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"Rare Earth: Why Complex Life Is Uncommon in the Universe"
By Peter D. Ward and David Brownlee
Copernicus Books
Nonfiction/Science
333 pages

By L.D. Meagher
Special to CNN.com

(CNN) -- The notion that life existed anywhere in the universe besides Earth was once laughable in the scientific community. Over the past thirty years or so, the laughter has died away. As the vast scale of the Universe has become clearer, the notion that life could have arisen only on Earth seems increasingly unlikely. The law of averages alone would suggest that there must be many places in the cosmos that harbor life.

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"Not so fast," say Peter D. Ward and Donald Brownlee. The two professors at the University of Washington argue that the recent trend in scientific thought has gone too far. They suggest that even if the universe is teeming with life, complex organisms are not likely to appear on many -- if any -- planets besides our own. They make their case in "Rare Earth."

The authors draw on a wide variety of scientific disciplines, from geology to paleontology to astrophysics, as they lay out the evidence that Earth may be a singular habitat for animal life. Indeed, they call this compendium of sciences "astrobiology," the study of life throughout the universe. They also admit that all of their conjectures about how life might evolve on other planets are based entirely on one example -- how life evolved on Earth. But they argue that example is rich enough in detail to provide clues about how the process might work everywhere.

Ward and Brownlee acknowledge that life arose fairly quickly on Earth, and they allow that simple life forms, of the singe-cell variety, might be common on many other planets. But they insist that the evolution that led to everything from butterflies to redwoods to humans is the result of a peculiar, and perhaps unique, sequence of events on Earth.

There is a long list of interrelated factors leading them to this conclusion. They include the presence of the planet Jupiter in an orbit sufficiently far away from Earth to deflect much of the cosmic debris floating around the solar system. If Jupiter weren't there, or if it were in a different orbit, a lot of that junk would be crashing into Earth and extinguishing complex plant and animal life. Only Earth, among the inner planets of the Solar System, has plate tectonics, a process that serves as a sort of regulator on our global temperature, as well as providing enough dry land for complex organisms to inhabit.

They also cite the happy accident of Earth's position in the galaxy -- not too close to the intense radiation of the central core, not so far away that it's left without the chemical building blocks of life. And there's the position of the galaxy itself -- not in a globular cluster, not in a metal-poor quadrant, but in just the right spot to foster the rise of complex life.

"The continued marginalization of Earth and its place in the Universe perhaps should be reassessed," the authors write. "We are not the center of the Universe, and we never will be. But we are not so ordinary as Western science has made us out to be for two millennia. Our global inferiority complex may be unwarranted."

"Rare Earth" poses many thought-provoking questions. Ward and Brownlee make their case forcefully. And yet, there's something not quite convincing about it. Perhaps it's because all of their conjectures about life on other planets are based on the only example available to examine -- life on Earth. Even an example as rich in diversity as terrestrial life certainly cannot encompass all of the possible ways evolution can work.

Still, as radio telescopes sweep the skies and earthbound researchers strain to pick up anything that might be a signal from extraterrestrial beings, "Rare Earth" may offer an explanation for why we haven't heard anything yet.

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