One possible solution now seems to be planet orbit evolution. We're finding
Jupiter sized planets, but they're all in funny orbits - very near the sun or
elliptical. Current theory holds that big planets can only form far from the
sun. Hence all these Jovians had to wander through the habitable zones on the
way to their current orbit, gravitationally slingshotting any potential lifebearing
planets into oblivion.
(This info is from a friend's FW of a NYT online article)
The solar system is fairly precisely set up to avoid this kind of thing.
Perhaps
this resulted not from stabilizing interactions during planet formation but
from
incredible blind luck.
Robin's "Great Filter" conception seems not to have reached broad public
recognition. The researchers were still viewing rarer life as a bad thing and
seem not to have considered the possibility that rarer intelligent life =
longer
plausible survival for intelligent life.