  Gamma-ray bursts (hereafter GRB) produce a flux of radiation detectable
across the observable Universe, and at least some of them are associated with
galaxies. A GRB within our own Ggalaxy could do considerable damage to the
Earth's biosphere; rate estimates suggest that a dangerously near GRB should
occur on average two or more times per billion years. At least five times in
the history of life, the Earth experienced mass extinctions that eliminated a
large percentage of the biota. Many possible causes have been documented, and
GRB may also have contributed. The late Ordovician mass extinction
approximately 440 million years ago may be at least partly the result of a GRB.
A special feature of GRB in terms of terrestrial effects is a nearly impulsive
energy input of order 10 s. Due to expected severe depletion of the ozone
layer, intense solar ultraviolet radiation would result from a nearby GRB, and
some of the patterns of extinction and survivorship at this time may be
attributable to elevated levels of UV radiation reaching the Earth. In addition
a GRB could trigger the global cooling which occurs at the end of the
Ordovician period that follows an interval of relatively warm climate. Intense
rapid cooling and glaciation at that time, previously identified as the
probable cause of this mass extinction, may have resulted from a GRB.
