Scientists since Darwin have known that whales are mammals whose ancestors walked on land. But one critical step was missing: The identity of the land ancestors of whales. Researchers have now discovered the skeleton of a 48-million-year-old mammal called an Indohyus. It is a fox-sized mammal that looked something like a miniature deer and is the closest known fossil relative of whales. Because Indohyus itself is not a whale, but a close cousin, the discovery suggests that the first whales were themselves aquatic, rather than evolving aquatic habits after they took to the water.