Peter D. Ward and Donald Brownlee. Rare Earth: Why Complex Life Is So Uncommon in the Universe. Copernicus, 2000.
For anyone who has followed Peter Brookcsmith's series in Fortean Times, on the rarity of aliens, this books deals with many of these arguments in much greater detail. Ward and Brownlee's central argument is that while very simple, single celled organisms may be common, appearing almost anywhere remotely possible, the conditions necessary for the appearance of complex multicellular animals (or animal-like organisms) are much more restrictive and onerous. Complex organisms, let alone technological intelligences, may be very scarce indeed.
The authors examine the various preconditions which may be needed to before animals could develop: These include being in the right place at the right time, (avoid: hyper-energetic galactic centres and metal-poor outer edges, or the metal-poor early universes, or the radioactive-element-poor late universes), not being in a double or multiple start system, having the right kind of star, having the right kind of planetary system, (avoid: inner gas giants, and, as new solar systems keep being discovered these look surprisingly common; have: outer gas giants to sweep up much cosmic debris), have the right planet (right tilt, need for moon sized satellite, not too much or too little water, right climatic history, importance of plate tectonics and so on) and the right kind of evolutionary history.
This is a pretty formidable list or prerequisites, and in effect the argument is that any complex life-bearing worlds would have to be pretty close to quasi-earths, with more or less the same properties and history. Not surprisingly this book looks set to be controversial, violating some pretty established dogmas. In a real sense it is a truly heretical and thus Fortean book, yet, of course, it will not be acclaimed as such by many Forteans, because its message is not one that is popular. The reaction in New Scientist and elsewhere hints at the essentially religious and faith based nature of the CETl argument.
Critics of Ward and Brownlee's have accused them o[ trying to reverse the moves away from anthropomorphism, and not accepting the 'principle of mediocrity'. Well that principle is a philosophical point or even a fashion statement, and not well established science, and equally one can argue that the 'Rare Earth' hypothesis is just a continuation of the disenchantment processes. Behind the CETI dream is the idea that technological species are somehow the goal of the universe and evolution, everything else is just a step on the ladder. What Ward and Brownlee propose is that animals and people are a happy accident in a universe, which if it is designed for anything, is designed for microbes.
To fully understand the implications for the ETH, remember our previous argument, that even given a galaxy filled with millions of rich and diverse biospheres, the chances of any other producing a creature sufficiently physically, psychologically and culturally similar to us to be engaged on our kind of project was remote, if that base is reduced to a few thousand or just a few hundred rich and diverse bio-spheres, then the odds against another similar technology must be very vast indeed.