Quaternary Megafauna Extinction: Why the Ice Age Giants Vanished

The Quaternary megafauna extinction was the disappearance of many of the world’s largest land animals, mostly between about 50,000 and 10,000 years ago, near the end of the last Ice Age. Woolly mammoths, giant ground sloths, sabre-toothed cats, Australia’s giant marsupials and many others vanished, along with roughly two-thirds of the world’s genera of large mammals. Scientists have debated for more than a century whether human hunters, climate change or both were responsible.

Background: a world of giants

The Quaternary period began about 2.6 million years ago and includes the Pleistocene epoch, with its repeated ice ages, and the Holocene, the warmer period of the last 11,700 years. For most of the Pleistocene, every continent except Antarctica supported many species of very large animals.

North America had mammoths, mastodons, camels, horses, giant beavers and the giant short-faced bear. South America had ground sloths the size of elephants, armoured glyptodonts and the gomphotheres, elephant relatives. Europe and northern Asia had woolly mammoths, woolly rhinoceroses, cave bears, cave lions and the giant deer. Australia had Diprotodon, giant kangaroos and the marsupial lion.

Scientists usually define megafauna as animals weighing more than about 44 kilograms (100 pounds) as adults. A 2004 review led by Anthony Barnosky estimated that around 90 of some 150 genera of such mammals worldwide died out in the late Quaternary.

A pattern across continents

The extinctions did not happen everywhere at once. Their timing and severity varied greatly, and this pattern is central to the debate.

Figures vary between studies and depend on how species are classified and dated.

Australia

Australia lost almost all its largest animals, including Diprotodon, the giant flightless bird Genyornis and the marsupial lion Thylacoleo. A 2001 dating study led by Richard Roberts suggested that many of these extinctions happened around 46,000 years ago, after the arrival of the first people, although some researchers argue for a longer, more gradual decline.

Northern Eurasia

In Europe and northern Asia, losses were staggered over tens of thousands of years. The cave bear, cave lion and woolly rhinoceros disappeared at different times, and the woolly mammoth lasted on remote Wrangel Island until about 4,000 years ago. The giant deer survived in western Siberia until about 7,700 years ago.

The Americas

The Americas suffered one of the most sudden waves. Mammoths, mastodons, horses, camels, ground sloths, glyptodonts and sabre-toothed cats all disappeared between roughly 13,000 and 10,000 years ago. This is close to the time when humans spread across both continents, including the hunters of the Clovis culture, although people were present at some sites, such as Monte Verde in Chile, by about 14,500 years ago.

Islands and Africa

On many islands, large animals survived until much later, disappearing only when people arrived. Madagascar’s giant lemurs and elephant birds vanished within the last 2,000 years, and New Zealand’s moa were wiped out within a few centuries of human settlement around AD 1300. Africa and southern Asia, where humans and their ancestors had lived for a very long time, kept far more of their megafauna, which is why elephants, rhinoceroses and hippopotamuses still survive there.

The case for human hunting

The idea that people caused the extinctions is known as the overkill hypothesis. It was developed from the 1960s by the American geoscientist Paul S. Martin, most influentially in a 1967 essay titled “Prehistoric Overkill”, who noted that extinctions on each continent seemed to follow soon after human arrival. In its boldest form, the “blitzkrieg” model, he proposed that hunters entering the Americas killed off the megafauna within a few centuries.

Supporters point to several lines of evidence. Large animals reproduce slowly, so even modest hunting can push populations into decline. Extinctions affected big animals much more than small ones, a pattern that climate alone does not easily explain. In North America, archaeologists have found mammoth and mastodon remains associated with stone spear points and butchery marks at a number of sites. And on islands, extinctions repeatedly followed human settlement, even long after the end of the Ice Age.

Critics reply that direct kill sites are rare, especially in Australia, and that some species began declining before humans arrived. Others note that the world’s climate was also changing dramatically during the same period.

Computer simulations have been used to test the idea. Several models suggest that small numbers of hunters, taking only a modest share of animals each year, could have driven slow-breeding species such as mammoths and ground sloths to extinction within centuries to a few thousand years. The results depend heavily on the assumptions used, including human population size and how readily animals learned to avoid people, so they show what was possible rather than proving what happened.

