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Just Finished: In The Blink Of An Eye: How Vision Sparked the Big Bang of Evolution.

Five hundred forty-three million years ago, an event occurred that we call the Cambrian explosion, in which thousands of new species appeared overnight. Before this event, the seas were full of slow-moving, soft-bodied forms such as worms and jellyfish. After this event, animals with hard parts, such as shells, spines, and exoskeletons existed. What caused this explosion?

The author suggests that the emergence of vision in the trilobites' immediate (soft-bodied) ancestors brought forth the world's first fast moving predators. In turn, the presence of fast-moving predators caused new survival traits to be selected for in their prey, namely defenses such as shells, spines, and exoskeletons. This also gave rise to the hard-bodied trilobites, which ruled the seas of the Cambrian era much as the fishes do today.

It sounds plausible. However, one big question remains. Sight has been independently developed many times since the Cambrian explosion. The earliest mollusks and chordates were blind. These phyla independently evolved vision on their own. But why didn't vision evolve before 543 million years ago? The author speculates that perhaps the ocean or atmosphere transmitted insufficient light prior to this date, and suggests this is a question for other scientists to answer.