後期更新世の絶滅

19世紀末から20世紀初頭にかけて、科学者たちが氷河期と間氷期が存在し、それらが特定の動物の繁栄や絶滅と何らかの形で関連していることに初めて気づいたとき、彼らは更新世の氷河期の終焉が絶滅の原因の一つであるのではないかと推測した。

氷河期の終焉に伴う最も明白な変化は気温の上昇である。15,000年前から10,000年前の間に、 地球の平均年間気温が6℃上昇した。これが絶滅の原因であると一般的に考えられていた。この仮説によれば、ウィスコンシン氷床を溶かすのに十分な気温上昇は、寒冷地に適応した哺乳類に十分な熱ストレスを与え、死に至らしめた可能性がある。氷河の寒さの中で体温を保つのに役立つ厚い毛皮は、余分な熱の放出を妨げ、哺乳類が熱疲労で死に至った可能性がある。表面積対体積比が低い大型哺乳類は、小型哺乳類よりも状況が悪かっただろう。過去56,000年を対象とした研究によると、最大16 ℃（29 °F）の気温変化を伴う急激な温暖化イベントが、大型動物の絶滅に重要な影響を与えた。古代のDNAと放射性炭素年代測定データは、局所的な遺伝子集団が同じ種内の他の集団、または同じ属内の他の集団に置き換えられたことを示している。集団の生存は避難場所の存在と長距離分散に依存していたが、これらは人間の狩猟によって阻害された可能性がある。^{[ 171 ]}  

他の科学者たちは、ますます極端な気候（夏は暑く冬は寒くなる）――「大陸性」と呼ばれるもの――や、降雨量の変化が絶滅の原因であると提唱している。植生は混交林と草原から草原と森林に分かれたことが示されている。 ^{[ 172 ] [ 173 ] [ 174 ]}これは利用可能な食料の種類に影響を与えた可能性がある。生育期間が短くなったことで、大型草食動物が絶滅し、他の多くの動物が矮小化した可能性がある。この場合、観察されているように、反芻動物は限られた量の高繊維の食物からより多くの栄養を抽出でき、草食動物に対する毒素に対処しやすいため、バイソンや他の大型反芻動物は馬、象、その他の単胃動物よりも生き残っただろう。 ^{[ 175 ] [ 176 ] [ 177 ]}つまり、一般的に、植生がより特殊化すると、食性の柔軟性が低い草食動物は、特定の地域内で生命を維持し繁殖するために必要な植生の組み合わせを見つけることが難しくなる可能性がある。大陸性気候の拡大は、降雨量の減少と予測の困難化をもたらし、エネルギーと栄養に必要な植物の入手可能性を制限した。^{[ 178 ] [ 179 ] [ 180 ]}この降雨量の変化により、繁殖に適した時間が制限されたことが示唆されている。^{[ 181 ] [ 182 ]}これは、交尾期間が長く柔軟性に欠ける大型動物に不均衡な影響を与える可能性があり、生育期の変化により十分な食料、水、またはシェルターが利用できない不利な季節に子を産む可能性がある。対照的に、小型哺乳類は、ライフサイクル、繁殖サイクル、妊娠期間が短いため、個体としても種としても気候の予測不可能性の増大に適応し、子孫の生存に有利な条件に合わせて繁殖努力を同期させることができたと考えられる。そうであれば、小型哺乳類は子孫の損失が少なく、状況が再び子孫の生存に有利になったときに繁殖努力を繰り返すことができたであろう。^{[ 183 ]}25,000年前から10,000年前までのヨーロッパ、シベリア、アメリカ大陸の環境条件を調べた研究では、氷河の融解と最大降雨量につながる長期にわたる温暖化現象が、大型草食動物を支えていた牧草地が草食動物に抵抗力のある植物を支える広範囲の湿地に変化する直前に発生したことがわかった。この研究は、水分によって引き起こされた環境変化が大型動物の絶滅につながり、アフリカの赤道横断的な位置により砂漠と中央森林の間に牧草地が存在し続けたため、そこで絶滅した大型動物の種が少なかったと提唱している。^{[ 171 ]}

東南アジアの証拠は、ヨーロッパ、オーストラリア、アメリカ大陸とは対照的に、気候変動と海面上昇がいくつかの草食動物種の絶滅の重要な要因であったことを示唆している。植生成長の変化と、初期の人類や哺乳類がこれまで孤立していた局所的な生態系へアクセスするための新たな経路は、特定の動物群にとって有害で​​あった。^{[ 184 ]}

ヨーロッパからの証拠の中には、気候変動が絶滅の原因であったことを示唆するものもある。個体の絶滅は環境変化の時期に起こる傾向があり、人間の移住とはあまり相関がなかったからである。^{[ 2 ]}

