Stand atop the steep white cliffs that surround the giant rivers of Siberia, and your feet will mark a pivotal point in the history of life on Earth: the 541-million-year-old geologic boundary between the Precambrian and Cambrian periods. The rocks below this dividing line contain scant fossil remains—ghostly impressions of soft-bodied organisms and a smattering of shelly forms. But break open any of the rocks just above the boundary, and they will be teeming with shells. A little higher up still, familiar fossil creatures such as trilobites appear. These changes document the so-called Cambrian explosion, one of the most significant, but still poorly understood, events in all of evolution.