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. | 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. | 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. | 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. | The collision between Earth and a large asteroid resulted in massive damage and generated enough heat to cause irreversible changes in Earth's atmosphere. | 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. |
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