Rare Earth: Why Complex Life is Uncommon in the Universe

_Rare Earth_ by Peter D. Ward and Donald Brownlee is an excellent, thorough, and thought provoking work analyzing the possibility of life on other worlds. Ward, a noted paleontologist, and Brownlee, an astronomer, sought to challenge the widespread notion that complex, even intelligent, life is common in the universe. This bias towards believing in intelligent life elsewhere is partly thanks to wishful thinking (aided by works of science fiction) and partly from the famous Drake Equation put forth by astronomers Frank Drake and Carl Sagan. This equation, one designed to estimate the number of advanced civilizations present in our galaxy, was based on educated guesses on how many planets there were in our galaxy, how many of those might harbor life, and of those what percentage advanced to become homes of civilized life forms. Using the best estimates at the time, Drake and Sagan in 1974 postulated that a million civilizations may exist in the Milky Way galaxy alone, and as our galaxy is but one of hundreds of millions of galaxies, according to them the number of intelligent alien species must be staggering. Ward and Brownlee did not think that credible, and put forth an alternative theory, the Rare Earth Hypothesis, using information gathered over the years in paleontology, geology, climatology, and astronomy. In a nutshell, they believe that microbial life is very common in the universe, perhaps more common than even Drake and Sagan anticipated, while complex life - animals and higher plants - is quite rare. As evidence exists that life first appeared on Earth 3.8 billion years ago, almost simultaneously with the end of heavy bombardment from outer space and the lowering of surface temperatures below the boiling point of water, it is possible that life could form very early on a planet, that it can and quite possibly will evolve on any planet if conditions are at least just barely tolerable. Additionally, with evidence that on Earth there are microbes that can survive temperatures above boiling water, as much as 169 degrees Celsius (and maybe up to 200), deep in the Earth's crust or in hydrothermal deep sea vents (as well as life found living in sea ice and snow), it would appear that microorganisms have very wide tolerances for a range of planetary conditions. Given that many believe life may have first evolved deep underground or deep beneath the sea, the authors feel that microorganisms may be quite common, perhaps even present elsewhere in our own solar system. Conversely, the conditions that allow animal and advanced plant life to develop (the authors largely concentrate on animals) are "rare if not unique," Earth being a "bastion of animals amid a sea of microbe-infested worlds." Ward and Brownlee have composed a long list of "low-probability" events or conditions that are necessary for the formation of animal life. I had long been familiar with the concept that Earth was lucky to be at just the right distance from its star, in the habitable zone, a region where heat from the sun provides a planetary surface temperature at which water neither freezes nor boils (the authors later narrow this concept to the animal habitable zone, the range of distances where not only is it possible for an Earth-like planet to have liquid surface water but also to maintain average global temperatures of less than 50 degree Celsius, as that is believed the upper limit at which animal life can exist). What was new to me was the concept of the galactic habitable zone, the notion being that complex life can only develop (and survive) on a planet in the right neighborhood of the galaxy basically, and indeed the right type of galaxy period. Earth-life as we know it could not arise at all if the sun was in a globular cluster, an elliptical galaxy, or a small galaxy, as most if not all stars in these formations are too metal-poor and often are too hot for life on inner planets. In systems low in metals (elements other than hydrogen and helium), there would not be enough solid matter to form a planet the size of Earth and even if one formed it would lack the metals needed to produce either magnetic fields or the internal heat sources to drive plate tectonics.Even in suitable galaxies an Earth-like planet could not exist in its center, as that is a region with many energetic processes that could eliminate life altogether; the high density of stars in the center make the danger of supernovae significant (our sun and planet are protected simply by the relative scarcity of stars around us). Other stellar members, such as neutron stars called magnetars, are considerably more common in the center (which can emit vast amounts of lethal X-rays and gamma rays). Nor could Earth exist at the galactic edge, as that region is too metal poor.The authors argued that plate tectonics is vital to the development of animal life for a variety of reasons. It promotes high levels of biodiversity, a major defense against mass extinctions; it provides the planet with a global thermostat by recycling chemicals essential in keeping the volume of carbon dioxide relatively uniform (and thus enabling liquid water to remain on a planet's surface); it creates the continents (not only vital for biodiversity purposes but also in terms of weathering and as source of marine nutrients); and it provides a magnetic field, a defense against lethal cosmic radiation and the loss of an atmosphere.There are a number of other factors that the authors look at; among them the importance of the Moon (extremely vital but perhaps a very rare thing for one of that size to exist around a terrestrial planet), the importance of a stable Jupiter at the right distance from Earth, and several perhaps unique events in Earth's history, such as Snowball Earth episodes and the Cambrian Explosion, vital events in the evolution of life that might be very unlikely to occur on an alien planet.

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.

Hipótesis sustentada con evidencia científica disponible en ese momento, punto de referencia también en la actualidad (con ciertas actualizaciones).

Libro en buen estado y entregado antes de lo comprometido

Un excellent livre, à lire petit à petit car il reste assez complexe mais très bien écrit

It's a bit dated now but it's still a classic. A truly mind boggling tour of all the main factors that enabled complex life to evolve on Earth, and therefore why it will be rare in the Universe.

Really good