.[13] There are three main hypotheses concerning the Pleistocene extinction:
  • The animals died off due to climate change associated with the advance and retreat of major ice caps or ice sheets.
  • The animals were exterminated by humans: the "prehistoric overkill hypothesis" (Martin, 1967).[14]
  • The extinction of the woolly mammoth (by whatever cause, perhaps by humans) changed the extensive grasslands to birch forests, and subsequent forest fires then changed the climate.[15] We now know that immediately after the extinction of the mammoth, birch forests replaced the grasslands and that an era of significant fire began.[16]
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.[14] 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.[17]

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 impact(s). Such a scenario has been proposed as a contributing cause of the 1,300-year cold period known as the Younger Dryas stadial.[citation needed] 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.[citation needed] The debate seems to be exacerbated by infighting between the Uniformitarianism camp and the Catastrophism camp.[citation needed]


The Chalicothere vanished in the early Pleistocene

Eucladoceros cranium fossil, Museo di Paleontologia di Firenze

Steppe Mammoth (Mammuthus trogontherii) dimensions

Megantereon model.

A Pachycrocuta bevirostris reconstruction

The modern Jaguar (Panthera onca), although now restricted to the Americas, originated in Asia, before colonising both sides of Beringia- Europe in the form of the European jaguar, and in the Americas as the predecessors of today's species.

Amphicyon reconstruction.

Paranthropus robustus family reconstruction, by Mauricio Antón.

Homotherium restoration- although Homotherium were extirpated in Africa 1.5 mya, they had spread throughout Eurasia and the Americas, remaining in South America at least until the Middle Pleistocene, and perishing in all other continents during the late Pleistocene.

Pelagornis sandersi comparison with the Andean condor (Vultur gryphus) and the Wandering albatross (Diomeda exulans)

Steppe bison (Bison priscus) cave art.

Reconstructions of 3 species of the Pleistocene bovid genus Pelorovis In the back is P. oldowayensis. Lying in front of it is P. turkanensis, with a horn-span of around 3 to 4 meters from tip to tip. Kneeling is the water buffalo-like P. antiquus.

Life reconstruction of the extinct Megaloceros

A woolly rhinoceros (Coelodonta antiquitatis) from a late Pleistocene landscape in northern Spain.

A cave hyena (Crocuta crocuta spelea) reconstruction.

Megalochelys atlas reconstruction

Musk oxen, present from Spain to Greenland during the late Pleistocene, were completely extirpated in Eurasia by the subatlantic Holocene- recent reintroductions from the Nearctic have substantiated their range throughout the Arctic.[18]

Wall drawing in the cave Les Combarelles in Dordogne (wild horse, cave bear, mammoth, cave lion).

Diprotodon became extinct around 50,000 years ago.

Africa and Asia

The Old World tropics were relatively spared 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 a megafaunal extinction events throughout the Pleistocene, particularly in Africa two million years ago, which coincide with key stages of human evolution and climatic trends.[19][20][21] The centre of human evolution and expansion, Africa and Asia were inhabited by advanced hominids by 2mya, with Homo habilis in Africa, and Homo erectus in both continents. By the advent and proliferation of Homo sapiens circa 298,000 BCE,[22] dominant species included Homo heidelbergensis in Africa, and (their descendants) Denisovans and Neanderthals with Homo erectus in Asia. There is evidence of an early migration event 268,000 BCE within Neanderthal genetics,[23][24] however the earliest dating for modern human inhabitation is 118,000 BCE in Arabia, China and Israel,[4][25][26][27] and 71,000 BCE in Indonesia.[28][29] Additionally, not only have these early Asian migrations left a genetic mark on modern Papuan populations,[30][31][32] the oldest known pottery in existence was found in China, dated to 18,000 BCE.[33] 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.[34]

Procoptodon goliath reconstruction

Thylacoleo carnifex rock art

Megalania priscus skeleton, Melbourne Museum

Reconstruction of the Late Pleistocene mekosuchine crocodile, Mekosuchus inexpectans, of prehistoric Fiji.

Comparison of the extinct Giant Fijian Iguana, Lapitiguana impensa, and two Viti Levu Giant Pigeons, Natunaornis gigoura, from prehistoric Fiji.

The American flamingo (Phoenicopterus ruber) was one of four species of flamingo present in Australia in the Quaternary, all of which are now either extinct or extirpated. Australia is now the only inhabited continent in the world without flamingoes.

