The Nature of Evolution: Slow and Steady?

As the story of the Cambrian Explosion emerged from the rock record it threw into question Darwin's picture of evolution as a slow and steady process which proceeded by small increments of change.

The sudden appearance of small shelly fauna and a rich trilobite assemblage at the Cambrian boundary only seemed sudden, workers reasoned, because the ancestors of these animals had not yet been discovered.

Photo from Louie Psihoyos' Hunting Dinosaurs, 1994.