I planned ahead for last month’s transatlantic roundtrip flight by looking for a good book to read on the plane. I’ve had Rare Earth on my Amazon wish list for some time, so I ironically popped over to Barnes and Noble to buy a copy before leaving the country.

The book is written by Peter Ward, Professor of Geological Sciences and Curator of Paleontology at the University of Washington in Seattle, and Donald Brownlee, Professor of Astronomy at the same university. Together they take a controversial stand in the new and radically cross-disciplinary field of astrobiology.

Astrobiology, sometimes referred to as the science with no data, concerns the study of life outside of Earth. Astronomers, paleontologists, chemists, biologists, and scientists of other disciplines are increasingly cooperating in the astrobiological inquiry as technology enables their reach farther outward and as origin-of-life research begins to prefer an off-world source for Earth’s first microbial life. (This is an account with which I am ultimately comfortable, although we normally use the phrase “out of this world” instead “off this world,” and my preferred account is beholden to theism rather than metaphysical naturalism. But I am getting ahead of myself.)

The book is eminently readable for the scientifically literate layman. Technical jargon that would only be accessible to the professional is mostly avoided, and explained well when necessary. Readers without a basic familiarity with astronomy, chemistry, and biology would probably find the book frustrating, but these readers would likely find it uninteresting as well. If this is the sort of stuff you like, you’ll get through this book with no problems.

Briefly summarized, the Rare Earth Hypothesis states that microbial life is common in the universe, but complex animal life is exceedingly rare, perhaps so rare as to occur only once in the cosmos. This hypothesis is defended by examining the history and present habitats of life on Earth, and by examining the features of the Earth and its surroundings that enable life.

Microbial life appears to have arisen on Earth just about as quickly as it possibly could have: fairly undisputed evidence of life’s existence appears within a few tens of millions of years of the oldest known rocks. Keeping in mind that the techniques used to date old igneous rocks actually measure the time of the rock’s transition from a liquid to a sold state, the planet seems to reveal that it played host to life very, very shortly after its surface quit being lava. This leads the authors to infer that microbial life originates quickly (however it originates) wherever conditions are even remotely amenable to its existence.

Ward and Brownlee further defend the Ubiquitous Microbes plank of the Hypothesis through a discussion of extremophiles. Extremophiles are microbes observed on earth to live at temperatures above 100°C or in conditions of extreme acidity, extreme salinity, high pressure, or other conditions normally considered to be sterile. These robust microbes provide evidence that microbial life, at least, does not require a lukewarm pond or carefully incubated nutrient agar in order to survive. Extremophiles lead the authors to believe that even a site less hospitable than the exquisitely appointed Earth might still host life, as long as it can maintain liquid water, has some kind of energy flux to drive metabolism, and possesses a few other sundries that are unusual but not nonexistent in the universe.

Animal life, they observe, seems more finicky. It does not appear to be comfortable above 40°C, and requires very stable and fairly specialized environmental conditions to survive and thrive. Ward and Brownlee occasionally caveat this to apply to “animal life as we know it,” but they seem to believe tentatively that animal life as we know it may for valid biochemical reasons be the only kind of animal life that is physically realizable in the cosmos (or capitalized “Universe,” as they prefer to say in the book). Animal life also seems to take a lot longer to develop, taking about two billion years for the first eukaryotes to appear and about 3.5 billion years for the really interesting things to happen in the Cambrian Explosion. Thus they conclude that the presence of animals should be rare in the cosmos.

But before there were even prokaryotes on Earth, just getting the right kind of planet in the right part of the universe was a chore more than worthy of a Titan. Aspiring planet creators need to choose a star with a long stable lifetime, yet with enough energy output to have a relatively broad habitable zone around it, where orbiting planets at least have a chance to maintain liquid water. The solar system needs an abundant supply of “metals” (a term astronomers use to describe elements heavier than helium, conspicuously including nonmetals heavier than helium), which ours happens to have in spades. The star also needs to be in the right part of the right kind of galaxy: not in a metal-poor elliptical or irregular galaxy; not too close to the middle of a spiral galaxy, where energy output would sterilize any life; and not too far out, where metals and are again scarce. The planet can’t exist too soon after the creation of the universe (again, all-important heavy elements are too rare), and it can’t arrive too late, when decelerating star formation will cause a paucity of the radioactive elements that drive the critical process of plate tectonics.

It helps the planet to have a Jupiter and a Moon as well. The Moon, which in our solar system is freakishly large compared to its companion planet, doesn’t just provide the ecologically significant tidal rhythm. More importantly, it helps stabilize the Earth’s remarkably consistent 23.5° axial tilt, which is key to both the stability and diversity of planetary climates. The authors assert that without the Moon, the Earth would likely flop around on its axis from time to time, even over time scales as short as thousands of years, with devastating climactic impact. Jupiter, for its part, adds stability to the orbits of other planets in the solar system and helps shepherd planet-sterilizing asteroids away from Earth. Asteroids capable of causing mass extinction events are believed still to have impacted Earth in the past, but they would be much more frequent and devastating without Jupiter to protect us.

