The Cambrian Explosion is one of the most spectacular episodes in Earth history, with the first traces of animals appearing in the fossil record at around 550 million years, and most modern major groups (phyla and classes) present by 510 Ma.
The event occurs after a long period, around one billion years, of limited evolutionary innovation, and immediately postdates the Snowball Earth event - a period of major climate perturbation when ice caps extended from the poles to the palaeo-equator. Furthermore, there are very few phyla and classes that evolve after this time interval. The coincidence of timing has proved irresistible for those in search of a causal mechanism for the origin of animals, and the deglaciation and return to equable climates has been hypothesised as a trigger for this major evolutionary innovation. Recent work in northern Greenland has shed light both on the nature of the Snowball Earth event and on the environmental conditions in which the major period of evolutionary diversification occurred. The Sirius Passet fossil locality on the northern tip of Greenland is the oldest of the global Cambrian localities with exceptional fossil preservation and offers new insights to the early evolutionary history of major animal groups, including the deuterostomes, a group that includes acorn worms, starfish and vertebrates.