History of research The megafaunal extinctions were already recognized as a distinct phenomenon by some scientists in the 19th century: Several decades later in his 1911 book
The World of Life (published 2 years before his death), Wallace revisited the issue of the Pleistocene megafauna extinctions, concluding that the extinctions were at least in part the result of human agency in combination with other factors. Discussion of the topic became more widespread during the 20th century, particularly following the proposal of the "overkill hypothesis" by
Paul Schultz Martin during the 1960s. By the end of the 20th century, two "camps" of researchers had emerged on the topic, one supporting climate change, the other supporting human hunting as the primary cause of the extinctions. 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. 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. 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. 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. A 2020 study published in
Science Advances found that human population size and/or specific human activities, not climate change, caused rapidly rising global mammal extinction rates during the past 126,000 years. Around 96% of all mammalian extinctions over this time period are attributable to human impacts. According to Tobias Andermann, lead author of the study, "these extinctions did not happen continuously and at constant pace. Instead, bursts of extinctions are detected across different continents at times when humans first reached them. More recently, the magnitude of human driven extinctions has picked up the pace again, this time on a global scale." On a related note, the population declines of still extant megafauna during the Pleistocene have also been shown to correlate with human expansion rather than climate change. There is evidence that the average size of mammalian fauna declined over the course of the Quaternary, a phenomenon that was likely linked to disproportionate hunting of large animals by humans. and by computer models by Mosimann and Martin, and Whittington and Dyke, and most recently by Alroy. In 2024 a paper was published in
Science Advances that added additional support to the overkill hypothesis in North America when the skull of an 18 month old child, dated to 12,800 years ago, was analyzed for chemical signatures attributable to both maternal milk and solid food. Specific isotopes of carbon and nitrogen most closely matched those that would have been found in the mammoth genus and secondarily elk or bison. A number of objections have been raised regarding the hunting hypothesis. Notable among them is the sparsity of evidence of human hunting of megafauna. 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. 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. The majority of North American taxa have too sparse a fossil record to accurately assess the frequency of human hunting of them.
Eugene S. Hunn suggests 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. Proponents of hunting as a cause of the extinctions argue that statistical modelling validates that relatively low-level hunting can have significant effect on megafauna populations due to their slow life cycles, and that hunting can cause top-down forcing
trophic cascade events that destabilize ecosystems.
Second-order predation 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. The second-order predation hypothesis has been supported by a computer model, the Pleistocene extinction model (PEM), which, using the same assumptions and values for all variables (herbivore population, herbivore recruitment rates, food needed per human, herbivore hunting rates, etc.) other than those for hunting of predators. It compares the overkill hypothesis (predator hunting = 0) with second-order predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that second-order predation is more consistent with extinction than is overkill (results graph at left). The Pleistocene extinction model is the only test of multiple hypotheses and is the only model to specifically test combination hypotheses by artificially introducing sufficient climate change to cause extinction. When overkill and climate change are combined they balance each other out. Climate change reduces the number of plants, overkill removes animals, therefore fewer plants are eaten. Second-order predation combined with climate change exacerbates the effect of climate change. (results graph at right). The second-order predation hypothesis is further supported by the observation above that there was a massive increase in bison populations. and specific
Paleoindian sites, according to the
Clovis theory However, this hypothesis has been criticised on the grounds that 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. The hypothesis further 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, which detractors cite as evidence against the hypothesis.
Landscape alteration One consequence of the colonisation by humans of lands previously uninhabited by them may have been the introduction of new fire regimes because of extensive fire use by humans.
Competition for water In southeastern Australia, the scarcity of water during the interval in which humans arrived in Australia suggests that human competition with megafauna for precious water sources may have played a role in the extinction of the latter.
Climate change landscape, featuring the
straight-tusked elephant (background right), the
narrow-nosed rhinoceros (far left),
steppe bison (background centre left),
wild horse (background centre) and
aurochs (background centre right). 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. 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. 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. 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. Increased continentality resulted in reduced and less predictable rainfall limiting the availability of plants necessary for energy and nutrition. It has been 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. A study looking at the environmental conditions across Europe, Siberia and the Americas from 25,000 to 10,000 YBP 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. Some evidence from Europe also suggests climatic changes were responsible for extinctions there, as the individuals extinctions tended to occur during times of environmental change and did not correlate particularly well with human migrations. In Beringia, megafauna may have gone extinct because of particularly intense paludification and because the land connection between Eurasia and North America flooded before the Cordilleran Ice Sheet retreated far enough to reopen the corridor between Beringia and the remainder of North America. Woolly mammoths became extirpated from Beringia because of climatic factors, although human activity also played a synergistic role in their decline. In North America, a Radiocarbon-dated Event-Count (REC) modelling study found that megafaunal declines in North America correlated with climatic changes instead of human population expansion. In the North American Great Lakes region, the population declines of mastodons and mammoths have been found to correlate with climatic fluctuations during the Younger Dryas rather than human activity. In the Argentine Pampas, the flooding of vast swathes of the once much larger Pampas grasslands may have played a role in the extinctions of its megafaunal assemblages. 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. 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. In addition, numerous species such as mammoths on
Wrangel Island and
St. Paul Island survived in human-free
refugia despite changes in climate. This would not be expected if climate change were responsible (unless their maritime climates offered some protection against climate change not afforded to coastal populations on the mainland). Under normal ecological assumptions island populations should be more vulnerable to extinction due to climate change because of small populations and an inability to migrate to more favorable climes. 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. 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. 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. as well as on Wrangel Island in the Siberian Arctic. 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. 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. 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. 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. 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. It may be observed that neither the overkill nor the climate change hypotheses can fully explain events:
browsers, mixed feeders and non-ruminant grazer species suffered most, while relatively more
ruminant grazers survived. However, a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by either Second Order Predation (see below) or
anthropogenic fire preferentially selects against browse species.
Other hypotheses Disease 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. 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. 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. 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. 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. 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.
Geomagnetic field weakening 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. 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. == Effects ==