An entire chapter is named for the “surprising importance of plate tectonics.” A major geological controversy just four decades ago, plate tectonics is now well-attested, and can be credited with several life-sustaining (and presumably rare) functions. By creating sizable landmasses with diverse climates (mountains, valleys, plains, etc.), tectonics provides a platform for the kind of biodiversity that helps insulate against mass extinctions. By exposing a fresh supply of silicate minerals like feldspar to chemical reaction with the atmosphere, tectonics helps regulate the climate. The movement of the Earth’s liquid core which drives tectonics enables the existence of the magnetic field, which shields us from all manner of harmful cosmic and solar radiation.

Most incredibly, the tectonic carbonate-silicate cycle actually exhibits a negative feedback characteristic that drives the temperature of the planet up or down as needed. It works like this: the warmer the Earth gets, the more efficiently the silicates emerging from the mantle are weathered. As these rocks are weathered, they react with CO₂ in the atmosphere, becoming sand and carbon-bearing limestone, and eventually being subducted back into the mantle. Carbon is “sequestered” in this process, thus decreasing atmospheric greenhouse gasses, and cooling the planet back towards the system’s set point. Alternatively, if the planet experiences a cooling trend, silicates are weathered less efficiently, and less carbon is subducted in the mantle. Greenhouse gas levels then increase, warming the planet back up again.

Considerable attention is given in the book to the early history of life on Earth. How the chapters on evolution advance the Rare Earth Hypothesis was not abundantly clear to me, except to say that if things happened as the naturalistic account would describe them, the Earth must be rare indeed. In the chapter on the Cambrian Explosion, the authors state, “For all of the animal phyla to appear in one single, short burst of diversification is not an obviously predictable outcome of evolution,” (p. 150, paperback edition) and for what it is worth, this layman would certainly agree. In any case, the Rare Earth Hypothesis may be somewhat controversial, but it is not so controversial as to question the naturalistic account for the origin and diversification of life. The book summarizes the broad narrative of the first 3.5 billion years of life on Earth, and explains some of the current thinking on key evolutionary questions clearly and lucidly. It does not depart from the naturalistic paradigm, as much as some readers might have liked it if it did.

The authors admit in the preface that the publication of the first edition in 2000 spurred vigorous discussion, some of which has “often left the realm of scientific discourse where we’d intended it to take place, and entered the areas of religion, ethics, and science fiction.” Fair enough. Testing whether their hypothesis is true or not is essentially beyond the reach of present technology (although they do suggest guidelines in a closing chapter), but it is a mark of a bold and influential idea to gain a hearing among those well outside of your discipline. Indeed, no question of enduring existential import is likely to be confined exclusively to a single area of intellectual inquiry with a label like Paleontology, Philology, or Eschatology. If you can have a department head in it, it’s probably too narrow to matter when it’s late and you can’t sleep. The questions that matter are going to have to be attacked in more ways than one, and the rarity of Earth is obviously one such question.

Engaging this book as an Evangelical–not desiring arbitrarily to shut off a rich means of intellectual inquiry, theology–I see much to embrace. The authors are unsurprisingly Darwinists, but only as a matter of professional course (and surely personal and scientific conviction), not in any way required by their broader Hypothesis. The timing, position, composition, and behavior of the galaxy, solar system, sun, and Earth are exceedingly rare regardless of one’s preferred abductive criteria for forming explanations of the origin and diversity of life. Beyond that question, and beyond my respected fellow Evangelicals who require that the Earth be young, this is very much our book.

It is not unlike the God of Evangelical revelation and tradition to craft a special place for his creatures to live, and to provide richly for their survival and eventual technological and social progress. It is consistent with the Old and New Testament teaching on evil that God would ordain calamities like earthquakes and volcanoes toward a greater good (although it is admittedly speculative to identify this greater good as a climatological control loop). Moreover, to affirm the Hypothesis is nearly to recovery the pre-Copernican centrality of Man that has remained so evident in revelation even while science has sojourned elsewhere for almost five centuries. Besides being a winning science text for the enjoyment and education of the lay science audience, this is a book that speaks strongly to the growing Evangelical intuition that science is reluctantly suggesting teleological conclusions. I thank Drs. Ward and Brownlee for their excellent work in writing it.

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• Posted by Tim on Wednesday, April 7, 2004 at 12:26:32 am PDT
• Category: Theology, Astronomy
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