No precedent Human activities are creating a mass extinction event. The intensity of this event is unprecedented during human times, but there have been several comparable events during Earth's history. Roopnarine and Angielczyk examined one of the largest, the Permian-Triassic Extinction (see the Perspective by Marshall). The structure and diversity of communities were key predictors of stability through the event. Furthermore, extinctions were not random, with smaller-bodied species being more prone to extinction. This pattern is in direct contradiction to the patterns seen in our current extinction. Thus, the current anthropogenically driven extinction is fundamentally different from previous catastrophic extinctions. Science , this issue p. 90 ; see also p. 38