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Rare Earth: Why Complex Life is Uncommon in the Universe

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There is a long tradition among humans that we are not aloneinthe universe - that there are other worlds with other intelligentbeings such as ourselves. This tradition is found in many religions and embodied in some scientific thought. The late Carl Sagan, for example, surmised the existence of one million civilizations capable of interstellar communications in the Milky Way galaxy. Ward and Brownlee take exception to these estimates. According to the authors, microbial life is common in the universe "but even the simplest animal life is exceedingly rare." Instead of millions of such civilizations in the Milky Way galaxy there might be just a few. There might be just one.

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.

This is one of the most wide-ranging and readable of all science books aimed at a non-specialist audience. The authors raise Big Questions in astronomy, planetary evolution, geology, climatology, ecology, and biological evolution, reminding us of how interdisciplinary the extraterrestrial life debate really is. The vast array of subjects they address could make this book an ideal discussion text for a course designed to introduce non-scientists to scientific issues and methods. The book, at least in its hardback edition, has an uncrowded format that makes it easy to read.

Ward and Brownlee accumulate a lengthy series of arguments in support of their position that, while simple life may be relatively common, complex life is rare. At first glance, these arguments seem persuasive. However, a closer reading reveals that the authors sometimes tilt the debate in their favor by choosing the argument that best supports their case even when the evidence is very limited or ambiguous, e.g. their statement that plate tectonics "may be vanishingly rare in the Universe as a whole." How can they know this when our sample is limited to our own solar system? Sometimes, the authors make unsupported blanket generalizations, e.g. "On every planet, sooner or later, a planetary catastrophe can be expected that either seriously threatens the existence of animal life or wipes it out altogether." Stating that "it just seems to have been by chance that our Jupiter formed as it did" is hardly scientific. Oddly, after stating that SETI is a futile effort if their hypothesis is correct, the authors go on to say that "There probably are other civilizations in the galaxy that have radio telescopes." In the end, the fundamental limitation to such arguments is that they are based on the only biosphere we know -- our own. This is summed up in the authors' statement that "It appears that Earth got it just right." Life as we know it may not exhaust all the possibilities.

There are some errors. Venus does not always present the same face to the Sun. Nicholas Copernicus was Polish, not Danish. Frank Drake's equation was developed for a meeting held in 1961. One hopes that the authors will clean up these details for a second edition.

These two authors have written a highly informative book to support their thesis that we might well be the only multi-celled organisms in the universe. It should be stressed that Ward and Brownlee feel strongly that there are probably simple, bacteria like creatures on other planets, but nothing more complex. This is a most interesting book even if you do not agree with their hypotheses. It provides an entertaining and accessible summary of the biological, cosmological, and geological science involved in the development of our home planet. W&B feel that the necessary conditions for complex life are so numerous that few, if any, planets elsewhere could meet the requirements.

Read this book and see: 1. Why the moon and Jupiter are essential for our existence. 2. Why a system of plate tectonics is vital for the development of life forms. 3. The effects that mass extinctions have had on evolution. 4. Why life may have originated in the deep ocean near hydrothermal vents. 5. Why earth is very lucky to be located on the far edge of our galaxy.

For the scientific oriented layman this book is a true gem.

This is the best book that I have seen on the subject of extraterrestrial life and the possibility of its occurrence beyond our solar system in a long time. In order to make the most accurate prediction possible the book makes use of the strongest evidence we have to date, our knowledge of our own solar system. Using very wide breadth, the authors rationalize the following simple theme, "The occurrence of simple microbial life is pervasive in the universe, but the occurrence of more complex, multi-cellular life is not.

From this book I now realize that the number of natural phenomena affecting the possible occurrence and evolution of complex life in one of our neighboring stars is far greater than I ever imagined. I thank the authors for enlightening me on this subject. Perhaps in future I won't be as disappointed as I was during the first Viking landing. I really did think they would find vegetation on Mars, and I'm still upset about it.

The authors of "Rare Earth" present their subject very well, although perhaps not in a style that would excite the average public, as did Carl Sagan and Issac Isamov in their science books. Indeed, I recommend that the authors of this text consider rewriting it in a more popular form, with plenty of illustrations. Hey, people love pictures, myself included.

