| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Reviews Rare Earth: Why Complex Life is Uncommon in the Universe, by Peter D. Ward and Donald Brownlee. Copernicus, New York, NY, 2000, 333 pp., $27.50 cloth. Many astronomers think that advanced life is widespread in our galaxy. The conditions that fostered evolution on Earth must exist in many other places, the reasoning goes, because the Milky Way contains hundreds of billions of stars around which hospitable planets might orbit. Chemistry and biology should produce rich stews of organisms on such worlds. And given enough time, those organisms could grow more complex and, ultimately, self-aware. With this view of a cosmic society, its just a matter of hunting in the right ways before we find our galactic brethren. But the leading effort to do so, the Search for Extraterrestrial Intelligence (seti), has thus far unveiled nothing but silence. In Rare Earth, two prominent scientists propose a reason for this failure: Our home planet and our solar system may be far from ordinary. The special circumstances that allowed animals and humans to thrive on Earth may arise almost nowhere else in the Milky Way, and indeed in the universe. Its a thought-provoking hypothesis that relies upon not one whit of religious determinism. Science rules the day in this volume, and the chain of logic is compelling. Geologist Peter Ward and co-author astronomer Donald Brownlee have earned their scientific stripes. Ward is an expert on mass extinctions, sporadic events in Earths history during which large fractions of all species suddenly vanished. Brownlee is a member of the National Academy of Sciences and leader of a nasa mission that will gather samples from a comet. Their thesis examines Earth not as an isolated planet but as a part of an ever-changing environment in space. The oasis we enjoy amid the hostile desert of space, they claim, bloomed in response to a surprisingly unlikely sequence of events. Some of these are well known. For instance, Earth orbits within the Suns “habitable zone,” a comfortable temperature regime that allows liquid water to flow on the surface. The authors compare this to sitting just the right distance from a campfire on a bitterly cold night. Our Sun is a stable and long-lived star, giving evolution billions of calm years to work its magic. Our atmosphere has developed into a life-friendly blanket, and our planets magnetic field protects us from radiation and other hazards of space. Ward and Brownlee argue that bacteria and other basic forms of life probably gain toeholds quite readily on worlds that have even a fraction of these features. They observe that microbes live in extraordinarily harsh settings, from scalding deep-sea vents to thin rinds within cold Antarctic rocks. Moreover, life probably flourished early in the planets history, not long after its birth 4.5 billion years ago. However, they note, “The formation of animal life was much more recent and protracted.” The hurdles that animals overcame even to exist on Earth are extensive, the authors claim, and they devote the bulk of their book to describing them. For example, the massive planet Jupiter orbits in just the right way in our solar system to fling asteroids and comets away from Earth, thus protecting it from frequent impacts that otherwise might sterilize life. Our gravitational waltz with the Moon stabilizes the tilt of Earths axis, preventing wild swings of temperature. Our planet has just the right ingredients for its outer crust to break up and move relentlessly; this plate tectonics regulates the gases in our air and sparks new species to arise by rearranging the continents. On a broader scale, we live in a peaceful part of a stable galaxy, far from damaging supernovas and other outbursts. Almost all other corners of the universe will lack one or more of these physical factors, Ward and Brownlee believe, creating “dead zones” where higher life cannot evolve. Scientists often lapse into jargon and convoluted arguments when they write for the public. Refreshingly, thats not the case here. Rare Earth reads like a breeze, with straightforward writing and dashes of clever imagery. The authors describe the sudden emergence of trilobites about 550 million years ago, seemingly without any complex animals preceding them: “It is as though an orchestra began playing without sounding a single tone to tune up.” A lovely sequence near the end of the book invokes the emotions that one might feel when gazing into a starry sky and realizing that we may have few companions. And yet its not likely that Rare Earth will change minds in the scientific community. There are too many hand-waving arguments, and much of the text relies too heavily upon controversial theories. One discussion centers around the “Snowball Earth” scenario, in which glaciers may have blanketed our entire planet several times during its history. The authors maintain that these rare events may have stimulated the origins of animal life, but its not widely accepted that such icy episodes even occurred. Another point notes that the huge blobs of stars called elliptical galaxies arent likely to host Earthlike planets, because they lack heavy elements. This ignores the growing consensus that many elliptical galaxies, and perhaps most, come from the collisions of smaller galaxies that are rich with such elements. The authors implicitly acknowledge these and other uncertainties by noting that they cannot prove their case, just as the adherents of seti cannot prove theirs. As such, Rare Earth mainly helps to frame a profound puzzle that may take decades or centuries to solve. Robert Irion Recommended Reading from the Editors' Desks The Triumph of Evolution and the Failure of Creationism, by Niles Eldredge. W.H. Freeman and Company, New York, NY, 2000, 223 pp., $24.95 cloth. Biologists generally hate to debate creationists. Its not that they fear losing, but that the debates