During the Pleistocene, large mammals like mammoths and saber-toothed cats roamed North America. By 11,000 years ago they were extinct. Large mammal extinctions occurred on other continents, including South America and Australia, during the Pleistocene.
When did it happen?
The extinctions in North America began about 12,900 years ago, at the start of a time interval called the Younger Dryas. Extinctions happened at about the same time in South America, but were earlier, about 41,000 years ago, in Australia. The timing and extent of the Pleistocene extinctions varies between continents.
Who became extinct?
Casualties of the extinctions in North America include mammoths, mastodons, camels, saber-toothed cats, giant ground sloths and giant armadillos. In South America, many of the same kinds of mammals disappeared, and Australian extinctions included giant kangaroos and wombats. Small mammals in all continents had much higher survival.
How did it happen?
The roles of climate change and humans in these extinctions have been debated for decades. It is possible that both played a role. Large mammals evolved in isolation from humans in North and South America and Australia. When humans colonized these continents, they decimated animals that were unused to being hunted. There is some archaeological evidence to support this hypothesis, and the fact that the extinctions followed first contact by humans is also consistent with it. Extinctions were much lower in Europe and Africa. There, large animals evolved alongside humans. They would have been used to being hunted and avoided humans.
There is also solid evidence for climatic change at the time of the extinctions in North and South America. The Younger Dryas was a roughly 1,000-year interval of cooling that halted climatic warming that followed the end of the last ice age. As the glaciers retreated, vast lakes of melt-water developed around the edges of the ice cap. The waters of one huge lake, Lake Agassiz, burst through ice-dams and flowed north into the Arctic Ocean and then into the North Atlantic. The inflow of freshwater slowed down ocean current systems and decreased the amount of heat transported north by currents like the Gulf Stream. This led to cooling and dryer climates over much of North America.
A new but controversial hypothesis suggests that explosions following break up of large comet in the Earth's atmosphere, 12,900 years ago, led to the outflow of melt-water from Lake Agassiz and the resulting climate change. The explosions also led to forest fires that destroyed mammal habitats in North America.
Why were the extinctions mostly of large animals?
Large animals have characteristics that make them particularly prone to extinction. They have larger food and habitat requirements than smaller animals, so their population sizes are low. In general, the risk of extinction goes up as population sizes become smaller. Also, large animals tend to have lower reproductive rates, producing fewer young per litter and have longer gestation periods. Once the populations of large animals start to decline, whether due to climate change, over-hunting or some other reason, their low reproductive rates make it more difficult for them to recover.