Over the last 2.6 million years of Earth's history, glaciers have come and gone, sometimes covering the northern parts of the world with ice for thousands of years at a time. Each time the climate has changed, individual species, including humans, adapted to different climates, hot or cold. And when one species died out, others evolved to take its place.
When the Earth was warming up from a frozen time called the last glacial maximum, the story should have been just the same. But it wasn't. Dozens of large mammals were alive and well all over the world when the warming started. Some of them had existed for tens of millions of years. Yet, between about 13,000 years ago and about 8,000 years ago, most of the biggest of them…
