Peter Ward and Donald Brownlee's "Rare Earth"

Peter Ward and Donald Brownlee, Rare Earth: Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000. Pp. 333, $27.50.

The question of whether life, especially intelligent life, is common in the universe is one that has fascinated authors and readers for many years. Carl Sagan, who was always optimistic about both these possibilities, argued that humans were not alone in the universe. He could not, of course, prove many of his points, but he presented his opinions with great conviction and with a considerable degree of literary flair.

Now a new book has come along that takes issue with many of these conclusions. In Rare Earth: Why Complex Life Is Uncommon in the Universe, Peter Ward and Donald Brownlee of the University of Washington argue that complex life--that is, multicellular eukaryotic plants and animals--may be rare. Not life itself--Ward and Brownlee stop short of suggesting that bacterial life is uncommon--but according to their analysis, anything one could see with the naked eye and, by implication, anything that could think is unlikely. For a generation that grew up watching "Star Trek," this is a rather disappointing conclusion.

Is the outlook for complex life really as bleak as Ward and Brownlee suggest? The arguments against complex life fall into two basic categories: those based on the nature of the physical universe (and of habitable planets in particular), and those based on the inherent difficulty of biological evolution. The most interesting parts of Rare Earth deal with issues of the first type. The problems in understanding biological evolution, while far from trivial, have been debated for many years. In particular, the extreme complexity of the eukaryotic cell, and the even greater complexity of the multicellular organisms that are constructed from it, continue to amaze most scientists, including this reviewer. But this same degree of amazement can be (and has been) expressed at the complexity of life itself. Even "simple" single-celled bacteria are extraordinarily complicated at the molecular level. We still do not have a satisfactory answer to the question of complexity. If complexity develops naturally out of disorder, then both simple and complex life may be equally common. If, however, complexity itself is hard to come by, then even simple life may be rare. Ward and Brownlee address this question but do not shed much new light on it. We simply do not know the answer at this time.

Rare Earth is most interesting in its discussion of the physical factors that may affect the habitability of planets and the distribution of habitable planets in the galaxy. The authors are to be commended for performing a thorough review of the literature on this topic. They raise most of the relevant issues and they have read most of the important recent papers dealing with them. However, the authors consistently take the most negative possible position on each issue. For this they can be partly forgiven--after all, their purpose was to write an interesting book, and an interesting book has to have a thesis. So it is perfectly fine for them to have taken a stand. Doing so, however, invites criticism--friendly criticism--because some of us are still Trekkies at heart and would like to continue to believe that alien life forms might exist. In this review, I hope to show that while the authors' stances on various issues are generally well-argued, alternative positions often are equally viable.

In no particular order, the main issues are the following.

Jupiter's Importance as a Shield Against Large Impacts

This argument derives largely from the work of George Wetherill (1994). Wetherill pointed out that Jupiter tends to shield the inner Solar System from cometary impacts. Comets originate either from the Oort Cloud (a large spherical shell surrounding the Solar System at great distance) or from the Kuiper Belt (a region within the plane of the Solar System beyond...