I just stumbled across this interesting theory from Andrew Parker. Andrew is a biologist exploring the Precambrian explosion, and his personal explanation for the phenomenal growth in diversity and complexity in the Precambrian era is that it was motivated by the emergence of the eye as an evolutionary adaptation. In his Light Switch Theory he argues that the evolution of the eye marked a massive step forwards in the evolutionary arms race that shifted the emphasis from internal to external evolution. This led to all the predominantly soft-bodied creatures that were prevalent at the time evolving harder bodies. The evolution of eyes switched on the light for predatory species, making them vastly more efficient at predation and creating the evolutionary bottleneck for diversification.