A reconstruction of normative vegetation cover at the Last Glacial Maximum, circa 18,000 years ago, based on fossil pollen samples recovered from lake and bog sediments.
Megafauna that disappeared in Africa or Asia during the Early and Middle Pleistocene include:

Megafauna that disappeared in Africa and/or Asia during the Late Pleistocene

The Pacific (Australasia and Oceania)

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.[68][69][70][71][72] Most evidence points to a 20,000 year period after human arrival circa 63,000 BCE,[5] but scientific argument continues as to the exact date range.[73] 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 not include any spate of extinctions post 1000 BCE (e.g. subatlantic New Zealand or Hawaii).
The extinctions in the Pacific included:
The woolly mammoth became extinct around 10,000 BCE – except for diminutive relict populations on St. Paul Island and Wrangel Island, which humans did not colonise until 3,600 BCE and 2,000 BCE respectively.

Cave bear (Ursus spelaeus)

Models of the straight-tusked elephant (Paleoloxodon antiquus).

Elasmotherium reconstruction


Hippopotamuses (Hippopotamus sp.) inhabited Great Britain until 80,000 BCE, whence due to glacial shifts, hippopotamuses were restricted to Southern-Eastern Europe, Mediterranean islands and finally Western Asia until 1,000 BCE.
Some extinct megafauna, such as the bunyip-like Diprotodon, may remain in folk memory or be the sources of cryptozoological legends.

Europe and Northern Eurasia


The Leopard (Panthera pardus) inhabited the entire expanse of Afro-Eurasia below the 54th parallel north, from modern day Spain and the UK in the west, to South Africa in the south, and Siberia, Japan and Sundaland in the east during the Late Pleistocene.

This geography 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.[82] 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 in particular northern Eurasia, being far colder and drier than today,[83] was largely hegemonized by the mammoth steppe, an ecosystem dominated by palatable high-productivity grasses, herbs and willow shrubs.[83][84] This supported an extensive biota of grassland fauna, and stretched eastwards from Spain in the Iberian Peninsula to the Yukon in modern-day Canada.[82][83][85][86] 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, antelope (Parabubalis, Procapra, Saiga, Spirocerus) and steppe pika. Carnivores included cave lion, Homotherium, cave hyena, grey wolf, dhole, and the arctic fox.[87][88][89]

At the edges of these large stretches of grassland could be found more shrub-like terrain and dry conifer forests and woodland (akin to forest steppe or taiga). The browsing collective of megafauna included woolly rhinoceros, Irish elk, moose, Cervalces, tarpan, aurochs, woodland bison, camels and smaller deer (Capreolus, Cervus, Moschus). 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 grasslands, subalpine conifer forests, alpine tundra and broken, craggy slopes, was occupied by several species of mountain-going animals like argali, chamois, ibex, mouflon, pika, wolves, leopards, Ursus sp. 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 steppes and Mediterranean savannahs hosted Stephanorhinus, gazelle, European wisent, asian ostriches, Leptobos, cheetahs, and onager. These biomes also contained an assortment of mammoth steppe fauna, such as saiga antelope, lions, Homotherium, cave hyenas, wolves, Pleistocene horse, steppe bison, Spirocerus, aurochs, and camels. Temperate coniferous, deciduous, mixed broadleaf and Mediterranean forests and open woodlands accommodated straight-tusked elephants, Praemegaceros, Stephanorhinus, wild boar, bovids such as European wisent, tahr and tur, species of Ursus such as the Etruscan bear, and smaller deer (Capreolus, Cervus, Dama, Haploidoceros) with several mammoth steppe species, such as lynx, tarpan, wolves, dholes, moose, Irish Elk, woodland bison, leopards, and aurochs. Woolly rhinoceros and mammoth occasionally resided in these temperate biomes, mixing with predominately temperate fauna to escape harsh glacials.[90][91] 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.[61][92]

The first possible indications of habitation by hominins are the 7.2 million year old finds of Graecopithecus,[93] and 5.7 million year old footprints in Crete- however established habitation is noted in Georgia from 1.8 million years ago, proceeded by Germany and France, via Homo erectus.[94][95] Prominent co-current and subsequent species include Homo antecessor, Homo cepranensis, Homo heidelbergensis, Neanderthals and Denisovans, preceding human habitation circa 38,000 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 Homotherium. However, the great majority of species were extinguished, extirpated or experienced severe population contractions between 13,000 BCE and 9,000 BCE,[96] 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.[97][98] 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 elephantids, megacerines and hippopotamuses, and giant avians, otters and rodents.


The 'Gallery of Lions', representations of the Eurasian Cave Lion in Chauvet-Pont-d'Arc Cave.

Homotherium restoration.

Reconstruction of the five phenotypes of Pleistocene wild horse. The coat colours and dimensions are based on genetic evidence and historic descriptions.

A model of an adult Neanderthal male head and shoulders on display in the Hall of Human Origins in the National Museum of Natural History in Washington, D.C.

Modern cheetah (Acinonyx jubatus) replaced giant cheetah (Acinonyx pardinensis) in Eurasia after the Middle Pleistocene, and inhabited a range from Eastern Europe and the Balkans to China. Today, the critically endangered Eurasian cheetah are now restricted to Iran.

