  The aim of the present work is to study the potential short-term atmospheric
and biospheric influence of Gamma Ray Bursts on the Earth. We focus in the
ultraviolet flash at the planet's surface, which occurs as a result of the
retransmission of the $\gamma$ radiation through the atmosphere. This would be
the only important short-term effect on life. We mostly consider Archean and
Proterozoic eons, and for completeness we also comment on the Phanerozoic.
Therefore, in our study we consider atmospheres with oxygen levels ranging from
$10^{-5}$ to 1% of the present atmospheric level, representing different
moments in the oxygen rise history. Ecological consequences and some strategies
to estimate their importance are outlined.
