I learned about this book watching the History Channel’s multi-part documentary about the creation of the Earth and the amazing journey it has been on ever since. The book covers the same story but in greater detail, with particular emphasis on the origin of life and the equally amazing journey it has been on, from basic microbial life to complex animal life to highly intelligent life capable of conscious thought—to homo sapiens, us. Are we really the stuff of stars, as Carl Sagan said? How do the most elementary particles—forged in the Big Bang—evolve over time to produce brain cells capable of rational thought, indeed, able to reflect on the creation of the universe? This question goes unaddressed in “Rare Earth” and is the 600-pound gorilla in the room. I don’t blame the authors. It’s a question with vast implications and well beyond the book's scope. Still, I couldn’t help thinking about it.
The premise of “Rare Earth” is that microbial life is common throughout the universe while animal life is rare. How can this be? Because microbial life—life in its most basic form—is extremely hardy. It can withstand extreme temperatures (from below freezing to above the boiling point of water) incredibly high pressure, does not require oxygen, and in some cases does not require sunlight. The authors believe that microbial life may not have originated on earth, but been transferred here by comets or possibly by asteroids that originated on a neighboring planet, probably Mars. Animal life, on the other hand, is extremely fragile. It can only survive in an atmosphere of plentiful oxygen, lots of water, minimal planet disruptions, and Goldilocks’ temperatures—neither too hot nor too cold. Microbial life arrived not long after earth’s formation and early on survived countless planet disruptions that would have destroyed all forms of animal life.
During the 600 million years it took to develop animal life, earth enjoyed a charmed existence. It was neither too close nor too far from a large stable sun, had a circular rather than elliptical orbit, was protected from astroids and comets by outer gas giants (notably Jupiter), and likewise protected from ultra-violate rays by a strong magnetic field, thanks to Earth’s largely iron core. During this time the oceans and the atmosphere were transformed by the introduction of oxygen. At the same time continents formed made of durable and relatively lightweight granite, which more or less floated on heavier molten rock. The floating continents, coupled with a few active volcanoes, helped regulate Earth’s temperatures and insured that the planet surface was continually being recycled. Add a generously large moon to regulate the tides, with the earth tilted on its axis just so to create seasons and further regulate temperatures, and the earth became a veritable garden of eden.
Still, all was not perfect. Over time, there were a few well-placed catastrophic events that destroyed all but the smallest and most adaptable forms of life. The most recent was a large asteroid or comet that struck earth 65 million years ago that put an end to the age of dinosaurs. A good thing, too, because with dinosaurs around mammals didn’t stand a chance of evolving into larger creatures, such as goats, pigs, oxen, horses, elephants, monkeys and apes and, as late as 50 thousand years ago, homo sapiens. These catastrophic events, while rare, served as a reset button—an occasional re-shuffling of the order of life on earth—without which the appearance of thinking homo sapiens would not have been possible. There are many more dimensions to earth’s charmed existence, including its location on the outer edge of the Milky Way galaxy, far from gamma ray explosions, with but a few non-threatening stars in the immediate galactic neighborhood, the presence of the right amount of carbon (neither too much nor too little), an iron rich planet composition, neither too much nor too little water, and eons of relatively uninterrupted time for life to emerge from the primordial ooze, develop into animal life and, with a few hiccups, produce life capable of rational thought.
All of these things must happen in order for a planet to produce complex life—an amazing string of events threatened at every turn, yet somehow defying the odds to not merely survive but thrive. Indeed, what are the odds? Thirty years ago, Carl Sagan said there were as many as a million planets in our galaxy capable of producing life. We have learned a great deal since then, including our first observations of distant solar system and planets, none of which act much like our own. The authors conclude it's probable that microbial life is common throughout the universe while a stable and long-lasting environment necessary for the evolution of animal life may not be—hence the rarified and charmed existence of earth. Are we alone? While the odds have been significantly reduced since Sagan made his prediction, the jury is still out.
About the book: it’s well organized, well-written and not at all hard to understand, if you don’t rush. To get the most out if it, careful reading is recommended. I spent about two hours a day for a week or so reading the book, learned a great deal, and enjoyed the experience. Five stars.
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