Of course, we cannot prove that the equivalent of our planet's animal life is rare elsewhere in the Universe. Proof is a rarity in science. Our arguments are post hoc in the sense that we have examined Earth history and then tried to arrive at generalizations from what we have seen here. We are clearly bound by what has been called the Weak Anthropic Principle – that we, as observers in the solar system, have a strong bias in identifying habitats or factors leading to our own existence. To put it another way, it is very difficult to do statistics with an N of 1. But in our defense, we have staked out a position rarely articulated but increasingly accepted by many astrobiologists. We have formulated a null hypothesis, as it were ... that life ... is out there, or that even simple worm-like animals are commonly out there. Perhaps in spite of all the unnumbered stars, we are the only animals, or at least we number among a select few. What has been called the Principle of Mediocrity – the idea that Earth is but one of a myriad or like worlds harboring advanced life – deserves a counterpoint. Hence our book.This could have been part of the introduction to “Privileged Planet.” In fact, it's not – it's from the preface to the first edition of “Rare Earth” by Peter Ward and Donald Brownlee. They put forward the idea that the environment in which earth appears is uniquely adapted to the presence of complex life. (Or at least, that's what I understand – I haven't got into the book yet.)
Gonzalez and Richards, in “Privileged Planet,” do the same, but note additionally that the factors that make the earth well-adapted to life (habitability) also make the earth well-adapted as an observation platform for the rest of the universe. Since there is no apparent obvious scientific reason for this to be the case, they conclude that this is a significant property of the universe.
Many of the observations that are made in PP echo those in RE, and conversely, Ward and Brownlee mention the influence of Gonzalez on their thoughts. And yet, whereas a great deal of fuss has been made by parts of the scientific community about PP (though admittedly not at the level of published papers or books), I am not aware of any such reaction against what Ward and Brownlee wrote, despite its fairly direct rebuttal of the Principle of Mediocrity.