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BOOK REVIEW

The Never-Ending Story

Antonio Lazcano

Origins of Existence: How Life Emerged in the Universe. Fred Adams. vi + 266 pp. The Free Press, 2002. $25. (Forthcoming in paper in October from Pi Press under a new title, Our Living Multiverse; $14.95.)

Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Andrew H. Knoll. x + 277 pp. Princeton University Press, 2003. $29.95.

The Origins of Life and the Universe. Paul F. Lurquin. xiv + 217 pp. Columbia University Press, 2003. $59.50, cloth; $27.50, paper.

"The gods had condemned Sisyphus to ceaselessly rolling a rock to the top of a mountain," wrote the French philosopher Albert Camus, "whence the stone would fall back of its own weight. They had thought with some reason that there is no more dreadful punishment than futile and hopeless labor." Like Sisyphus, those trying to discover the origins of life and study the earliest stages of biological evolution have an uphill quest: Over and over it happens that a theory or explanation believed to be well established has to be abandoned or rethought in the light of new findings.

There are several examples of this: Half a century ago Stanley L. Miller demonstrated that amino acids and other biochemically significant compounds could be easily synthesized under conditions simulating those of the primitive atmosphere. But nowadays geoscientists tend to doubt that the conditions of the prebiotic environment were as reducing as Miller and others had assumed. A second example is that the startling discovery of the catalytic properties of RNA molecules helped overcome the chicken-and-egg problem posed by the relationship between proteins and DNA. However, it is now generally accepted that RNA is a frail polymer unlikely to have undergone prebiotic synthesis and accumulation. A third instance is that paleontological evidence seemed to push the antiquity of life back to the earliest Archaean times. But several authors have contested the biological origin of the structures and chemical signatures described in the early Archaean Australian Warrawoona formation and the similarly ancient Akilia island rocks, respectively. Finally, the molecular traits that eukaryotes and heat-loving extremophiles share appeared initially to point toward an archaebacterial origin for the nucleocytoplasm. But as more and more completely sequenced cellular genomes have become available, their analysis has shown that an extensive amount of horizontal transfer of genes occurred, leading many to wonder whether we will ever untangle the weblike phylogenies of early cell evolution.

Despite these various setbacks, interest in how life came into being shows no sign of diminishing, as demonstrated by the ample rate of publication of new books on the subject, including the volumes reviewed here, all of which address the transition between nonliving matter and the earliest biological systems. They differ significantly in depth, emphasis and scope.

In this microbial ecosystemClick to Enlarge Image

Life on a Young Planet, by paleontologist Andrew Knoll, focuses on the first three billion years of biological history on our planet, with considerable emphasis on the information provided by the fossil record. By contrast, physicist Fred Adams and geneticist Paul F. Lurquin attempt in their respective books to describe more grandiose schemes: They start with physics and the Big Bang, continue through the origin of life on Earth and conclude, almost inevitably, with chapters on the purpose of it all.

There is much to admire in Adams's Origins of Existence and Lurquin's The Origins of Life and the Universe. Both are engaging, well-written books that provide considerable information on fundamental concepts of modern physics. However, surveys of the entire history of the universe tend to lean on sweeping generalizations and to present a lineal progression that cannot but elicit occasional doubts.

The take-home message of Origins of Existence is that "Life does not arise by random arrangement of amino acids but rather by some physical process that drives the system toward complexity." This is a view more popular among physicists than among life scientists; presentations of it tend to be glossy and promotional, and they usually fail to take into account the epistemological uniqueness of the methodologies employed by evolutionary biology, which examines systems whose complexity is well beyond that of systems traditionally studied by physics. Adams's book shares those characteristics. His is a lengthy narrative that traces the history of life to the appearance of the first atoms of hydrogen soon after the Big Bang, continues with formation of other key elements such as carbon, oxygen, nitrogen and sulfur in the cauldron of stellar interiors or in supernova explosions, and culminates with the emergence of complex life-forms; change and continuity define this story line. But enhanced complexity is not evidence of development along fixed lines—and one should not ignore the differences in the nature of the evolutionary processes in each stage.

