Framing causal questions about the past: The Cambrian explosion as case study.

About 540 million years ago, a rapid radiation of animal phyla radically changed the Earth's biota in a geological eye-blink. What caused this "Cambrian explosion"? Over the years, paleontologists have pointed to a wide array of different physical mechanisms as the causal "trigger" for the explosion. More recently, some paleontologists have proposed complex causal pathways to which multiple physical mechanisms are said to have contributed. Despite their variety, these answers share an assumption that a single explanation can in principle be constructed that identifies some factor or confluence of factors as the cause of the Cambrian explosion. That assumption is unjustifiable. The Cambrian explosion had multiple causes, and different aspects of the event are best explained by different causes. These different causes cannot, even in principle, be integrated into a single causal explanation. We can learn much about the causes of the Cambrian explosion-or for that matter about any historical event-but only by attending more carefully to how we frame our causal questions about the past.