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Peter D. Ward and Donald Brownlee
Rare Earth : Why Complex Life is Uncommon in the Universe
Hardcover Edition -- 2000, Paperback Edition -- 2004
Second Edition, First Paperback Edition, 368 pages
Published 2004 by Copernicus Books
ISBN 0-387-95289-6 / ISBN 0387952896
13 chapters plus appendices, B&W illustrations,
17-page 2-column index, detailed bibliography.
The Rare Earth Hypothesis, put forth by Peter Douglas Ward and Donald Brownlee, states that single-celled organisms probably exist on many other planets, but the probability that multicellular animals exist on other planets is much smaller than we had previously believed.
In the first place, elliptical galaxies and globular clusters contain very few metal atoms, which are required in order to have solid planets. There is little hope for life to evolve on gas planets which have neither liquid nor solid phases. Bacteria might survive if transported there, but their initial development there is not expected. Should the reader take an interest only in our own spiral Milky Way galaxy, and perhaps our neighboring spiral Andromeda galaxy, and not care about any of the others, nevertheless, it is only a restricted region of our own galaxy that has a significant number of metal atoms. [Page 29]
Apart from the elemental composition, most stars cannot have planets which develop life. A "suitable star" the authors explain, is "one that will burn long enough to let evolution work its wonders, one that does not pulse or rapidly change its energy output, one without too much ultraviolet radiation, and, most important, one that is large enough."  If the star is too small, planets will orbit too closely, causing tidal lock -- the same extremely hot hemisphere of the planet always facing the star, and the same extremely cold hemisphere always facing away.
Assuming that liquid water is necessary for life to evolve, it is fortunate that temperatures on earth are in the narrow range between the freezing and boiling points. This is the result of several coincidences -- besides the avoidance of tidal lock. Our distance from the sun lies, not only in the narrow habitable zone (HZ) but also in the narrower animal habitable zone (AHZ) and a continuously habitable zone (CHZ). [16-20] Having a moderate temperature also requires that that planet's orbit around the sun is nearly circular and not too elliptical, which would produce great temperature swings. It's fortunate that the earth's rotation with respect to its solar orbit is tilted (obliquity) at an optimum angle, and at a nearly constant angle, to produce seasons which moderate the climate.  Again, bacteria might be able to live on a planet with an extreme average temperature or a wide temperature swing, if they were to be deposited there -- some organisms on earth today are extremophiles [3, 55] -- but for them to initially develop there would be much more problematic.
It was fortunate that collisions by comets brought water to the earth. It is likewise fortunate that these comets didn't bring so much water that we don't have any dry land at all. Only shallow water permits the formation of limestone, the major process which removes carbon dioxide from the atmosphere. [59-60, 262]
We are lucky in several respects that our planet is one with a molten core. Plate tectonics, driven by the heat emitted by the decay of radioactive atoms, permits the formation of continents, regulates the atmosphere's temperature by removing carbon dioxide from the air and putting it into carbonate rocks , and promotes the biodiversity which assists evolution. The planet's magnetic field, which can be generated only by a molten iron and/or nickel core, deflects deadly cosmic rays away from the planet's surface, so that life on earth wouldn't be sterilized as soon as it had begun. [194, 247]
Collisions by asteroids can cause mass extinctions, and, if sufficient in magnitude, can rip away a planet's atmosphere and oceans -- if it had possessed these in the first place.  It is a matter of luck that the mass of (and hence the gravitational pull by) Jupiter was sufficient to cleanse the solar system of most earth-crossing asteroids. But we're lucky that the gravitation of Jupiter isn't so great as to prevent the earth from having a stable orbit, possibly attract and devour the earth, jerk the earth out of the solar system altogether, or prohibit the accretion of solid particles into an earth in the first place.
In this manner, Ward and Brownlee describe the formation and the survival of complex life as the result of many fortunate accidents. The book provides a summary of these factors, and several others, in two charts, entitled "Dead Zones in the Universe" and "Rare Earth Factors," which are inserted between the Introduction and the first chapter.
As I read the text for the first time, I considered that it's feasible for a person to win the lottery, but almost unthinkable that the same person might win the lottery a hundred times.
After discussing the cosmological facts, the authors discuss several of the paradoxes of biology.
