The abrupt appearance of animal fossils at the end of the Precambrian can be explained in two ways—either as the initial explosion of animal life, or as the explosive appearance of fossilisable animals. Evidence from the oxygen‐binding proteins of living animals supports the latter view, since it points to a billion year history for the lower invertebrate phyla. It was the rise of large, muscular and/or mineralised animals that caused the Cambrian explosion, and the best explanation for their appearance seems to be a sharp increase in the oxygen content of the atmosphere.