Jonathan Silverlight wrote:

In his story "Wrong Way Street" Larry Niven wrote that the Moon helped there, by stripping off most of the Earth's atmosphere. Was that ever a serious theory? I'd guess it's been discarded in favour of the idea that Venus has a thick atmosphere because it's closer to the Sun and hence hotter.
And would the Earth still have a thick atmosphere if it hadn't been blown off by the collision that formed the Moon?

I had heard that the difference is that the Earth has oceans of water that ate most of the
carbon dioxide out of the atmosphere.  Making limestone out of it. Venus either never had,
or lost most of its water, thus this didn't happen there. 
It may be that the Moon wasn't that essential for multicullar life on Earth.  The Sun also
makes tides, though smaller.  Idea being that tides help the evolution of seashore life
that doesn't mind a few hours being high and dry.    If so, than the odds of finding
ET should improve some, though in my previous ramblings in this thread the
odds still seem pretty long.  SETI hasn't found anything, and there's no credable
evidence of UFOs and such.  It's not like the european explorers of old finding
people living on every sizable land location (except Antartica) on Earth.  All those
people had a common ancester from hundreds of thousands of years ago.   They
had Kon Tiki style rafts and such to get to new locations.  But man has never
gotten outside Earth's gravity field yet (the Moon is still inside that, and there's no
Moon camps right now). 
Oh, there's likely ET out there somewhere, but it's likely that we have to go to
another galaxy to find them.  Man may well have spread thru the Milky Way
Galaxy in a hundred thousand years, and that's a pretty short time in the
history of the universe.   If  ET is elsewhere in the Milky Way, he should
have shown up here by now.....