Despite the obstacles to understanding the origin of life, there has been no shortage of discussion about how it took place. Based on the results of drilling projects that have demonstrated the existence of microbial communities far underground, Adams favors the view that life evolved, not in a primordial pond on the Earth's surface, but rather, deep inside our planet. Assuming that other bodies in our solar system and elsewhere may harbor similar environments, Adams argues that "life is likely to emerge and thrive in these common habitats."

There are, of course, alternative viewpoints. It is possible, for instance, that the microbes found in a wide variety of harsh environments, including subterranean hydrogen-powered ecosystems, tell us more about the surprising adaptability of life to a wide range of conditions than about the emergence of the first biological systems.

This more conventional description of the emergence of life is advocated by Lurquin. As its title suggests, The Origins of Life and the Universe is divided into two halves, but they are well connected with chapters that discuss at some length and with considerable clarity the prebiotic environment and some of the basic results of laboratory simulations of the primitive Earth.

It is a pity, however, that the book was not checked for chemical accuracy. The claim that cyanoacetylene can react with hydrogen cyanide "to form nitrogenous bases found in RNA" is inexact. And in a diagram, the synthesis of adenine from hydrogen cyanide, which has been described by Gerald Joyce and Leslie Orgel as "the rock of faith" of prebiotic chemistry, is erroneously presented as the outcome of the oligomerization not of five but of six molecules of hydrogen cyanide.

Despite such inaccuracies (and a slightly out-of-place chapter in which the teachings of the ill-famed Raelians—members of a sect that believes human life was first created in the laboratory by people from another planet who had mastered genetics and cell biology—are used to introduce the panspermia hypothesis), this is a clear, well-written text that is enjoyable to read. It reflects not only Lurquin's longtime interest in the origin and evolution of cells, but also his lengthy experience as a teacher. His students have been lucky.

Knoll's Life on a Young Planet takes up where Lurquin leaves off. Largely based on Knoll's direct personal experience as one of the world's foremost experts on the Precambrian fossil record, it is a balanced, excellent account of current theories and discussions of the origin and early evolution of life. A distinguished scientist and Harvard professor, Knoll is able to convey difficult scientific issues with a minimum of jargon, using a brisk and witty prose full of literary references that reflect his American education. It is perhaps not surprising that the views and proposals made by scientists working in the United States tend to be overrepresented in his book. But as shown by the manifold references not only to senior investigators but also to their students, postdocs and other associates, Knoll's sense of fairness is beyond doubt. He is a gifted storyteller with a knack for choosing the right anecdote. These tales enrich the book's unity of purpose and consistency of theme and demonstrate repeatedly that the writer is an active scientist in the field and a mainstream interpreter of events.

"If my theory be true," wrote Charles Darwin in The Origin of Species, "it is indisputable that before the lowest Silurian [that is, Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures." What thrived during those lengthy epochs was a microbial biosphere that rapidly diversified into major domains and several kingdoms, occupied all available ecological niches, developed gene transfer and sexuality, gave origin to symbiotic associations that evolved into nucleated cells, separated into protists with strange lifestyles and eventually gave rise to animals, plants and fungi—all of which have left traces in the sediments.

A browse through Knoll's book will enlighten both the cognoscenti and those unfamiliar with the complexities of reading a fossil record. As Knoll argues, proper understanding of bygone Precambrian organisms and their communities requires an eclectic approach combining traditional paleontological techniques with geochemical evidence, studies of extant metabolic pathways and comparative morphologies, and molecular phylogenies of both genes and genomes. Life on a Young Planet is written with such skill that each section throws light on the others. Knoll manages to present a multidisciplinary field in an interdisciplinary volume. The illustrations and photographs are excellent. However, the reconstructions included here are but pale shadows of the complexity of the primitive environments and the conditions into which life appeared and evolved.

These three books provide insights into our planet's infancy and the conditions that led to the appearance of life and its extraordinary divergence, but much information has been lost forever. Like Sisyphus, we may never find respite at the top of the hill, but as Paul Lurquin reminds us in a footnote, Camus concluded his essay by stating that "The struggle itself toward the heights is enough to fill a man's heart. One must imagine Sisyphus happy." As the elegant writing and optimistic tone of these books suggest, Camus probably had it right.

 

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