Somehow, the early and simple living cells evolved from prokaryotes to eukaryotes. The latter have, in addition to protective membranes, organelles which are bounded by membranes, including a membrane-enclosed nucleus which contains the DNA. [90 et. seq.] Only eukaryotic cells are able to form vacuoles to devour food, and able to perform sexual reproduction. Afterwards, a mysterious "jump"  from single-celled organisms to the simplest multicellular animals took place. This is the "ancient dichotomy."  It required the appearance of cells which perform numerous specialized tasks, and the appearance of a protective outer skin, the epithelium.  After a long delay, the number of animal phyla flourished in a relatively short span of time, known as the Cambrian explosion , perhaps in response to such factors as the optimum quantities of oxygen and nutrients, the end of an ice age, and the sliding of the earth's crust upon its fluid interior. [142-147] Again, we are the byproduct of many fortunate accidents.
Ward and Brownlee propose specific methods for testing their Rare Earth hypothesis. Before these tests can be undertaken, science must first detect life of any sort, simple or complex, on distant planets. The principal means to do so is spectral analysis of the gases in the atmospheres of planets. For example, only a planet with life could have a significant amount of both nitrogen and oxygen. Without life processes, the oxygen and nitrogen would have long ago combined into nitric acid. 
Later, the authors reconsider the coefficients of the Drake Equation for estimating the number of advanced civilizations in the universe, in light of information available today but not available when astronomer Frank Drake first proposed it. 
Rare Earth, originally published in hardcover in 2000 , is out in paperback in 2004. A new three page preface to the paperback edition precedes the three page preface to the first edition.
Most reviewers have interpreted the case presented in Rare Earth as a dampening factor to the recent rise in optimism in the search for extraterrestial intelligence . The reviewer for Discover magazine called the book "a wet blanket for E.T. enthusiasts," and the New York Times remarked, "Maybe we are alone in the universe, after all."
On the other hand, there are probably some scientists and philosophers out there who are thinking, "If I didn't know better, this book is almost enough to scare me into believing in a God."
Regardless of the conclusions that the reader may draw, the book is a good example of how scientific debate ought to proceed, with a clear statement of the authors' assertion and a concise organization of the accompanying array of evidence.
- - - - - - Book review by M. Lepore for crimsonbird.com, 2006
Press release received from the publisher:
Maybe we really are alone.
That's the thought-provoking conclusion of Rare Earth , a book that is certain to have far-reaching impact in the consideration of our place in the cosmos.
While it is widely believed that complex life is common, even widespread, throughout the billions of stars and galaxies of our Universe, astrobiologists Peter Ward and Donald Brownlee argue that advanced life may, in fact, be very rare, perhaps even unique.
Ever since Carl Sagan and Frank Drake announced that extraterrestrial civilizations must number in the millions, the search for life in our galaxy has accelerated. But in this brilliant and carefully argued book, Ward and Brownlee question underlying assumptions of Sagan and Drake's model, and take us on a search for life that reaches from volcanic hot springs on our ocean floors to the frosty face of Europa, Jupiter's icy moon. In the process, we learn that while microbial life may well be more prevalent throughout the Universe than previously believed, the conditions necessary for the evolution and survival of higher life -- and here the authors consider everything from DNA to plate tectonics to the role of our Moon -- are so complex and precarious that they are unlikely to arise in many other places, if at all.
Insightful, well-written, and at the cutting edge of modern scientific investigation, Rare Earth will fascinate anyone interested in the possibility of life elsewhere in the Universe, and offers a fresh perspective on life at home which, if the authors are right, is even more precious than we may ever have imagined.
About the authors ...
Peter D. Ward is Professor of Geological Sciences and Curator of
Paleontology at the University of Washington in Seattle. His
previous books include:
Time Machines : Scientific Explorations in Deep Time
The Call of Distant Mammoths : Why the Ice Age Mammals Disappeared
On Methuselah's Trail : Living Fossils and the Great Extinctions and Rivers in Time
Donald Brownlee, a member of the National Academy of Sciences, is Professor of Astronomy at the University of Washington in Seattle. He has been a member of numerous important NASA teams, and specializes in the study of the Solar System's origin, comets and meteorites, and the underlying subject of this book, astrobiology.
Peter D. Ward and Donald Brownlee are co-authors of The Life and Death of Planet Earth : How the New Science of Astrobiology Charts the Ultimate Fate of Our World
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