Complex life did not start in the Cambrian - it was there in the Ediacaran, the period that preceded the Cambrian. And the physical and chemical environment that prevailed in the early to middle Cambrian may well have arisen at earlier times in Earth history. So what exactly was the Cambrian explosion? And what made it happen when it did, between 541 and 530 million years ago? Many explanations have been proposed, but, as Paul Smith explains in the podcast, they tend to rely on single lines of evidence, such as geological, geochemical, or biological. He favors explanations that involve interaction and feedback among processes that stem from multiple disciplines. His own research includes extensive study of a site where Cambrian fossils are exceptionally well preserved in the far north of Greenland. Smith is Director of the Oxford University Museum of Natural History and Professor of Natural History at the University of Oxford.