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Re: Impacts and ETs

  One reason for the rare Earth notion is the supposed
need for a stabilizing moon. Dinosaurian history
suggests otherwise. During the Mesozoic, the whole
world was warm. Since solar declination had relatively
little effect on temperature, obliquity shifts
probably wouldn't have either.

  I suggest we put off any conclusions regarding the
frequency of ET intelligence in the Galaxy until after
he Kepler and Terrestrial Planet Finder missions.

--- David Marjanovic <david.marjanovic@gmx.at> wrote:

> > Read paleontologist Peter Ward's book, "Rare
> Earth".  Some of your
> > questions are addressed (although not necessarily
> answered) there.
> Peter D. Ward & Donald Brownlee: Rare Earth. Why
> Complex Life Is Uncommon in 
> the Universe, Copernicus/Springer 2000
> Quotes from near the beginning...
> "Maybe we really are alone."
> "To put it another way, it is very difficult to do
> statistics with an _N_ of 
> 1."
> "Perhaps in spite of all the unnumbered stars, we
> are the only animals, or 
> at least we number among a select few."
> "The idea of a million civilizations of intelligient
> creatures in our galaxy 
> [Sagan, 1974] is a breathtaking concept. But is it
> credible? The solution to 
> the Drake Equation includes hidden assumptions that
> need to be examined. 
> Most important, it assumes that once life originates
> on a planet, it evolves 
> toward ever higher complexity, culminating on many
> plaets in the development 
> of culture. That is certainly what happened on our
> Earth. Life originated 
> here about 4 billion years ago and then evolved from
> single-celled organisms 
> to multicellular creatures with tissues and organs,
> climaxing in animals and 
> higher plants. Is this particular history of life --
> one of increasing 
> complexity to an animal grade of evolution -- an
> inevitable result of 
> evolution, or even a common one? Might it, in fact,
> be a very rare result?
>         In this book we will argue that not only
> intelligent life, but even 
> the simplest of animal life, is exceedingly rare in
> our galaxy and in the 
> Universe. We are not saying that _life_ is rare --
> only that _animal_ life 
> is. We believe that life in the form of microbes or
> their equivalents is 
> very common in the universe, perhaps more common
> than even Drake and Sagan 
> envisioned. However, _complex_ life -- animals and
> higher plants -- is 
> likely to be far more rare than is commonly assumed.
> We combine these two 
> predictions of the commonness of simple life and the
> rarity of complex life 
> into what we will call the Rare Earth Hypothesis."
> "What if the Earth, with its cargo of advanced
> animals, is virtually unique 
> in this quadrant of the galaxy -- the most diverse
> planet, say, in the 
> nearest 10,000 light-years? What if it is utterly
> unique: the only planet 
> with animals in this galaxy or even in the visible
> Universe, a bastion of 
> animals amid a sea of microbe-infested worlds?"
> Read the rest yourself. :-) 

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