The controversy between the people at SETI and the authors is unfortunate. I believe both sides have a lot to gain from each other's work. In my opinion it was a mistake for the authors to have included, near the very end of the book, references to SETI. The antagonism created was predictable. I myself am a participant in the seteathome project, having completed 225 work units so far. However, I do not allow my excitement over the possibility of receiving an alien signal stand in the way of my objectivity in the face of the best hard evidence we have to date. Besides, I take it not as fact, but simply as the best prediction I have heard anyone say so far. It is the closest to fact that we can get at this time. Hopefully soon we will get closer.

Rare Earth presents the interesting and not widely disseminated view that complex life forms are probably very rare in the universe. In other words, we haven't seen any ET's because there aren't any-or very few. The book makes this argument reasonably effectively and supports it with some science.

I was disappointed that the science seemed pretty shallow-not really well done. There are just plain factual errors, e.g., the repeated assertion that Venus and Mercury present the same face towards the sun all the time, which leads to the elimination of a whole class of stars as potential homes for Earth type planets. Some science was confusingly presented, some just irrelevant, e.g., the discussion of periodite as distinct from basalt.

One is tempted to dismiss these as minor flaws, but they are particularly irritating in light of the authors' hubris about our current knowledge. For instance, I think they too easily dismiss the possibility of complex life developing or surviving in some way we haven't thought of. Such a possibility could overcome the need for the presence or absence of certain chemicals, the necessity of protection from ultraviolet radiation, or relax temperature limits, etc.

Another instance: in considering the consequences of the cessation of plate tectonics they predict, with no qualifications, that the continents will disappear, Earth will become a "waterworld", sea life will be aversely affected, carbon dioxide levels will drop to zero, the greenhouse effect will be diminished and Earth will freeze. There are no alternative hypothesis, no consideration of possible breaks in their chain of reasoning. The role of water vapor, clouds, the effects of increased areas of evaporation, the adaptability of life are not dealt with at all. This unqualified confidence seemed particularly unsuited to scientists who, as a group, find it necessary to change some aspects of their creation story every five months and can't come close to predicting next week's weather.

The argument that the moon's role in stabilizing the obliquity of the Earth's spin axis was essential for the existence of long lasting complex life was unconvincing. It was hardly an argument, more just an assertion. I came away saying, "Maybe....but, well, maybe not." Another assertion, not even an argument, "the critical 40 degree C mark that is the upper temperature limit of animal life." Living in Phoenix Arizona, where the temperature reaches 50 degrees C, I'm tempted to say, "I don't think so."

It was a little jarring to find several mentions of the current "mass extinction" mixed in with the statement, "The fossil record suggests that there are more species of animals and plants alive on Earth today than at any time in the past." No data is presented or alluded to; the nuances and profound uncertainties regarding the current stage of life on Earth are just not acknowledged-just boilerplate about rain forests and topsoil. The idea that intelligent life is very rare is important. This book presents that idea fairly well, but with some distressing and regrettable problems.

I discovered this title in a review in the Boston Sunday Globe a few years ago and decided to try reading the book. To my great pleasure, I found that the authors had done their job well. Although they are pretty sure, given the present state of our knowledge, that advanced life is almost improbable elsewhere in the universe, they continually provide views apposing theirs....so the book is not merely a vehicle to broadcast their own bias [note especially their summation: pp 282-287] They also assert quite strongly that since these studies are just beginning, future discoveries could completely alter the picture as they see it. In addition, the writing, especially considering the material they are discussing, is surprisingly easy to read and understand and totally lacking the 'jargon' so prevalent in today's academic world. I highly recommend this book to anyone who really wants to consider the implications of their thesis.

This is one of the most astonishing books I have ever read. Its thesis, that complex life similar to life on the Earth, is probably very rare in the galaxy, if not the universe. The authors meticulously build a case for Earth's special place in the cosmos, showing how bacterial life might be common but that anything bigger than simple organisms might not be able to survive the ravages of time and comets and wandering axial tilt. As other readers have pointed out, this doesn't necessarily mean that silicon-based life or some other kind of life can't exist in the universe, but it does demonstrate that Fermi's Dilemma might in fact be true. As a science fiction writer, I was stunned at how much this book could change the way science fiction is written. No more Star Trek universes with complex life in every system, no more "federations" or "foundations" or "assemblies" of thousands of inhabited worlds. According to the thesis Ward and Brownlee present, habitable worlds might be few and far between. Humans may very well find no other worlds beyond the Earth upon which to live. RARE EARTH will make it difficult to read books or watch movies that envision a galaxy filled with wonderful and curious creatures, where humans spread out across the galaxy as if they were so many islands in Polynesia. And if the Rare Earth hypothesis is true, then we are also obliged to get our act together here and now if we are going to become a space-going species. I think this book puts a lot of science fiction writers out of business. It certainly makes a lot of science fiction, including my own, seem downright silly.

