According to one prominent extinction theory, a rapidly warming climate some 14,000 years ago transformed the open, grassy areas used by mammoths and other large grazers into less productive shrubland. Without enough food, these herbivores disappeared, followed by the predators that hunted them.

Other researchers posit that it was actually a period of abrupt cooling from around 12,900 years ago to 11,700 years ago—and not the warm period that preceded it—that initiated these animals’ demise. Mathew Stewart, a zooarchaeologist at Griffith University in Australia, writes in an email that dozens of studies, including a 2021 paper that he co-authored, have pointed to terminal cooling and habitat change as the main driver of the megafauna extinctions.

Whether they blame cooling or warming or both, proponents of the climate change theory emphasize the dearth of megafauna kill sites in the archeological record. In addition, they point out that megafauna in some places appear to have disappeared before the arrival of humans and that some species clearly not targeted by humans, such as a spruce tree and a snake, also went extinct at the end of the last Ice Age.

First popularized in the 1960s by University of Arizona paleoecologist Paul Martin, the so-called overkill hypothesis theorizes that the earliest Americans hunted these animals to death (or at least hunted the big herbivores to death, leaving the big carnivores to starve).

Large animals reproduce slowly and require a lot of food and space. Moreover, North America’s megafauna may have been unafraid of the human newcomers, to their own detriment. “The fact is you don’t have to hunt down every living mammoth to cause an extinction,” Lindsey says. “You just have to hunt slightly more than are being born each year.”

Proponents of the overkill hypothesis point out that whenever humans arrived anywhere in the world, be it Australia, Hawaii, Madagascar, South America or North America, extinctions inevitably followed. Climate change couldn’t be the main factor, they argue, because North America’s mammalian behemoths had already survived several other ice ages without dying off. A 2023 paper found “no support for an extinction driven primarily or even secondarily by climate.”

“Humans definitely had something to do with the extinctions,” Lindsey says. She notes, however, that widescale slaughter may be an oversimplified explanation. “There are other very significant impacts humans can have on a fauna or an ecosystem other than just hunting and eating animals,” Lindsey says.

In 2023, Lindsey, Dunn and several colleagues published a paper claiming that human-ignited fires helped trigger vegetation changes in southern California that ultimately wiped out saber-toothed cats, dire wolves, wild horses and other megafauna from the area prior to their extinction elsewhere. The paper stresses that California faces similar conditions today, with megafires, rapid warming and drought.

As in the Ice Age, Dunn says, large animals and island species are currently most under threat from humans.

It has been suggested that a comet, asteroid or meteor struck Earth 12,900 years ago and caused the global cooling period that killed off the Ice Age mammals, much as an extraterrestrial impact likely vanquished the dinosaurs some 66 million years ago. However, Lindsey calls this theory “widely discredited,” pointing out that it doesn’t explain why megafauna on different continents went extinct at different times.

In 1997, two researchers proposed that, rather than over hunting or destroying too much habitat, early Americans must have accidentally introduced a “hyperdisease” that killed off most of the Ice Age’s large animals. Disease was certainly present. In 2023, for example, scientists found evidence of skeletal disease in the remains of saber-toothed cats and dire wolves. Nonetheless, most experts doubt that even an extremely virulent virus could have eliminated so many species at once.

Lindsey says it isn’t credible “that there would be a disease that would transfer across different families of animals, but then leave some animals alone within a family or even within a genus.”

Stewart is likewise dismissive. “Alternative hypotheses such as hyper disease or an extraterrestrial impact hypothesis have either been disproven and largely forgotten or remain fringe ideas pushed by select people outside mainstream academia,” he writes. “Climate change/habitat change and human activities/overhunting, or some combination of these factors, remain the main hypotheses.”

Increasingly, Dunn says, researchers believe it was probably a mix of climate change and humans that did in the Ice Age megafauna. “I think the human brain likes to simplify and just have one cause,” she says. “And that’s not how things are in reality. Everything in ecology is really complex.”

For now, the mystery remains unsolved, in part because outside of certain places, like La Brea Tar Pits, too few high-quality remains of Ice Age megafauna have been found. There’s also little information on the Clovis people, who were known to hunt big game at the end of the last Ice Age, and even less information on pre-Clovis Americans.

Stewart says he’s “not against the idea” that humans played a key role in wiping out the megafauna. “However, the fossil record is too patchy and too poorly dated for most places around the globe to adequately test this,” he writes.