Saiga antelope (Saiga sp.) inhabited a range from England and France to the Yukon in the Late Pleistocene, diversifying into two species. S. borealis is now extinct and the critically endangered S. tatarica is now limited to steppe in Kazakhstan and Mongolia.
(80,000–4,000 years ago)
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 Eurasia during the Quaternary extinction event include-

North America and the Caribbean


Short faced bear (Arctodus simus)

Chendytes lawi et Oncorhynchus rastrosus

Life restoration of the Yukon wild horse, Equus lambei.

Reconstruction of the Western Camel, Camelops hesternus

Life restoration of Cervalces scotti.

Bison latifrons, fossil buffalo skeleton (public display, Cincinnati Museum of Natural History & Science, Cincinnati, Ohio, USA).

Californian Turkey (Meleagris californica) and Megafaunal Californian Condor (Gymnogyps amplus) fossil displays at La Brea Tar Pits

Reconstruction of the Cuban Giant Owl, Ornimegalonyx oteroi, of Pleistocene Cuba, with the carcass of a large solenodon.

Life restoration of the Shasta Ground Sloth, Nothrotheriops shastensis.

Glyptotherium reconstruction

Mixotoxodon larenis reconstruction.

Reconstruction of Smilodon fatalis

Reconstruction of Panthera leo atrox

Columbian Mammoth (Mammuthus columbi) reconstruction

Graphical reconstruction of Mammut americanum.

The Dhole, now restricted to the southern portions of Asia, was present from the Iberia to Mexico during the Late Pleistocene.

A Chacoan peccary (Catagonus wagneri), believed to be the closest surviving relative of the extinct Platygonus.

Size comparison of Smilodon fatalis (purple), S. popular (green), and S. gracilis (orange) with modern human for scale. Each grid segment=1 square meter.

Life restoration of Arctotherium bonariense.

Reconstruction of Canis dirus with two possible aspects according to its probable geographic origin: North American or South American.

Cuvieronius reconstruction

Reconstruction of a Macrauchenia mother and calf, from Pleistocene South America.

A Toxodon skull in an exhibition commemorating the 200th anniversary of Charles Darwin's birth, Esplanada dos Ministérios, Brasília.

An illustration of Megatherium.

Doedicurus clavicaudatus reconstruction, distributed in the temperate savannahs and woodlands of South America.

Fossil reconstruction of Panochthus frenzelianus with metal model.

Fossil skull of Hippidion, a genera of native South American horse which went extinct in the early Holocene (6,000 BCE).

The disputed Late Pleistocene remains of a phorusrhacid in Uruguay are similar in size to the above Pliocene age Procariama simplex.

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,[125][126][127][1] and Panama, the crossroads of the American Interchange.[2] Most other extinctions are poorly constrained in time, though some definitely occurred outside of this narrow interval.[128] 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 unequivacolly approximately 22,000 BCE north of the glacier,[6] and 13,500 BCE south,[129][130] 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.[125][131] The megafaunal extinctions include forty one genera of herbivores (H), and twenty carnivores (C). North American extinctions 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, waipiti (elk), Odocoileus sp.) (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), margay (C), and jaguarundi (C). All save the pronghorns and giant anteaters were descended from Asian ancestors that had evolved with human predators.[159] 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.[160][161][162] Because ancient bison evolved into living bison,[163][164] 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-Indian 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 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.[165] Critics have disputed this by arguing that llamas, alpacas, and bison were domesticated.[166]

South America

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.[167][168][3] 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,[169][170] and the colder, arid interior resulted in the advance of temperate lowland woodlands, tropical savannas and deserts at the expense of rainforests.[171][172][173][174][175] Within these open environments, megafauna diversity was extremely dense, with over 40 genera recorded from the Guerrero member of Luján Formation alone.[176][177][178][179] 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.[180][181][182][183] 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).[184] 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.[125][130][185] 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 Tapirus genus, 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.

Later extinctions

There is no general agreement on where the Holocene, or anthropogenic, extinction begins, and the Quaternary extinction event which includes climate change resulting in the end of the last ice age ends, or if they should be considered separate events at all.[211][212] 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,[213] 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.[214][215] Only during the most recent parts of the extinction have plants also suffered large losses.[216]

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


A Woolly Mammoth hunt

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.[217][218][219] 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. 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.[citation needed] This is particularly true of island fauna, which display a disastrous lack of fear of humans.[citation needed] Of course, it is impossible to demonstrate this naïveté directly in ancient fauna.

