The origin and evolution of animals have remained hotly debated issues ever since Darwin drew attention to the relative paucity of fossils from the Precambrian, which ended 543 million years ago (Mya). On the one hand, a growing collection of exquisitely preserved fossils of soft-bodied animals from the Cambrian has highlighted the existence of Cambrian representatives of most of the living animal phyla. This has given rise to the "Cambrian Explosion" hypothesis that most animal phyla arose ～543 Mya within a short period (see the figure). On the other hand, studies of increasingly large molecular data sets suggest that many of the same phyla arose in the Precambrian (4—6), leading to considerable and occasionally intemperate debate on the timing of divergence of animal phyla. The article by Rokas et al. on page 1933 in this issue presents a new aspect to this controversy.
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