Why Did the Woolly Mammoth Go Extinct? Climate, Hunters and the Last Island Herd

The woolly mammoth was a close cousin of the Asian elephant, about the same size, and adapted to the steppe that covered the northern continents during glacial periods: a cold, dry, treeless grassland richer than the tundra that replaced it. It had a coat of coarse outer hair up to a metre long over dense underwool, a thick layer of fat, small ears and a short tail to limit heat loss, and long curved tusks that it used to sweep snow off the grass. Its blood carried a variant of haemoglobin that released oxygen normally at low temperatures. Frozen carcasses from the Siberian permafrost, some with stomach contents intact, have given more detail about it than about any other extinct animal.

The last ice age ended abruptly. Between about 15,000 and 10,000 years ago the climate warmed, became wetter, and lurched through several sharp reversals, and the mammoth steppe, which depended on cold, dry conditions, broke up. Forests spread north across Europe and Siberia and boggy tundra replaced the grassland in the far north, and mammoths, which needed vast quantities of grass, lost their food. Ancient DNA from sediments across the Arctic shows the steppe plants declining and the mammoth populations shrinking with them, and the survival of mammoths on islands, where the steppe vegetation persisted longest, fits.

The difficulty for a pure climate explanation is that mammoths had survived previous warm periods, including one 125,000 years ago that was warmer than today, by retreating north and waiting. Something was different the last time.

The difference was people. Modern humans spread across northern Eurasia between 45,000 and 30,000 years ago and reached the Americas around 15,000 years ago, and everywhere they arrived, large animals began disappearing soon afterwards. Mammoth bones with butchery marks and spear points are found at sites across Europe and Siberia, mammoth ivory was carved and traded, and at several Ukrainian sites huts were built from hundreds of mammoth bones. Elephants breed slowly, one calf every several years, so a population can be pushed into decline by a hunting rate that would hardly affect deer. The argument that humans did it is called the overkill hypothesis, and its strongest evidence is the timing across continents:

• •Australia lost its giant animals around 45,000 years ago, soon after humans arrived and long before the end of the ice age
• •The Americas lost theirs around 13,000 to 11,000 years ago, within a few thousand years of human arrival
• •Islands lost theirs whenever humans arrived, whether that was 10,000 years ago or, in New Zealand and Madagascar, within the last thousand
• •Africa and southern Asia, where large animals had evolved alongside humans for a million years, kept most of theirs

The consensus now is that neither cause was sufficient alone. Climate change fragmented the mammoth's range and shrank its populations into scattered refuges, and human hunting, which the animals had never faced at that scale, finished off populations that in earlier warm periods would have recovered. Computer models that combine the two reproduce the timing of the extinctions on each continent far better than either alone. The mainland mammoths of Siberia were gone by about 10,000 years ago; a population on St Paul Island in the Bering Sea lasted until 5,600 years ago and died when its only freshwater lake shrank; and the Wrangel Island herd, isolated as sea levels rose, lasted to about 4,000 years ago.

The Wrangel mammoths' genomes have been sequenced, and they show an animal in genetic trouble, inbred, with a lost sense of smell and defective coats. What killed the last of them is not known; a bad winter, a disease or the arrival of a boat would all have been enough for a herd of a few hundred.

Because so much mammoth DNA survives in the permafrost, the mammoth has become the test case for de-extinction. A company founded in 2021 aims to edit the genome of the Asian elephant, its closest living relative, to carry the mammoth's cold-adaptation genes and produce a hairy, cold-tolerant elephant within a decade, with the stated goal of restoring grazing to the Arctic. Whether the result would be a mammoth, whether the Arctic wants one, and whether the money would be better spent on the elephants that still exist are all disputed.

Most dates in the mammoth story come from radiocarbon dating of bones, tusks and teeth. Living things take in carbon, including a small, steady share of radioactive carbon-14, which decays at a known rate after death. Measuring what remains gives an age, which is then calibrated against tree rings and other records because the amount of carbon-14 in the air has varied. The method works back to roughly fifty thousand years, which covers the mammoth's final chapter well, but every date comes with a margin of error.

A deeper problem is that the youngest fossil found is almost never the last animal that lived. As a population dwindles, its members become rare, and the chance that one of the last few died where its bones would be preserved and later discovered becomes small. Palaeontologists therefore treat the youngest dated bone as a minimum age for survival and use statistics to estimate how much longer the species may have lasted. Newer methods that extract traces of DNA from ancient sediments can detect mammoths without any bones at all, and some such studies suggest small populations persisted later than the fossils show, a claim still debated.

The woolly mammoth was the last and best known member of a larger group. Mammoths first evolved in Africa several million years ago and spread into Eurasia, giving rise to a series of species adapted to different climates. The steppe mammoth, one of the largest elephants ever, lived in Eurasia before the woolly mammoth appeared. In North America the Columbian mammoth, taller and less shaggy, roamed further south, into what is now the southern United States and Mexico, and genetic studies suggest it was partly descended from interbreeding with woolly mammoths.

Mammoths also show what happens to large animals stranded on islands. On the Channel Islands off California, mammoths that had swum out from the mainland evolved into the pygmy mammoth, standing not much taller than a person. Dwarf elephants appeared in a similar way on Mediterranean islands such as Sicily and Crete. Island dwarfing is common in big mammals, because a small island cannot feed many giants, and smaller bodies need less food. The Wrangel Island herd was once thought to be dwarfed too, but that idea has since been largely set aside.

A mammoth tusk is an incisor tooth that kept growing throughout the animal's life, adding new layers of ivory at its base as the older ivory was pushed outward. Cut along its length, it shows growth bands rather like the rings of a tree, and each band holds chemical traces of the food the animal ate and the water it drank at the time. Elements such as strontium vary with the underlying rock of a landscape, so by matching the chemistry of each layer to maps of local geology, researchers can follow where an individual animal travelled, season by season, through its whole life.

In 2021 a team published exactly such a life history for a male woolly mammoth that died in Alaska about 17,000 years ago. Over its 28 years it had moved widely across the region, covering a total distance roughly equal to going nearly twice around the Earth. The chemistry showed its range changing as it matured, as young male elephants change their habits when they leave the herd, and in its final months a shift in nitrogen isotopes that is typical of starvation, in the far north of Alaska where it died. A single tusk can therefore show how one mammoth lived, moved and coped with hard times.

Woolly mammoths went extinct because the end of the last ice age destroyed the cold, dry grassland they depended on, and because modern humans, spreading across their range at the same time, hunted a slow-breeding animal that had always previously recovered from warm periods by retreating north. The two together explain the timing on every continent. The mainland populations were gone by 10,000 years ago and the last herd, on Wrangel Island, survived until about 4,000 years ago.