For hundreds of millions of years, an abundance of large animals, the megafauna, was a prominent feature of the land and oceans. However, in the last few tens of thousands of years—a blink of an eye on many evolutionary and biogeochemical timescales—something dramatic happened to Earth’s ecology; megafauna largely disappeared from vast areas, rendered either actually or functionally extinct. Only in small parts of the world do megafauna exist at diversities anything close to their previous state, and, in many of these remaining regions, they are in a state of functional decline through population depletion and range contraction.

The last ice age peaked around 20,000 years ago. Glaciers covered huge areas of North America, Europe, South America, and Asia. The last ice age was during the Pleistocene epoch which was a geological period starting 2.6 million years ago and ending 11,700 years ago. This epoch saw many glacial and interglacial periods. When the climate was cooler, the glaciers advanced. When it was warmer, they retreated. During the last ice age, global temperatures were about 11 degrees Fahrenheit (or 6 degrees Celsius) cooler than today. This last ice age began 115,000 years ago and ended 11,700 years ago.

With cooler temperatures and a lot of ice cover, there was less precipitation and less rainfall. Snow and ice reflected more of the sun’s rays, only adding to the cold. In addition, sea levels were much lower as more of the oceans were locked up as ice. This resulted in land bridges that had previously been covered by the sea. These bridges allowed species to access islands from the mainland and even cross from one continent to another.