And in the thousands of subdivisions of the chordates, high intelligence developed in only one, the primates, and even there only in one small subdivision. Sagan applies physicalist thinking to this problem. He constructs two linear curves, both based on strictly deterministic thinking. Such thinking is often quite legitimate for physical phenomena, but is quite inappropriate for evolutionary events or social processes such as the origin of civilizations. Also, the case of Earth shows how impossible the origin of any civilization is unless high intelligence develops first.
Earth furthermore shows that civilizations inevitably are short-lived.
It is only a matter of common sense that the existence of extraterrestrial intelligence cannot be established by a priori arguments. But this does not justify SETI projects, since it can be shown that the success of an observational program is so totally improbable that it can, for all practical purposes, be considered zero. All in all, I do not have the impression that Sagan's rebuttal has weakened in any way the force of my arguments.