This is certainly a great book. It is bound to age quickly since most of the book is based on really ground-breaking discoveries and propositions that are many times challenged shortly after their publication. The book presents a paradoxical theory that says that life is widespread on the universe but most of the times it doesn't go much beyond the very simple organisms on the first evolutionary steps.

Complex life as we see on earth, the book says, must be extraordinary rare if not unique. The idea is revolutionary and the book is written in a very compelling way.

The idea certainly goes against the mainstream challenging consolidated ideas like the Drake Equation. The Drake Equation, devised by Dr. Frank Drake in 1961 is a famous way to estimate the number of intelligent civilizations able to communicate within our own galaxy. This important estimate is one of the pillars for projects like the SETI (Search for Extra Terrestrial Intelligence).

One of the most interesting ideas presented in the book in my opinion is the possibility of exchange of microorganisms between planets and moons within our solar system. Meteor or comet impacts in planets would eject rocks that could carry live microorganisms to neighbor planets. This would happen more often in smaller planets subject to smaller gravitational force and with thinner atmospheres like Mars. Life as we see it on earth, the book says, might actually have started in Mars. However the book doesn't consider the artificial process of seeding life what is for me the lost factor on Drake's equation, the colonization factor. An advanced civilization, even a unique one, could very well have spread life on the universe bypassing the first evolutionary steps and making complex life feasible even on planets that are for a shorter period of time at the solar system habitable zone. If nobody did it before we, humans, might very well start doing in the near future.

Many are, according to the book, the reasons why earth displays such a lush biodiversity. Things that we many times take for granted like plate tectonics, Earth's magnetic field, the presence of other planets like Jupiter and Mars, the size and stability of our Sun and Earth's basic composition are paramount to our very existence. The odds of reproducing such favorable conditions are, according to the book, slim.

The book covers a wide range of subjects from astronomy to biology passing through geology, paleontology, climatology, and many other ologies. But don't panic, the book is written in a way that you can read it and enjoy it even if you lack the basic knowledge in one or more of the subjects covered. This is done by constantly restating the key points presented in every chapter.

This is a fascinating book on a fascinating topic. The biggest problem is that we try to study or even predict the nature of life in other planets based on our unique sample, Earth. As any statistician would agree it's hard to make good statistics on only one sample.

Leonardo Alves, November 2000

Astrobiology, the subject of this excellent book, is a science still in eager anticipation of its first object of contemplation. Professors Ward and Brownlee from the University of Washington, the former a geologist, the latter an astronomer, argue very strongly that such an object will not be what we would call an animal or a metazoan. Certainly the word "intelligent" will not apply to the first extraterrestrial. Their thesis is that our earth is indeed rare, the product of innumerable rare events and unusual conditions, so much so that we should expect to find similar circumstances elsewhere only very rarely. Acknowledging the recent discoveries of extremophiles, single-celled organisms that can live in very harsh environments, the authors aver that microbial life, on the other hand, should be plentiful. They believe, nonetheless, that the leap from microbes to complex life forms is a very rare event indeed.

Their argument is based on all the things that can go wrong. They cite a formidable litany of hindrances including the need for a planet to be the right size at the right distance from a star of the right size and composition, to the need for plate tectonics (for biotic diversity and the recycling of materials) and a large moon (to stabilize the planet's tilt and temperature), to a Jupiter to sweep the system relatively free of deadly planet-crossing bodies, and especially to a very long time period in which to evolve, etc.

