American Scientist

Rare Earth: Why Complex Life Is Uncommon in the Universe. Peter D. Ward and Don Brownlee. 336 pp. Copernicus, 2000. $27.50.

The Emergence of Life on Earth: A Historical and Scientific Overview. Iris Fry. 256 pp. Rutgers, 2000. $24 (paper).

Darwin never really discussed the actual origin of life in his famed book On the Origin of Species. He was more concerned with the mechanisms of evolutionary change as seen in the diversity of life today, as well as the few trophies of nature found in fossils. His next most (semi)popular volume also would not examine this question but instead dealt a harder blow to the ego of humanity by putting us under the microscope of evolutionary detection. Contemporary writers, armed with more information and separated from churchly matters, are in close reach of answering, or at least understanding more fully, how life originated.

Peter Ward and Don Brownlee's Rare Earth: Why Complex Life Is Uncommon in the Universe is a stellar example of clear writing on a complex issue. For to ask "What is life?"or about the astronomical and geological forces that helped or hindered life's origin requires patience with the intended audience.

The authors offer "a long grocery list of ingredients seemingly necessary to make a planet teeming with life." The first is the formation of the planet itself, its location in a solar system (its potential habitable zone), its debris-catching neighbor (in our case Jupiter) and even its location in the galaxy, where "celestial catastrophes" like supernovae, impacting bodies, extreme radiation and heat would prove fatal for any beginning life, never mind its ability to sustain itself.

In our solar system are examples of lifeless planets. And we can see why. Proximity to the sun, rotational axis, orbital motion and large-scale impacts—all can prove detrimental. Impacts were rather common in the early solar system. Until most of the impacting agents subsided, life did not have a chance to take off. (How many other planets had the right stuff early on in their evolution but were globally wiped out?) Yet some of these early colliding agents may have, in fact, contributed some of the necessary biochemical seeds of life. These seeds may have been amino acids, from which proteins evolved. Then the problem of complexity arose, more commonly referred to as the chicken-and-egg conundrum: Proteins had to be already present to assemble the molecules whose job it is to assemble proteins in the first place. Ward and Brownlee favor a hypothesis with an RNA catalyst forming first. Yet an RNA scenario would limit the choices of origination, because RNA is more temperature-sensitive than DNA. The RNA-first idea would rule out such suspects for the origination of life as thermophilic microbes like those found near hydrothermal vents. Rather, mesophiles, organisms that accept warm but not hot temperatures, would be more suitable. Eventually, as complex genes developed, all three taxonomic domains—Archaea, Bacteria and Eucarya—emerged. Realizing that eukaryotic-cell development took time, on the order of a billion years, the authors note that "the jump from single-celled organisms to organisms of multiple cells requires numerous evolutionary steps," even more so for animals. Not to mention alterations in atmospheric conditions, specifically the shift toward the higher ratio of oxygen similar to today.

Ward and Brownlee summarize many of the current lines of early biotic research, particularly the consequences of the near-fatal global glaciations 2.4 billion years ago and 800–650 million years ago, "when Earth teetered dangerously close to becoming too cold for any life." The challenges our planet has faced throughout its history should also provide exemplary "challenge[s] to astrobiology." For example, was the Cambrian Explosion, a biological event some 600 million years ago that saw the rapid emergence of all the phyla we see today (perhaps even more), "inevitable once a certain level of biological organization had evolved"? The fact that it took 3 billion years from the emergence of life to reach this level of multicellular organization suggests "that forming animal life is a much more difficult ? project than the initial formation of nonanimal life," considering all the hoops and near-fatal consequences.

Maintaining diversity, however, is still a tricky business, as seen in mass extinction. Admittedly, mass extinction may have provided avenues or spaces for previously submissive taxa to radiate, but, as Ward and Brownlee report, the near global calamities are obvious. Among the culprits: the famed asteroid/comet impact scenario, minor alterations in the axial spin of the planets, energy output of the sun, radiation emissions, ice and runaway greenhouse gases. Thus, the events—astronomical, geological (including plate tectonics, apparently unique to our planet) and biological—tabulated require sober realization that higher forms of animal life are rare.

In reformulating the famous Drake equation, which tabulates the mathematical potential of intelligent life in the universe, Ward and Brownlee have their own "Rare Earth Equation," which puts more restrictive parameters on the existence of higher animals on other worlds. Each component in the equation is multiplied after the other so that if each component nears zero, the entire equation will have a lower value. Through our galactic, planetary, geological and biological history, we may have emerged, through time, as the only voice in the stellar choir.

In The Emergence of Life on Earth, Iris Fry's focus is a little different—more biological, less astronomical and geological. The author contends that "life based on carbon and water is anything but a rare phenomenon" and stays away from the issue of higher or lower life forms.

From the Greeks to Aquinas, the emergence of species was a nonissue, according to the church and the notion of spontaneous generation. With spontaneous generation, no matter how much the natural philosophers poked and prodded, an intelligent designer was still behind the cause. Although Darwin implied greatly as to a mechanism of species transmutation, the issue of spontaneous generation would not see its downfall until the experimentation of Pasteur in the latter half of the 19th century. With this, the possible mechanisms of life's origin remained in limbo but only for a short time. The publications of Alexander Oparin and J. B. S. Haldane in the 1920s "proposed for the first time," Fry notes, "specific hypotheses about the geographical conditions on an ancient Earth and the constituents of the early atmosphere that made this synthesis possible." Oparin, by understanding colloid chemistry, found that when certain polymers reach a critical level, two distinct reactions occur. As microdroplets form, they gather more substances and "a sort of primitive metabolism" takes place. The droplets grow and eventually divide. Down the evolutionary line, "while the first creatures exploited the chemical energy stored in organic substances in the environment, those that followed were forced to rely on alternative means to produce energy," a form of natural selection. Haldane proposed that the prebiotic chemical soup was stirred and changed through ultraviolet radiation.

The origin-of-lifers got a boost in the 1950s with the work of Stanley Miller, who was able to synthesize organic molecules in a laboratory version of the primordial world. But were the originators of life proteins or replicating molecules? Like Ward and Brownlee, Fry, in much more exhausting detail, describes the chicken-and-egg problem, the evolution of possible solutions, and, coming to the fore, the so-called RNA world hypothesis.

It is at this point that Fry hints at matters seemingly external to the issue of the scientific pursuits in the origin-of-life question. Whereas Ward and Brownlee used earth history to an astrobiological end, Fry notes that as the issue of the origin of life became more possible to discover, hence more publicly known, the social implications, primarily the religious overtones, emerged as relevant. The chicken-and-egg problem has provoked many creationists to label this as a crisis in science, whereas scientists "consider it a challenge that calls for new ideas about the mechanisms responsible for the emergence of life under prebiotic conditions."

Fry then outlines several developments that include the inorganic "scaffolding" model from which organic molecules could have arisen and the recently discovered ribosymes that "catalyze the cutting and joining of segments of RNA." This latter discovery may constitute some of the nuts and bolts of the RNA world.

The final third of The Emergence of Life on Earth examines sociological and religious implications. Despite the pursuit's being unfathomable to a creationist who believes the "intelligent design" doctrine, the search for how life emerged requires a scientific methodology, and testing empirical evidence promotes the search for knowledge.

The Emergence of Life on Earth does not describe the roles external forces played in evolution, like those outlined by Ward and Brownlee, but this may be an unfair comparison. And even if Fry has provided far more detail than the general reader can consume, the intensity of the research outlined, the novel solutions and the sociological implications won't be underestimated by anyone. These are two worthwhile journeys back to the beginning of life.