Over 80,000 years ago, somewhere on a southern California beach, a mammoth wanders across the sand. The beast doesn’t stop at the water’s edge. Step by step, its trunk held high, the towering proboscidean walks into the waves until its feet no longer touch the bottom. One, two, three, kick, the elephant keeps paddling, the air-filled sinuses of its great skull helping to keep the mammoth’s head near the surface as it moves further and further out from the coastline. It has a long swim ahead. In the distance, rising from the surface 12 miles to the west, are a small set of islands. That’s where the mammoth will again emerge onto dry land.

We know that such an event must