  We discuss some the effects of local gamma-ray bursts on the earth's
atmosphere. A rough calculation of the fraction of ozone destruction by
catalytic $NO_{x}$ cycles is given, which in turn serves to argue how the large
flux of gammas from these events would have indirectly provoked major
extinction of living organisms. We give specific examples of these features,
and tentatively identify the Cambrian explosion seen in the actual fossil
record as an event caused by a GRB.