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.[220][221] 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.[222][223]

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. 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[224][225][226] and by computer models by Mosimann and Martin,[227] and Whittington and Dyke,[228] and most recently by Alroy.[229]

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.[230] This was compared against climate reconstructions for the last 90,000 years.[230] 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.[230][231] The study, however, found an apparently low extinction rate in the fossil record of mainland Asia.[231]

Overkill hypothesis


The timing of extinctions follows the "March of Man"

The overkill hypothesis, a variant of the hunting hypothesis, was proposed 40 years ago 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:
  • In predator-prey models it is unlikely that predators could over-hunt their prey, since predators need their prey as food to sustain life and to reproduce.[232] This criticism has been rejected by many ecologists because humans have the widest dietary choice of any predator and are perfectly capable of switching to alternative prey or even plant foods when any prey species becomes rare. Humans have indisputably hunted numerous species to extinction, which renders any argument that human predators can never hunt prey to extinction immediately invalid.
  • There is no archeological evidence that in North America megafauna other than mammoths, mastodons, gomphotheres and bison were hunted, despite the fact that, for example, camels and horses are very frequently reported in fossil history.[233] Overkill proponents, however, say this is due to the fast extinction process in North America and the low probability of animals with signs of butchery to be preserved.[234] Additionally, biochemical analyses have shown that Clovis tools were used in butchering horses and camels.[235] A study by Surovell and Grund[236] concluded "archaeological sites dating to the time of the coexistence of humans and extinct fauna are rare. Those that preserve bone are considerably more rare, and of those, only a very few show unambiguous evidence of human hunting of any type of prey whatsoever."
  • A small number of animals that were hunted, such as a single species of bison, did not go extinct. This cannot be explained by proposing that surviving bison in North America were recent Eurasian immigrants that were familiar with human hunting practices, since Bison first appeared in North America approximately 240,000 years ago[160][161][162] and then evolved into living bison.[163][237] Bison at the end of the Pleistocene were thus likely to have been almost as naive as their native North American megafaunal companions.
  • The dwarfing of animals is not explained by overkill. Numerous authors[who?], however, have pointed out that dwarfing of animals is perfectly well explained by humans selectively harvesting the largest animals, and have provided proof that even within the 20th century numerous animal populations have reduced in average size due to human hunting.
  • Eurasian Pleistocene megafauna became extinct in roughly same time period despite having a much longer time to adapt to hunting pressure by humans. However, the extinction of the Eurasian megafauna can be viewed as a result of a different process than that of the American megafauna. This makes the theory less parsimonious since another mechanism is required. The latter case occurred after the sudden appearance of modern human hunters on a land mass they had never previously inhabited, while the former case was the culmination of the gradual northward movement of human hunters over thousands of years as their technology for enduring extreme cold and bringing down big game improved. Thus, while the hunting hypothesis does not necessarily predict the rough simultaneity of the north Eurasian and American megafaunal extinctions, this simultaneity cannot be regarded as evidence against it.
  • Eugene S. Hunn points out that the birthrate in hunter-gatherer societies is generally too low, that too much effort is involved in the bringing down of a large animal by a hunting party, and that in order for hunter-gatherers to have brought about the extinction of megafauna simply by hunting them to death, an extraordinary amount of meat would have had to have been wasted.[238] It is possible that those who advocate the overkill hypothesis simply have not considered the differences in outlook between typical forager (hunter-gatherer) cultures and the present-day industrial cultures which exist in modernized human societies; waste may be tolerated and even encouraged in the latter, but is not so much in the former. It may be noted that in relatively recent human history, for instance, the Lakota of North America were known to take only as much bison as they could use, and they used virtually the whole animal—this despite having access to herds numbering in the millions.[239] Conversely, "buffalo jumps"[240] featured indiscriminate killing of a herd. However, Hunn's comments are in reference to the now largely discredited theory of hunter-prey equilibrium reached after thousands of years of coexistence. It is not relevant to hunters newly arrived on a virgin land mass full of easily taken big game. The well-established practice of industrial-scale moa butchering by the early Maori, involving enormous wastage of less choice portions of the meat, indicates that these arguments are incorrect.[224]
  • The hypothesis that the Clovis culture represented the first humans to arrive in the New World has been disputed recently. (See Settlement of the Americas.) However, they were certainly the first to leave abundant widespread evidence of their presence.

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 the Pleistocene ice age might be an explanation for the extinctions.

Critics object that since there were multiple glacial 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.[241] 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.[242][243][244]

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.[245]

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.[246]

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 16 °C (29 °F) 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.[247]

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.[248][249][250][251][252][253]

In addition, numerous species such as mammoths on Wrangel Island[254] 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.[250][251][253] 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.[255][256][257] 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.[258][259][260] Axelrod[261] and Slaughter[262] 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.[263]

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.[247]

