  It is shown that, contrary to an existing claim, the near equality between
the lifetime of the sun and the timescale of biological evolution on earth does
not necessarily imply that extraterrestrial civilizations are exceedingly rare.
Furthermore, on the basis of simple assumptions it is demonstrated that a near
equality between these two timescales may be the most probable relation. A
calculation of the cosmic history of carbon production which is based on the
recently determined history of the star formation rate suggests that the most
likely time for intelligent civilizations to emerge in the universe, was when
the universe was already older then about 10 billion years (for an assumed
current age of about 13 billion years).