Their argument is tantalizing and difficult to fault. We have only one example of the rise of complex life, and it seems reasonable to conclude that the factors that allowed this life to evolve are necessary. But I am troubled. If I look at the factors that led to my birth, my birth as exactly me, the whole process seems rather miraculous. If my parents had never met, if my mother had had a headache that night, if, in fact, she or my father had behaved in any way differently from exactly how they did behave during the time up and including my conception, another little sperm might have fertilized the egg and I would not exist. Or, take a hand at bridge. If you are dealt 13 spades, you can be pretty sure that something fishy is going on since the odds against getting exactly 13 spades are astronomical. However, the odds against being dealt any other exact hand at bridge are also astronomical, in fact the odds are identical. But some combination of cards must be dealt! So the fact that the development of complex life on this planet seems miraculous in the sense that everything had to be just right for it to develop may be a misconception. I think it's akin to the anthropic principle. If our solar system were different perhaps some other creatures would be here rhapsodizing over just how miraculous all the coincidences were that produced them!

The authors, of course, are talking about carbon-based "life as we know it." Since it is massively difficult to speculate on life as we don't know it, especially when considering that 90% of the universe, the so-called mysterious dark matter, is still totally beyond our ken, they are wise to qualify the argument. I would add that life may take on forms and modes that we wouldn't recognize as being "alive." It may be, for example, that the stars are "alive." We know that they are born, they grow, reproduce and die. They spew their seeds, the heavy elements out into space where they attract other elements and form new stars. Such life would be "life as we wouldn't recognize it." There is also the possibility that life may take on forms and modes totally beyond our comprehension, giving us "life as we can't possibly know it." An example would be my friends "the energy beings" who live, reproduce and disappear in a small fraction of a nanosecond, too quick for us ever to be aware of them.

Yes, one may, I hope, speculate. Certainly that is what the authors are doing here. Regardless of how carefully they conform to what is currently known in a scientific sense, and regardless of how carefully those speculations are expressed, they are postulating from a sampling of one. But the authors know this. On page 282 they write, "The great danger to our thesis...is that it is a product of our lack of imagination. We assume in this book that animal life will be somehow Earth-like. We take perhaps the jingoistic stance that...lessons from Earth are not only guides but also rules." Nonetheless they say that "evidence and inference" lead them to believe that the earth is very rare indeed.

There is some redundant repetition from chapter to chapter, which can happen with two authors, and there are the usual "spell-checker" typos, i.e., "out" for "our," etc., that occasionally mar the text, but no more than usual. On page 267, however, they estimate that there are between 200 and 300 million stars in our galaxy. Of course the number is more like 100 billion. They probably meant that there are 200 to 300 million "sun-like" stars in our galaxy. These quibbles aside, I think that the significant value of this book is that, in the process of arguing for their rare earth, Ward and Brownlee are able to share with us their formidable understanding of the latest findings in a number of sciences, especially geology, biology and astronomy, and apply that knowledge to astrobiology. The material on tectonics and mass extinctions is especially interesting. Their recreation of the impact and aftermath of the object that hit the earth at the end of the Cretaceous is strikingly vivid. More than this, it is their enthusiasm in presenting the material that makes this excellent book so very interesting and readable.

Rare Earth is a fascinating book that argues that intelligent life is rare in the universe. The value of this book does not lie in the cogency of its arguments but rather in the panoply of interesting facts and theories the authors marshal to argue their point. The authors contend that simple unicellular organisms are ubiquitous in the universe, if not potentially in our solar system. They base their argument on the Draconian environments here on earth where extremeophiles survive. They contend, however,that only a set of unique circumstances could allow unicellular organisms to develop into complex metazoans. The unique conditions include the position of a planet in a solar system, the position of a solar system in a galaxy, and the close proximity of both a large planet and a large moon. Furthermore a planet requires earth-like plate tectonics and maybe a couple of ice ages thrown in that dwarf our previously known ice ages in severity and longevity.(They describe the new theory of "snowball earth"). The weakness of their argument lies in their emphasis on the conditions necessary for life rather than the process. The formation of a complex molecule, RNA, necessary for life, and DNA, necessary for reproduction, might be a far more difficult process than the evolution of microbes to complex organisms. They also make their argument from a strictly anthropic point of view,ignoring the possibility of completely different forms of life. It seems axiomatic that the more complex the life form becomes in any habitat,the more specialized the habitat must become to sustain that life. In any case, intelligent life can be extraordinarily rare but be far from unique due to the vastness of our universe. New theories of inflation, especially chaotic inflation, suggest that the visible universe with its one hundred billion galxies is merely a speck in the much larger cosmos. In summary, even though I don't always agree with the authors, this book is too interesting and informative not to read.