Arguments against the continentality hypotheses

Critics have identified a number of problems with the continentality hypotheses.
  • Megaherbivores have prospered at other times of continental climate. For example, megaherbivores thrived in Pleistocene Siberia, which had and has a more continental climate than Pleistocene or modern (post-Pleistocene, interglacial) North America.[264][265][266]
  • The animals that became extinct actually should have prospered during the shift from mixed woodland-parkland to prairie, because their primary food source, grass, was increasing rather than decreasing.[267][266][268] Although the vegetation did become more spatially specialized, the amount of prairie and grass available increased, which would have been good for horses and for mammoths, and yet they became extinct. This criticism ignores the increased abundance and broad geographic extent of Pleistocene Bison at the end of the Pleistocene, which would have increased competition for these resources in a manner not seen in any earlier interglacials.[241]
  • Although horses became extinct in the New World, they were successfully reintroduced by the Spanish in the 16th century—into a modern post-Pleistocene, interglacial climate. Today there are feral horses still living in those same environments. They find a sufficient mix of food to avoid toxins, they extract enough nutrition from forage to reproduce effectively and the timing of their gestation is not an issue. Of course, this criticism ignores the obvious fact that present-day horses are not competing for resources with ground sloths, mammoths, mastodons, camels, llamas, and bison. Similarly, mammoths survived the Pleistocene Holocene transition on isolated, uninhabited islands in the Mediterranean Sea[269] and on Wrangel Island in the Siberian Arctic[270] until 4,000 to 7,000 years ago.
  • Large mammals should have been able to migrate, permanently or seasonally, if they found the temperature too extreme, the breeding season too short, or the rainfall too sparse or unpredictable.[271] Seasons vary geographically. By migrating away from the equator, herbivores could have found areas with growing seasons more favorable for finding food and breeding successfully. Modern-day African elephants migrate during periods of drought to places where there is apt to be water.[272]
  • Large animals store more fat in their bodies than do medium-sized animals[273] and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.
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.[274][275] 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.[276] However, a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by either Second Order Predation (see below)[277][278] or anthropogenic fire preferentially selects against browse species.[citation needed]

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.[279][280][281] 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.[279]

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.[282] 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

  • Generally speaking, disease has to be very virulent to kill off all the individuals in a genus or species. Even such a virulent disease as West Nile Virus is unlikely to have caused extinction.[283]
  • The disease would need to be implausibly selective while being simultaneously implausibly broad. Such a disease needs to be capable of killing off wolves such as Canis dirus or goats such as Oreamnos harringtoni while leaving other very similar species (Canis lupus and Oreamnos americanus, respectively) unaffected. It would need to be capable of killing off flightless birds while leaving closely related flighted species unaffected. Yet while remaining sufficiently selective to afflict only individual species within genera it must be capable of fatally infecting across such clades as birds, marsupials, placentals, testudines, and crocodilians. No disease with such a broad scope of fatal infectivity is known, much less one that remains simultaneously incapable of infecting numerous closely related species within those disparate clades.

Second-order predation


Combination Hypotheses: Climate Change, Overkill + Climate Change, Second-Order Predation + Climate Change

Overkill Hypothesis and Second-Order Predation

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:
  • After the arrival of H. sapiens in the New World, existing predators must share the prey populations with this new predator. Because of this competition, populations of original, or first-order, predators cannot find enough food; they are in direct competition with humans.
  • Second-order predation begins as humans begin to kill predators.
  • Prey populations are no longer well controlled by predation. Killing of nonhuman predators by H. sapiens reduces their numbers to a point where these predators no longer regulate the size of the prey populations.
  • Lack of regulation by first-order predators triggers boom-and-bust cycles in prey populations. Prey populations expand and consequently overgraze and over-browse the land. Soon the environment is no longer able to support them. As a result, many herbivores starve. Species that rely on the slowest recruiting food become extinct, followed by species that cannot extract the maximum benefit from every bit of their food.
  • Boom-bust cycles in herbivore populations change the nature of the vegetative environment, with consequent climatic impacts on relative humidity and continentality. Through overgrazing and overbrowsing, mixed parkland becomes grassland, and climatic continentality increases.

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[284][285] (results graph at left).

The PEM 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.[277] (results graph at right).

The second-order predation hypothesis is supported by the observation above that there was a massive increase in bison populations.[286]

Second-order predation and other theories

  • Climate Change: Second-Order Predation accounts for the changes in vegetation, which in turn may account for the increase in continentality. Since the extinction is due to destruction of habitat it accounts for the loss of animals not hunted by humans. Second-Order Predation accounts for the dwarfing of animals as well as extinctions since animals that could survive and reproduce on less food would be selectively favored.
  • Hyperdisease: The reduction of carnivores could have been from distemper or other carnivore disease carried by domestic dogs.
  • Overkill: The observation that extinctions follow the arrival of humans is consistent with the Second-Order Predation hypothesis.