The case for climate change

The last glacial period saw sharp climate swings. The Last Glacial Maximum, about 26,000 to 19,000 years ago, was followed by rapid warming, a cold reversal called the Younger Dryas (about 12,900 to 11,700 years ago) and then the warmer Holocene.

These changes transformed habitats. The cold, dry “mammoth steppe” of northern Eurasia and Alaska, a grassland rich in herbs, gave way to wet tundra, bogs and forest. Animals adapted to open, grassy landscapes lost much of their range, and their populations became smaller and more fragmented.

Climate clearly mattered for some species, such as the giant deer in Ireland, which disappeared during the Younger Dryas when there is little evidence of people on the island. But the megafauna had survived many earlier warm periods during the Pleistocene without comparable extinctions, which suggests that something about the last one, perhaps the presence of humans, was different.

Other ideas and today’s consensus

Other explanations have been proposed. The “hyperdisease” hypothesis, put forward in 1997, suggested that people or their animals spread deadly infections, but no such disease has been identified. The Younger Dryas impact hypothesis, published in 2007, proposed that a comet or asteroid caused sudden cooling and extinctions about 12,900 years ago; it has been strongly disputed and is rejected by many specialists.

Today most researchers accept that both humans and climate played a part, with their relative importance varying from region to region. In Australia and the Americas, many argue that human impact was decisive. In northern Eurasia, climate change seems to have played a larger role, with people adding pressure on populations that were already shrinking.

Consequences

The loss of the megafauna reshaped ecosystems. Large herbivores had grazed and trampled vegetation, spread seeds and recycled nutrients across vast distances. Without them, some grasslands became forest or shrubland, fires may have become more frequent in places, and the predators and scavengers that depended on giant prey also disappeared.

Some plants may still bear traces of their lost partners. Ecologists Daniel Janzen and Paul Martin suggested in 1982 that some tropical fruits evolved to be eaten by large animals that no longer exist; later writers have applied the idea to fruits such as the avocado and the Osage orange. For humans, the extinctions removed important game animals and may have contributed to shifts in hunting and, eventually, to new ways of life.

Discovery and debate

The idea that whole species could vanish was itself established through Ice Age animals. In 1796 the French naturalist Georges Cuvier showed that mammoth bones belonged to an extinct species. During the voyage of HMS Beagle in the 1830s, Charles Darwin collected the remains of giant ground sloths and other extinct mammals in South America and puzzled over why they had disappeared.

In The Geographical Distribution of Animals (1876), Alfred Russel Wallace wrote that we live in a zoologically impoverished world, from which “all the hugest, and fiercest, and strangest forms have recently disappeared”. Much of the modern debate began with Paul Martin’s work in the 1960s and continues today with improved radiocarbon dating, ancient DNA and studies of preserved dung fungus spores that track how many large herbivores were present over time.

Timeline

• c. 50,000 to 40,000 years ago: Most Australian megafauna disappear after humans arrive.
• c. 26,000 to 19,000 years ago: Last Glacial Maximum.
• c. 13,000 to 10,000 years ago: Mammoths, mastodons, horses, ground sloths and sabre-toothed cats vanish from the Americas.
• c. 12,900 to 11,700 years ago: The Younger Dryas cold period.
• c. 7,700 years ago: The last giant deer die out in western Siberia.
• c. 4,000 years ago: The last woolly mammoths die out on Wrangel Island.
• c. AD 1300 to 1450: New Zealand’s moa are hunted to extinction.
• 1796: Georges Cuvier demonstrates that the mammoth is extinct.
• 1876: Alfred Russel Wallace describes the “zoologically impoverished” modern world.
• 1967: Paul S. Martin sets out the Pleistocene overkill hypothesis.

Why it matters

The Quaternary megafauna extinction created the world we live in, a world with far fewer giant animals than the one in which our species evolved. Understanding its causes matters today too, because it is one of the earliest examples of how humans and climate change together can drive species to extinction.