Almost as controversial as the "how" and the "how long" of attaining life is the "where." In what physical environment did the first life on Earth originate? Answering the question of where is also an important aspect of assessing the possibility and frequency of life on other planets.
The first, most famous, and longest-accepted model was proposed by Charles Darwin, who in a letter to a friend suggested that life began in some sort of "shallow, sun-warmed pond." This type of environment, be it of fresh water or perhaps in a tide pool at the edge of the sea, still remains a viable candidate. Other scientists early in the twentieth century, such as J.B.S. Hal-dane and A. Oparin, agreed with Darwin and expanded on this idea. They independently hypothesized that the early Earth had a "reducing" atmosphere (one that produces chemical reactions the opposite of oxidation,- in such an environment, iron would never rust). The atmosphere at that time may have been filled with methane and ammonia, forming (because it was filled with the chemicals necessary to create amino acids) an ideal "primordial soup" from which the first life appeared in some shallow body of water. Until the 1950s and into the 1960s, it was thus believed that the early Earth's atmosphere would have allowed commonplace inorganic synthesis of the organic building blocks called amino acids by the simple addition of water and energy, as shown in the famous experiments of Miller and Urey in 1952. All that was needed was a convenient place for all the various chemicals to accumulate. The best place for this seemed to be a fetid pond or a wave-lapped tide pool at the edge of a shallow, warm sea.
Yet as we learn more about the nature of our planet's early environments, tranquil ponds or tide pools seem less and less likely to be plausible sites for the first life—or even to have existed at all on the surface of the early Earth. What Darwin could not appreciate in his time (nor could Hal-dane and Oparin, for that matter) was that the mechanisms leading to the accretion of Earth (and of other terrestrial planets) produced a world that, early in its history, was harsh and poisonous, a place very far removed from the idyllic tide pool or pond envisioned in the nineteenth and early twentieth centuries. In fact, we now have a very different view of the nature of the early Earth's atmosphere and chemistry. It is widely believed among planetary scientists that carbon dioxide, not ammonia and methane, dominated the earliest atmosphere and that the overall conditions may not have favored the widespread synthesis of organic molecules on Earth's surface. It seems more reasonable that the rain of asteroids and comets delivered these compounds essential for life.
But if not in a pond or tide pool, where could these components have come together to produce life? Here is an alternative view from microbiolo-
gist Norman Pace, one of the great pioneering microbiologists interested in life's evolution:
We can now imagine, based on solid results, a fairly credible scenario for the terrestrial events that set the stage for the origin of life. It seems fairly clear, now, that the early earth was, in essence, a molten ball with an atmosphere of high-pressure steam, carbon dioxide, nitrogen, and other products of volcanic emissions from the differentiating planet. It seems unlikely that any landmass would have reached above the waves (of a global ocean) to form the "tide pools" invoked by some theories for the origin of life.
Pace looks for an entirely different setting—one of heat and pressure, such as in the deep-sea volcanic vents.
The "where" of life's origination is obviously controversial, and as pointed out by University of Washington astronomer Guillermo Gonzalez, the favored habitats appear to depend on a given scientist's discipline. In his delightful 1998 essay "Extraterrestrials: A Modern View," Gonzalez noted,
The kind of origin of life theory a scientist holds to seems to depend on his/her field of specialty: oceanographers like to think it began in a deep sea thermal vent, biochemists like Stanley Miller prefer a warm tidal pool on the Earth's surface, astronomers insist that comets played an essential role by delivering complex molecules, and scientists who write science fiction part time imagine that the Earth was "seeded" by interstellar microbes. The fact that life appeared soon after the termination of the heavy bombardment about 3.8 billion years ago tells little about the probability of the origin of life—it could have been a unique event requiring extraordinary conditions. However, there are a few very basic ingredients that are required by any conceivable kind of life, overactive imaginations notwithstanding.
Our vision of the "cradle of life" has obviously changed since Darwin's time. How do scientists now envision planet Earth at the time of life's first appearance? Even around 4 billion years ago, about 500 million years after initial accretions, Earth would have seemed a very foreign world to us. For instance, there was little land area, because there were few or no continents. Volcanism and the eruption of lava from the interior of the planet, however, were far more common than today. The deep-sea ridges, places where new oceanic crust is created on the sea floor, are estimated to have been three to five times longer than they are today, and hydrothermal activity along these ridges may have been as much as eight times greater than in the present-day world. All of this suggests a very energy-rich, volcanic world, with huge amounts of deep-earth chemicals and compounds spewing forth in the oceanic environment. The chemistry of seawater would have been enormously different than it is now. The ocean was what we would call "reducing" (in contrast to the present-day oxidizing oceans) because there was no free oxygen dissolved in the seawater. The temperatures of the oceans may have been far higher than today, ranging from warm to hot—perhaps hot enough to scald us if we were there. Finally, there may have been 100 to 1000 times as much carbon dioxide in the atmosphere as there is today.
The extremophiles may thus yield the most important clue uncovered to date. Darwin and de Duve imply that life originated on Earth's surface (although de Duve hedges a bit on this question with his comment that environments within the planet may be involved as well). Yet most views of Earth's surface at the time of the first formation of life paint a very bleak picture. Lethal levels of ultraviolet radiation polluted the surface, and the impacts of giant comets with Earth periodically vaporized the planet's oceans. The boiling of the seas would have repeatedly sterilized Earth's surface. But what about beneath the surface, in the subterranean regions now inhabited by the extremophilic archaeans and bacteria? These deep Hadean environments may have served as bomb shelter-like refuges, protecting deep extremophiles from the fury at the planet's surface. Could the deep subsurface have served not only as refuge but also as cradle of life early in Earth history? New analyses of the "Tree of Life," or phylogenetic history of life on our planet, support this possibility. But before we examine the Tree of Life and what it tells us, we need to consider one more possible origin of life on Earth.
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