Arguments against the second-order predation hypothesis

  • The model specifically assumes high extinction rates in grasslands, but most extinct species ranged across numerous vegetation zones. Historical population densities of ungulates were very high in the Great Plains; savanna environments support high ungulate diversity throughout Africa, and extinction intensity was equally severe in forested environments.
  • It is unable to explain why large herbivore populations were not regulated by surviving carnivores such as grizzly bears, wolves, pumas, and jaguars whose populations would have increased rapidly in response to the loss of competitors.
  • It does not explain why almost all extinct carnivores were large herbivore specialists such as sabre toothed cats and short faced bears, but most hypocarnivores and generalized carnivores survived.
  • There is no historical evidence of boom and bust cycles causing even local extinctions in regions where large mammal predators have been driven extinct by hunting. The recent hunting out of remaining predators throughout most of the United States has not caused massive vegetational change or dramatic boom and bust cycles in ungulates.
  • It is not spatially explicit and does not track predator and prey species separately, whereas the multispecies overkill model does both.
  • The multispecies model produces a mass extinction through indirect competition between herbivore species: small species with high reproductive rates subsidize predation on large species with low reproductive rates.[229] All prey species are lumped in the Pleistocene Extinction Model.
  • Everything explained by the Pleistocene Extinction Model also is explained by the multispecies model, but with fewer assumptions, so the Pleistocene Extinction Model appears less parsimonious. However, the multispecies model does not explain shifts in vegetation, nor is it able to simulate alternative hypotheses. The multispecies model therefore necessitates additional assumptions and hence is less parsimonious.

Arguments against the second-order predation plus climate hypothesis

  • It assumes decreases in vegetation due to climate change, but deglaciation doubled the habitable area of North America.
  • Any vegetational changes that did occur failed to cause almost any extinctions of small vertebrates, and they are more narrowly distributed on average.

Comet hypothesis

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.[287][288][289]

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. However, if the bolide struck the Laurentide ice sheet as hypothesized by Firestone et al. (2007), we would not see the typical impact crater. A spike in platinum was found in the Greenland ice cores by Petaev et al. (2013), which they view as a global signal.[290] Platinum has more recently been found in sediments by Christopher Moore (2017).[291]
  • There is a lack of evidence for a population decline among the Paleoindians at 12,900 ± 100 calBP as might be expected.[292][293][294]
  • There is evidence that the megafaunal extinctions that occurred across northern Eurasia, North America and South America at the end of the Pleistocene were not synchronous as the bolide theory would predict. The extinctions in South America appear to have occurred at least 400 years after those in North America.[295][296][297]
  • Additionally, some island megafaunal populations survived thousands of years longer than populations of the same or related species on nearby continents; examples include the survival of woolly mammoths on Wrangel Island until 3700 BP,[295][296] and the survival of ground sloths in the Antilles until 4700 cal BP.[295][296][297]
  • Several markers for the proposed impact event are disputed by most scientists. They have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets,[298] and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates.[299][300] An analysis of a similar Younger Dryas boundary layer in Belgium also did not show evidence of a bolide impact.[301] Researchers have also not found any extraterrestrial platinum group metals in the boundary layer; this absence is inconsistent with the hypothesized impact event.[302][290]
at

Neanderthal extinction

From Wikipedia, the free encyclopedia

Neanderthal extinction began around 40,000 years ago in Europe, after anatomically modern humans had reached the continent. This date, which is based on research published in Nature in 2014, is much earlier than previous estimates, and it was established through improved radio carbon dating methods analyzing 40 sites from Spain to Russia. The survey did not include sites in Asia, where Neanderthals may have survived longer.[1]

Hypotheses on the fate of the Neanderthals include violence from encroaching anatomically modern humans,[2] parasites and pathogens, competitive replacement, competitive exclusion, extinction by interbreeding with early modern human populations,[3] and failure or inability to adapt to climate change. It is unlikely that any one of these hypotheses is sufficient on its own; rather, multiple factors probably contributed to the demise of an already widely-dispersed population.[4][5]

Coexistence prior to extinction


Neanderthal tools

modern human tools
In research published in Nature in 2014, an analysis of radiocarbon dates from forty Neanderthal sites from Spain to Russia found that the Neanderthals disappeared in Europe between 41,000 and 39,000 years ago with 95% probability. The study also found with the same probability that modern humans and Neanderthals overlapped in Europe for between 2,600 and 5,400 years.[1] Modern humans reached Europe between 45,000 and 43,000 years ago.[6] Most recent numbers based on improved radiocarbon dating indicate that Neanderthals disappeared around 40,000 years ago, which overturns older carbon dating which indicated that Neanderthals may have lived as recently as 24,000 years ago,[7] including in refugia on the south coast of the Iberian peninsula such as Gorham's Cave.[8][9] Inter-stratification of Neanderthal and modern human remains has been suggested,[10] but is disputed.[11]

Possible cause of extinction

Violence

Some authors have discussed the possibility that Neanderthal extinction was either precipitated or hastened by violent conflict with Homo sapiens. Conflict and warfare are virtually ubiquitous features of hunter-gatherer societies, including conflicts over limited resources, such as prey and water.[12] It is therefore plausible to suggest that violence, including primitive warfare, would have transpired between the two human species.[13] The hypothesis that early humans violently replaced Neanderthals was first proposed by French palaeontologist Marcellin Boule (the first person to publish an analysis of a Neanderthal) in 1912.[14] Several finds in both Homo-sapiens and Neanderthal bones indicate inter-species aggression from injuries (grooves in the bones themselves) that could only have come from spear or other projectile tips crafted with prevalent tool-making methods contemporary to the time.[15][unreliable source?]

