Rare Earth: Why Complex Life is Uncommon in the Universe [Englisch] [Taschenbuch]

One of the things I liked most about this book its very nice summary of the history of earth. Chapter 1 has some interesting information about recent discoveries regarding the environments in which extremophiles live. It might seem incredible to us, but extremophiles actually thrive in very high temperatures, pressures, and pH levels that we would find terribly fatal. The wide range of environments in which the simplest life forms can live gives rise to the greater probability of finding them throughout the universe. Extremophiles not only thrive in such environments, they can also tolerate brief forays into space aboard debris ejected from meteor impacts, and they can escape harsh surface conditions by living deep under ground.

The second chapter introduces us to the concept of habitable zones. For extremophiles the habitable zones are quite large, so planets harboring such life can be found in a wider range of orbits around a wider range of stars. More complex life, however, requires far smaller ranges in environmental conditions, leading to a much-reduced habitable zone. Habitable zones must also exist over sufficiently long periods of time for life to evolve. In other words, the habitable zone has both spatial and a temporal components. The upshot is that habitable zones for complex life are pretty small, and may not exist at all unless the star is quite similar to ours.

Chapter three describes some of the critical components for a habitable earth, including the creation of hydrogen and helium (and a bit of lithium) in the Big Bang, and the creation of heavier elements in stars. The earth needed to form from an inter-stellar accumulation rich in metals with the right amount of water. The authors argue that such conditions are rare in the universe, and that planets such as ours are rare as well.

Chapter four discusses the conditions on earth after the initial bombardment stopped, during the earth's early formation. This chapter has some very interesting material suggesting how bacteria, archaea, and eucarya form the earliest tree of life. Chapter 5 continues by describing how Eucarya are differentiated from the archaeans and the bacteria. Eucarya include the animals, ciliates, fungi, plants, flagellates, and microspordia that constitute the complex life that the authors claim is so rare in the universe. The key piece of evidence regarding eucarya is that they took a long time to evolve in earth's history. The message in this fact, according to the authors, is that eucarya require a more specialized environment in which to evolve - a narrower habitable zone - and that this environment must persist for long periods of time. All of which argues against this type of life being common in the universe.

Chapter 6 is called "Snowball Earth," and describes the earliest known ice ages on earth, which date to 2.4 billion and 800 to 650 million years BP. These ice ages, in contrast to the one a few tens of thousands of years ago, literally covered the entire earth and froze the oceans over. These ice ages could have helped to stimulate the evolution of animals, and (just as importantly) they also show that major environmental changes - changes that can cause mass extinction - have and can occur on earth. Timing and the extreme nature of the events are critical to the evolution of life.

The real centerpiece of this book is chapter 7, which discusses the enigma of the Cambrian explosion. Perhaps no other event in earth's history has generated as much debate and speculation as the comparatively sudden rise in complexity of animal species that accompanied the Cambrian explosion. The authors argue that the triggers for this explosion (and they offer several possible candidates) are necessary for the evolution of complex life, but they are also rare - adding further emphasis to their hypothesis that complex life in the universe is rare.

Chapter 8 describes mass extinctions and threats to animal life. Chapter 9 raises some interesting issues about the importance of plate tectonics and argues that the evolution of animal life requires plate tectonics to maintain biodiversity and to stabilize global climate. Chapter 10 discusses the importance of the moon in creating tides, contributing to orbital stability (and possibly contributing to plate tectonics) and Jupiter, which cleans the solar system of planet-crossing projectiles that could sterilize earth. Again, the point is that these conditions are rare, and so planets with complex life are also rare.

Chapter 11 describes ways in which the rare-earth hypothesis might be tested, and chapter 12 reassesses the odds of complex life in the universe via a modified form of the Drake equation. I found chapter 12 a bit of a disappointment, though, since the authors never really stick there necks out and make a claim for how many planets with complex life might exist in the galaxy. The final chapter discusses the philosophical and ethical implications of the rare-earth hypothesis, including the role people play in the current extinction crisis.

This is a first-rate book. The notes are detailed, the index complete, the text clear and understandable. The argument is lucid and in many (though not all) ways compelling. And, of course, it's hard to think of subject matter more relevant.

Folks, the fact is that we have barely even begun to explore the habitablility of our Universe beyond planet Earth. Don't be mislead into thinking the task is finished. Despite much self-lauding on how much we've discovered, we've only been at it for 50 years! We know very little even of the other planets in our own solar system, and we are NOT sure that none of them harbor complex life.

Outside our own Solar System our knowledge is even less complete. The best we can do at this point is look for fluffy, hot giant planets. While the technology to detect Earths around other stars is certainly coming along, it is not here yet. Ergo, we know nothing about the frequency of Earths orbiting other stars, let alone whether they are habitable.

We are very special here--anyone who has walked in the rain, sung a song or been in love will agree. But whether we are ALONE is a whole other matter, one that neither Ward nor Brownlee is prepared to answer given the current body of knowledge. Therefore I find the argument of this book quite misleading to the general public and mildly insulting to the authors' colleagues.