In "The Origin of Species," Darwin describes how black bears in North America often swim "for hours with widely open mouth, thus catching, almost like a whale, insects in the water." Darwin was making a hypothetical point about how evolution might work -- the swimming bear, he suggested, might be the first step in the evolution of whales.
As it happens, although the idea that whales evolved from land animals was a typically farsighted one, it was one of the surprisingly few things that Darwin was wrong about -- whales didn't evolve from bears. This week the story of whale evolution, one of the most remarkable and informative in all of biology, has been definitively told.
After millions of years of gradual adaptation, vertebrates left the oceans. In the millions of years that followed, they lost fishlike features and became specialized for terrestrial life. Some of the most radical changes occurred in mammals, which evolved to become hairy and warmblooded, delivering live young, feeding them with milk and often living in social groups.