オーストラリアでは、いくつかの研究が、大型動物の絶滅は大陸への人類の移住以前に始まっており、気候変動がその要因であると示唆している。^{[ 185 ]}

ベーリンジアでは、特に激しい湿地化と、コルディレラ氷床がベーリンジアと北アメリカの残りの部分との間の回廊を再び開くほど十分に後退する前にユーラシアと北アメリカの間の陸地が水没したために、大型動物が絶滅した可能性がある。^{[ 186 ]}ケナガマンモスは気候要因によりベーリンジアから絶滅したが、人間の活動もその減少に相乗的な役割を果たした。^{[ 187 ]}北アメリカでは、放射性炭素年代測定イベントカウント（REC）モデリング研究により、北アメリカの大型動物の減少は人間の人口増加ではなく気候変動と相関していることが判明した。^{[ 188 ]}

北米五大湖地域では、マストドンやマンモスの個体数減少は、人間の活動ではなく、ヤンガードリアス期の気候変動と相関関係にあることが分かっている。^{[ 189 ]}

アルゼンチンのパンパでは、かつてははるかに広かったパンパ草原の広大な地域が洪水に見舞われたことが、大型動物群の絶滅の一因となった可能性がある。^{[ 9 ]}

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. Proponents of climate change as the extinction event's cause like David J. Meltzer suggest that the last deglaciation may have been markedly different from previous ones.^[190] Also, one study suggests that the Pleistocene megafaunal composition may have differed markedly from that of earlier interglacials, making the Pleistocene populations particularly vulnerable to changes in their environment.^[191]

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 most of the same large mammals survived similar temperature increases.^{[192][193][194]} In addition, numerous species such as mammoths on Wrangel Island and St. Paul Island survived in human-free refugia despite changes in climate.^[195] 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.

Critics have also 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.^{[196][197][198]} 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.^{[199][198][200]} 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.^[191] 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. This criticism ignores the 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 until 4,000 to 7,000 years ago,^[201] as well as on Wrangel Island in the Siberian Arctic.^[202] Additionally, 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.^[203] 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.^[204] Large animals also store more fat in their bodies than do medium-sized animals and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.^[205]

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.^{[206][207][208]}

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.^[209]

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.^[210] However, a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by either Second Order Predation (see below)^[166][211] or anthropogenic fire preferentially selects against browse species.

The hyperdisease hypothesis, as advanced by Ross D. E. MacPhee and Preston A. Marx, attributes the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans.^{[212][213][214]} In more recent times, disease has driven many vulnerable species to extinction; the introduction of avian malaria and avipoxvirus, for example, has greatly decreased the populations of the endemic birds of Hawaii, with some going extinct.^[215] 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.^[212] A similar suggestion is that pathogens were transmitted by the expanding humans via the domesticated dogs they brought with them.^[216] A related theory proposes that a highly contagious prion disease similar to chronic wasting disease or scrapie that was capable of infecting a large number of species was the culprit. Animals weakened by this "superprion" would also have easily become reservoirs of viral and bacterial diseases as they succumbed to neurological degeneration from the prion, causing a cascade of different diseases to spread among various mammal species. This theory could potentially explain the prevalence of heterozygosity at codon 129 of the prion protein gene in humans, which has been speculated to be the result of natural selection against homozygous genotypes that were more susceptible to prion disease and thus potentially a tell-tale of a major prion pandemic that affected humans of or younger than reproductive age far in the past and disproportionately killed before they could reproduce those with homozygous genotypes at codon 129.^[217]

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.

As with other hypotheses, a number of counterarguments to the hyperdisease hypothesis have been put forth. 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 fever is unlikely to have caused extinction.^[218] 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. On the other hand, this objection does not account for the possibility of a variety of different diseases being introduced around the same era. 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 there it would require that the population remain immunologically naive despite this constant transmission of genetic and pathogenic material. The dog-specific hypothesis in particular cannot account for several major extinction events, notably the Americas (for reasons already covered) and Australia. Dogs did not arrive in Australia until approximately 35,000 years after the first humans arrived there, and approximately 30,000 years after the Australian megafaunal extinction was complete.

Around 41,500 years ago, the Earth's magnetic field weakened in an event known as the Laschamp event. This weakening may have caused increased flux of UV-B radiation and has been suggested by a few authors as a cause of megafaunal extinctions in the Late Quaternary.^[219] The full effects of such events on the biosphere are poorly understood, however these explanations have been criticized as they do not account for the population bottlenecks seen in many megafaunal species and nor is there evidence for extreme radio-isotopic changes during the event. Considering these factors, causation is unlikely.^[220][221]