  We use a statistical model to investigate the detectability (defined by the
requirement that they are in causal contact with us) of communicating
civilizations within a volume of the universe surrounding our location. If the
civilizations are located in our Galaxy, the detectability requirement imposes
a strict constraint on their epoch of appearance and their communicating
lifespan. This, in turn, implies that the fraction of civilizations of which we
can find any empirical evidence strongly depends on the specific features of
their temporal distribution. Our approach shed light on aspects of the problem
that can escape the standard treatment based on the Drake equation. Therefore,
it might provide the appropriate framework for future studies dealing with the
evolutionary aspects of the search for extraterrestrial intelligence (SETI).
