Advances in space exploration and space science have led to the realization that bombardment is a cosmic and geological process that has affected the Earth throughout its history. The remains of craters formed by the impact of asteroids, meteoroids, and comets scar the Earth's surface. Over 170 craters and geological structures discovered so far show strong signs of an impact origin. Impacting bodies not only excavate craters, they also set in train a sequence of events that causes changes in climate, ecosystems, and the crust and mantle. Thus, the bombardment hypothesis has an enormous impact on the Earth and life sciences. Bombardment is a regular cosmic and geological process that has operated throughout the entire course of Earth history. However, the bombardment hypothesis has had a chequered career and has ignited heated debates. This chapter will explore some of these debates by discussing the history of the bombardment hypothesis from its hesitant beginnings to the final acceptance of cosmic catastrophism.
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