Evidence for a geologic trigger of the Cambrian explosion

The oceans teemed with life 600 million years ago, but the simple, soft-bodied creatures would have been hardly recognizable as the ancestors of nearly all animals on Earth today. Then something happened. Over several tens of millions of years — a relative blink of an eye in geologic terms — a burst of evolution led to a flurry of diversification and increasing complexity, including the expansion of multicellular organisms and the appearance of the first shells and skeletons. The Great Unconformity is visible in the Grand Canyon at the base of a rock cliff above where the canyon walls slope down to the Colorado River. The flat-lying layered sedimentary rocks of the 525-million-year-old Cambrian Tapeats Sandstone rest on metamorphic rocks of the 1,740-million-year-old Vishnu Schist. (Photo: Jack Share) The results of this Cambrian explosion are well documented in the fossil record, but its cause — why and when it happened, and perhaps why nothing similar has happened since — has been a mystery. New research shows that the answer may lie in a second geological curiosity — a dramatic boundary, known as the Great Unconformity, between ancient igneous and metamorphic rocks and younger sediments.