QUESTION:
Of the three inner planets, the Earth is the only one with a substantial
moon.  Would life on Venus and Mars have had a better chance to begin and
continue if those other worlds had moons similar to ours?

ANSWER from Jack Farmer on August 6, 1997:
This is a good question, and unfortunately, one that we really don't have a
good answer for. While it is true that the moon probably sheltered the Earth
to some degree during heavy bombardment, it is not clear what effect this
would have had on the origin of life. To a large extent, that depends on how
fast life originates. Under some scenarios, life is envisioned as having
developed under heavy bombardment scenario similar to the moon, and even
with the possibility of one or more life extinguishing events (giant
impacts). Having a less intense bombardment history only makes the origin
and persistence of life easier. The suggestion still persists, however,
based on the rooting of the RNA-tree in extreme thermophily, that at least
one late giant impact probably forced the early biosphere through a high
temperature bottle-neck.

The problem with trying to extrapolate to Mars and Venus is knowing what the
impact fluxes were in different parts of the early solar system. Some
studies have suggested impact rates were comparable for the inner planets.
Assuming equal rates, some have suggested that it may have actually been
easier to originate life on Mars because of fewer large impacts (smaller
gravitational field) and more smaller impacts reaching the surface because
of a more attenuated atmosphere, (depth of gardening of the surface would
have been shallower and clement subsurface environments would have been less
affected). The real problem is that we lack the constraints needed to really
answer this question.