Parasites and pathogens

Another possibility is the spread among the Neanderthal population of pathogens or parasites carried by Homo sapiens.[16] Neanderthals would have limited immunity to diseases they had not been exposed to, so diseases carried into Europe by Homo sapiens could have been particularly lethal to them while Homo sapiens were relatively resistant. If it were relatively easy for pathogens to leap between these two similar species, perhaps because they lived in close proximity, then Homo sapiens would have provided a pool of individuals capable of infecting Neanderthals and potentially preventing the epidemic from burning itself out as Neanderthal population fell. An examination of human and Neanderthal genomes and adaptations regarding pathogens or parasites may shed light on this issue.

Competitive replacement


Sapiens and Neanderthal skulls

Species specific disadvantages

Slight competitive advantage on the part of modern humans has accounted for Neanderthals' decline on a timescale of thousands of years.[17]

Generally small and widely-dispersed fossil sites suggest that Neanderthals lived in less numerous and socially more isolated groups than contemporary Homo sapiens. Tools such as Mousterian flint stone flakes and Levallois points are remarkably sophisticated from the outset, yet they have a slow rate of variability and general technological inertia is noticeable during the entire fossil period. Artifacts are of utilitarian nature, symbolic behavioral traits are undocumented before the arrival of modern humans in Europe around 40,000 to 35,000 years ago.[18][19]

Jared Diamond, supporter of competitive replacement points out in his book The Third Chimpanzee that the genocidal replacement of Neanderthals by modern humans is comparable to patterns of behavior that occur whenever people with advanced technology clash with less advanced people.[20]

Division of labor

In 2006, two anthropologists of the University of Arizona proposed an efficiency explanation for the demise of the Neanderthals.[21] In an article titled "What's a Mother to Do? The Division of Labor among Neanderthals and Modern Humans in Eurasia",[22] it was posited that Neanderthal division of labor between the sexes was less developed than Middle paleolithic Homo sapiens. Both male and female Neanderthals participated in the single occupation of hunting big game, such as bison, deer, gazelles and wild horses. This hypothesis proposes that the Neanderthal's relative lack of labor division resulted in less efficient extraction of resources from the environment as compared to Homo sapiens.

Anatomical differences and running ability

Researchers such as Karen L. Steudel of the University of Wisconsin have highlighted the relationship of Neanderthal anatomy (shorter and stockier than that of modern humans[citation needed]) and the ability to run and the requirement of energy (30% more).[23][not in citation given]

Nevertheless, in the recent study, researchers Martin Hora and Vladimir Sladek of Charles University in Prague show that Neanderthal lower limb configuration, particularly the combination of robust knees, long heels and short lower limbs, increased the effective mechanical advantage of Neanderthal knee and ankle extensors, thus reducing the force needed and the energy spent for locomotion significantly. The walking cost of the Neanderthal male is now estimated to be 8–12% higher than that of anatomically modern males, whereas the walking cost of the Neanderthal female is considered to be virtually equal to that of anatomically modern females.[24]

Other researchers, like Yoel Rak, from Tel-Aviv University in Israel, have noted that the fossil records show that Neanderthal pelvises in comparison to modern human pelvises would have made it much harder for Neanderthals to absorb shock and to bounce off from one step to the next, giving modern humans another advantage over Neanderthals in running and walking ability.[25]

Modern humans' advantage in hunting warm climate animals

Pat Shipman, from Pennsylvania State University in the United States, argues that the domestication of the dog gave modern humans an advantage when hunting.[26] The oldest remains of domesticated dogs were found in Belgium (31,700 BP) and in Siberia (33,000 BP).[27][28] A survey of early sites of modern humans and Neanderthals with faunal remains across Spain, Portugal and France provided an overview of what modern humans and Neanderthals ate.[29] Rabbit became more frequent, while large mammals – mainly eaten by the Neanderthals – became increasingly rare. In 2013, DNA testing on the "Altai dog", a paleolithic dog's remains from the Razboinichya Cave (Altai Mountains), has linked this 33,000-year-old dog with the present lineage of Canis lupus familiaris.[30]

Interbreeding


Human-Neandertal mtDNA

Neanderthal DNA extraction
Interbreeding can only account for a certain degree of Neanderthal population decrease. A homogeneous absorption of an entire species is a rather unrealistic idea. This would also be counter to strict versions of the Recent African Origin, since it would imply that at least part of the genome of Europeans would descend from Neanderthals, who left Africa at least 350,000 years ago.

