  An astrophysical model is proposed to answer Fermi's question. Gamma-ray
bursts have the correct rates of occurrence and plausibly the correct
energetics to have consequences for the evolution of life on a galactic scale.
If one assumes that they are in fact lethal to land based life throughout the
galaxy, one has a mechanism that prevents the rise of intelligence until the
mean time between bursts is comparable to the timescale for the evolution of
intelligence. Astrophysically plausible models suggest the present mean time
between bursts to be $\sim 10^8$ years, and evolutionarily plausible models
suggest the rise of intelligence takes $\sim 10^8$. Hence, this model suggests
that the Galaxy is currently undergoing a phase transition between an
equilibrium state devoid of intelligent life to a different equilibrium state
where it is full of intelligent life.