are essentially incoherent. The creationists argue for a metaphysics: God created the world. The evolutionists argue for a methodology: science. It is no surprise that the arguments fly right past each other and no one gets convinced of anything except the dishonesty and foolishness of the other side. Eldredge makes the best of a bad situation in The Triumph of Evolution, taking the opportunity to promote his own view of how evolution works and to review the history of evolutionary biology. He also makes a plea for peace, claiming that preventing or minimizing ecological crises should be above ideological debate on the agendas of both religious fundamentalists and evolutionary biologists. He rightly questions the sense of arguing on the porch while the house burns to the ground. Living On The Wind: Across the Hemisphere with Migratory Birds, by Scott Weidensaul. North Point Press, New York, NY, 1999, 436 pp., $15.00 paper. Humdrum as it may appear, the familiar robin flitting about your front lawn is a bona fide member of the avian jet-set clique. Robins, like many other birds, are migratory, chasing food and warmer climes south in the winter, and flying north in the summer. Scott Wiedensaul undertakes the same journey in their wake, tracking robins, hawks, herons and other birds in their transglobal wanderings, as he pieces together the natural history of avian migration. The story Weidensaul tells is a gratifyingly rich one. The fundamentals of migratory behavior are covered well, often in encyclopedian detail. How birds navigate, for instance, remains largely a mystery, but Weidensaul canvasses the possibilities, from a sort of avian orienteering using the Earths weak magnetic field, to genetically instructed dead reckoning. His focus throughout the book, however, remains resolutely aimed at the importance of understanding what he terms the “ecological whole of migration.” Migratory birds routinely span continents in their travels, but our understanding of their life habits rarely extends so far. Swainsons hawk, which has suffered thousands of deaths from pesticide exposure in its winter home in the Argentine pampas, makes a ready example. While “mountains of studies” document the birds diet in its summer home in North America, virtually nothing is known about its prey in Argentina, hampering conservation efforts. The hawks story is more common than not; for the most part, our knowledge of birds natural history remains piecemeal, fragmented by political boundaries and limited scientific resources. Living on the Wind makes a fine start of filling in the gaps and ensuring that migratory birds remain aloft for generations to come.
The Redwood Forest: History, Ecology, and Conservation of the Coast Redwoods, edited by Reed F. Noss. Island Press, Washington, DC, 1999, 352 pp., $60.00 cloth, $30.00 paper. Coast redwoods seem immortal and invulnerable when we stand in the midst of a fog-draped grove. While individual trees may indeed live for two millennia, redwood forests as we observe them today—mixed with Douglas-fir, tanoak, and a broad community of flora and fauna—have only been around for about 4,000 years. The dynamic forests have responded to and persisted through countless environmental changes and pressures, recently retreating, for instance, from the southern end of their range while expanding northward. Coast redwoods have been the beneficiaries of a conservation success story. Until 1900, redwood champions focused only on the Sierra Nevadas giant sequoias, but the sequoias coastal cousin became the focal species for Californias young state park system. Leading the charge for much of the past century has been the Save-the-Redwoods League, the organization that conceived this comprehensive coast redwood compendium. This book sees the forest for the trees. Editor Reed Noss calls it “a case study of an endangered ecosystem,” which attempts to compile whats known about the biology and ecology of coast redwood forests. The 32 contributors to The Redwood Forest are all experts on some aspect of this ecology, while Noss is a leading conservation biologist. The treatment may be too technical for some, but readers should find answers to many pressing questions, including “How does water get to the tops of tall trees?” By providing a rich and rigorous natural history context, the volume can also serve as a scientific guide for the current century of redwood conservation. As a species, the coast redwood now faces little imminent danger of extinction, but the same isnt true for all components of the intricate forest system. This book should help forest managers think as broadly as possible. Declares Noss, “...a lineage that has persisted for so long and in the face of so much global turmoil demands our admiration—and our careful stewardship.” Driftwood Valley: A Woman Naturalist in the Northern Wilderness, by Theodora C. Stanwell-Fletcher, introduction by Wendell Berry. Oregon State University Press, Corvallis, OR, 1999, 338 pp., $17.95 paper. When Driftwood Valley was published in 1946, its tales of wolves and wildness found an enthusiastic audience immediately. The book enjoyed 27 reprintings and won the John Burroughs Medal for nature writing. More than 50 years and two editions later, the book has been republished and still offers readers hair-raising stories of bushwacking, canoeing through rapids, and other on-the-edge adventures in remote British Columbia. A must-read classic for natural-history buffs, Driftwood Valley takes readers back to a time when wolves, moose, and grizzlies were commonplace in Canadian wilderness, and when the forests themselves seemed infinite. Although its occasional condescension toward native peoples seems outdated, the books unpretentious narrative style is so convincing youll want to put on a sweater when Stanwell-Fletcher talks about sub-zero snow camping. |
Fall 2000
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