ice age

"Historically, this migration has been going on since the ice age," he said.

Meanwhile experts have pointed to important biological differences between the wolf on the cover of Time and the dire wolf that roamed and hunted during the last ice age.

Their research sheds new light on both the rapid expansion of humans throughout the Americas and the extinction of large ice age mammals.

Their extinction coincided with the end of the last ice age, a time when the planet quickly warmed and glaciers retreated northward.

Researchers believe they have found possible evidence that polar bears lived in Scotland during the last ice age.