There have been many attempts to explain the causes of mass extinctions. A. Asteroid impacts, evolutionary developments, and changes in Earth's climate and in the positions of the continents have all been proposed as possible causes of mass extinctions. B. Researchers have observed 26-million-year cycles in extinction rates of a number of fossil groups that could all be attributed to the same cause. C. According to the Alvarez hypothesis, much of the iridium originally present on Earth was thrown into the atmosphere as a result of an asteroid impact that also caused a mass extinction. D. The unusual distribution of iridium on Earth and the presence of craters and heat-shocked quartz are central to the theory that an asteroid impact caused the late Cretaceous event. E. The collision between Earth and a large asteroid resulted in massive damage and generated enough heat to cause irreversible changes in Earth's atmosphere. F. There was a particularly large mass extinction that occurred around 250 million years ago at the end of the Permian period, whose cause could not be determined.