The most vocal proponent of the hybridization hypothesis is Erik Trinkaus of Washington University.[31][32] Trinkaus claims various fossils as hybrid individuals, including the "child of Lagar Velho", a skeleton found at Lagar Velho in Portugal.[33] In a 2006 publication co-authored by Trinkaus, the fossils found in 1952 in the cave of Peștera Muierilor, Romania, are likewise claimed as hybrids.[34]

Genetic studies indicate some form of hybridization between archaic humans and modern humans had taken place after modern humans emerged from Africa. An estimated 1–4% of the DNA in Europeans and Asians (e.g. French, Chinese and Papua probands) is non-modern, and shared with ancient Neanderthal DNA rather than with sub-Saharan Africans (e.g. Yoruba and San probands).[35]

Modern-human findings in Abrigo do Lagar Velho, Portugal allegedly featuring Neanderthal admixtures have been published.[36] However, the interpretation of the Portuguese specimen is disputed.[37]

Jordan, in his work Neanderthal, points out that without some interbreeding, certain features on some "modern" skulls of Eastern European Cro-Magnon heritage are hard to explain. In another study, researchers have recently found in Peştera Muierilor, Romania, remains of European humans from ~37,000–42,000 years ago[38]who possessed mostly diagnostic "modern" anatomical features, but also had distinct Neanderthal features not present in ancestral modern humans in Africa, including a large bulge at the back of the skull, a more prominent projection around the elbow joint, and a narrow socket at the shoulder joint. Analysis of one skeleton's shoulder showed these humans, like Neanderthals, did not have the full capability for throwing spears.[39]

The Neanderthal genome project published papers in 2010 and 2014 stating that Neanderthals contributed to the DNA of modern humans, including most humans outside sub-Saharan Africa, as well as a few populations in sub-Saharan Africa, through interbreeding, likely between 50,000 and 60,000 years ago.[40][41][42] Recent studies also show that a few Neanderthals began mating with ancestors of modern humans long before the large "out of Africa migration" of the present day non-Africans, as early as 100,000 years ago.[43] In 2016, research indicated that there were three distinct episodes of interbreeding between modern humans and Neanderthals: the first encounter involved the ancestors of non-African modern humans, probably soon after leaving Africa; the second, after the ancestral Melanesian group had branched off (and subsequently had a unique episode of interbreeding with Denisovans); and the third, involving the ancestors of East Asians only.[44]

While interbreeding is viewed as the most parsimonious interpretation of the genetic discoveries, the authors point out they cannot conclusively rule out an alternative scenario, in which the source population of non-African modern humans was already more closely related to Neanderthals than other Africans were, due to ancient genetic divisions within Africa.

Among the genes shown to differ between present-day humans and Neanderthals were RPTN, SPAG17, CAN15, TTF1 and PCD16.

Neanderthal DNA Comparison (SharedDNA)

Climate change


Snowbound, painting of Neanderthals in a blizzard, Charles R. Knight, 1911

Neanderthals went through a demographic crisis in Western Europe that seems to coincide with climate change that resulted in a period of extreme cold in Western Europe. "The fact that Neanderthals in Western Europe were nearly extinct, but then recovered long before they came into contact with modern humans came as a complete surprise to us," said Love Dalén, associate professor at the Swedish Museum of Natural History in Stockholm. If so, this would indicate that Neanderthals may have been very sensitive to climate change.[45]

Natural catastrophe

A number of researchers have argued that the Campanian Ignimbrite Eruption, a volcanic eruption near Naples, Italy, about 39,280 ± 110 years ago (older estimate ~37,000 years), erupting about 200 km3 (48 cu mi) of magma (500 km3 (120 cu mi) bulk volume) has contributed to the extinction of Neanderthal man.[46] The argument has been developed by Golovanova et al.[47][48]

Although Neanderthals had encountered several Interglacials during 250,000 years in Europe[49] inability to adapt their hunting methods caused their extinction facing H. sapiens competition when Europe changed into a sparsely vegetated steppe and semi-desert during the last Ice Age.[50] Studies of sediment layers at Mezmaiskaya Cave suggest a severe reduction of plant pollen.[48] The damage to plant life would have led to a corresponding decline in plant-eating mammals hunted by the Neanderthals.[48][51][52]
at
Subscribe to: Posts (Atom)

Bayesian inference

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Bayesian_inference Bayesian inference ( / ...

  • Islamic State and the Levant
    From Wikipedia, the free encyclopedia Islamic State of Iraq and the Levant الدولة الإسلامية في العراق والشام   ( ...
  • Heart Sutra
    From Wikipedia, the free encyclopedia A reproduction of the palm -leaf manuscript in Siddham script ...
  • Slum
    From Wikipedia, the free encyclopedia ...