Important evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation: why do they happen when they do? What shapes them, and why do they eventually come to an end? However, much less attention has been paid to the idea of a ‘null hypothesis’—that certain features of such diversifications arise simply through their statistical structure. Such statistical features also appear to influence our perception of the timing of these events. Here, we show in particular that study of unusually large clades leads to systematic overestimates of clade ages from some types of molecular clocks, and that the size of this effect may be enough to account for the puzzling mismatches seen between these molecular clocks and the fossil record. Our analysis of the fossil record of the late Ediacaran to Cambrian suggests that it is likely to be recording a true evolutionary radiation of the bilaterians at this time, and that explanations involving various sorts of cryptic origins for the bilaterians do not seem to be necessary.

诸如寒武纪爆炸之类的重要进化事件激发了许多解释的尝试：为什么它们在发生时才发生？它们是什么形状的，为什么它们最终会消失？但是，人们很少关注“零假设”的概念，即这种多样化的某些特征只是通过其统计结构产生的。这样的统计特征似乎也影响了我们对这些事件发生时间的认识。在这里，我们特别表明，对异常大的进化枝的研究导致某些类型的分子钟对进化枝年龄的系统高估，并且这种效应的大小可能足以解决这些分子钟与化石之间令人费解的错配记录。