The one-eyed man is king.

An Australian biologist has come up with a theory that the Cambrian explosion was a rapidly-escalating arms race catalyzed by the development of the first creature with vision.

Without giving it a lot of thought, it seems plausible, and intriguing. As he says, imagine all of these creatures evolving in a darkened room, when all of a sudden, someone flicks the light switch. It would be a radical and sudden change in the environment and evolutionary pressures.