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Re: Impacts and ETs

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. :-)