Gamma ray bursts (GRBs) are the most violent and energetic explosions in the Universe. This has led to speculation that they may be responsible for at least some of the mass extinction events witnessed here on Earth.
Due to the short duration of GRBs, most astronomers believe that the damage to the Earth if it were located in the beam of a nearby GRB would be limited. Calculations have suggested that perhaps the most serious effect of a 10 second burst located within our own Milky Way galaxy would be the destruction of at least half of the ozone layer, a situation from which it would take several years to recover. A damaged ozone layer would lead to an increase in the amount of ultraviolet radiation reaching the Earth, and could disrupt the food chain and kill off much of the surface (and near-surface) life on Earth. The result would almost certainly be a mass extinction event.
Trying to link a GRB to a specific mass extinction event in the Earth's past is a highly speculative endeavour, especially since the rate of GRBs in the local Universe is unknown. However, scientists believe that the Earth may have been hit by a GRB in the last billion years and have suggested the Ordovician-Silurian extinction event of 450 million years ago as the mass extinction most likely to have been caused by such a scenario.