"Thus, some studies support an episodic or punctuational view of life as a whole (as opposed to punctuated equilibrium, which considers change associated with speciation in just one lineage at a time) and emphasize the importance of the rare crises, as did Sepkoski and Raup. Detailed evidence does not necessarily favor a regular or periodic history of evolution, however, and certainly not one divided into simple 26-million-year increments.
"Overall, scientists have found little evidence that extraterrestrial impacts coincided with most of the major and minor extinctions, the Cretaceous-Tertiary boundary being a major exception. There are a few other hints of other impacts associated with extinctions in the geologic record, but most of these are poorly substantiated or very localized and there are many extinction events that do not correlate with any evidence of impact. Likewise, there are many records of impact (craters, for instance) that seem to have produced little or no biotic effect. So the search for a single common cause to all biological crises has come up rather empty."