Much of this molecular diversity occurs because electric sparks and ultraviolet radiation trigger the formation of highly reactive chemical species, such as hydrogen cyanide (HCN) and formaldehyde (CH2O), which readily link to other molecules. Miller suspected, for example, that most of the amino acids produced in his experiments arose by a chemical process known as Strecker synthesis, in which hydrogen cyanide reacts with formaldehyde and ammonia.
Enthusiasm grew as other scientists discovered promising new chemical pathways. In 1960, John Oró of the University of Houston turned scientific heads when he discovered that a concentrated hydrogen–cyanide solution, when heated, produced lots of adenine, one of the missing purines and a crucial biomolecule that also plays a role in metabolism. Other chemists conducted similar experiments, starting with relatively concentrated solutions of formaldehyde (CH2O), a molecule thought to be common in some prebiotic environments. These experiments produced a rich, though random, variety of sugars, including a modest yield of ribose. Gradually, through such specialized experiments, gaps in the prebiotic inventory of life's molecules were filled in.
The experiments of Oró and others, relying as they did on relatively concentrated solutions of reactive organic molecules, raised some eyebrows. The Miller-type spark experiments by themselves had never yielded hydrogen cyanide in sufficient concentrations to produce much adenine, or enough formaldehyde to make much ribose. But discoveries in the mid-1960s in the lab of Salk Institute pioneer Leslie Orgel pointed to a plausible fix. The early Earth was not uniformly hot; it probably had ice caps and may have periodically experienced more extensive “ice ages,” as well. Orgel realized that such conditions might promote intriguing organic reactions. When an organic-rich water solution freezes, pure water ice crystals grow, while the dwindling reservoir of residual liquid becomes an increasingly concentrated organic brine (and recall that a higher concentration of “agents” may facilitate emergence).
Orgel and co-workers exploited this idea by slowly freezing flasks of dilute HCN solutions to -20°C. The procedure produced tiny volumes of extremely concentrated hydrogen cyanide, which reacted over weeks to months to produce small linkages of up to four HCN molecules. This curious phenomenon became the inspiration for one of the longest experiments in the history of origins research. Sometime in the mid-1970s, Miller, now a professor at the University of California, San Diego, and his co-workers repeated the Orgel protocol and stored their frozen flasks in the back of a freezer. In the late 1990s, more than two decades later, they removed the frozen solutions, which had developed curious dark concentrated clumps that were rich in organics. Analysis revealed an abundant production of adenine. It takes a lot of time for reactions to proceed at ultracold temperatures, but the primitive Earth had time to spare.
These remarkable results seem to defy convention: Heat, not cold, normally drives chemical reactions. You don't make a cake by freezing batter. Nevertheless, additional freezing experiments have produced amino acids and other interesting biomolecules by this counterintuitive process of concentration. In this curious way, prebiotic cycles of freezing and thawing may have enhanced the emergence of biomolecules and thus provided a pathway to life.
DOUBTS
As exciting and important as the Miller–Urey results may be, seemingly intractable problems remain. Within a decade of Miller's triumph, serious doubts began to arise about the true composition of Earth's earliest atmosphere. Miller and Urey had exploited a highly reactive atmosphere of methane, ammonia, and hydrogen, which seemed a plausible early atmosphere to them. But by the 1960s, new geochemical calculations along with data from ancient rocks pointed to a much less reactive early atmosphere of nitrogen and carbon dioxide, two gases that do almost nothing of interest in a Miller–Urey apparatus.
Miller and his supporters continue to counter with a pointed argument, difficult to dismiss. Life's biomolecules match those of the original Miller–Urey experiment with great fidelity, they say. Doesn't that fact alone argue for an atmosphere rich in reactive methane? Harold Urey is said to have often quipped, “If God did not do it this way, then He missed a good bet.” Nevertheless, most geochemists now discount the possibility of more than a trace of atmospheric methane or ammonia at the time of life's emergence.
Added to this atmospheric concern is the fact that the molecular building blocks of life created by Miller and his colleagues represent only tiny steps on the long road to life. Living cells require that such small molecules be carefully selected and then linked together into vastly more complex structures—cell membranes, protein catalysts, DNA, RNA, and other so-called macromolecules. The prebiotic ocean was an extremely dilute solution of many thousands of different organic molecules, most of which play no known role in life. By what emergent processes were just the right molecules selected and organized?
The Miller–Urey scenario suffers from yet another nagging problem. Macromolecules tend to fragment, rather than form, when subjected to the energetic insults of lightning and the Sun's ultraviolet light. These so-called ionizing forms of energy are great for making reactive molecular fragments that combine into modest-sized molecules like amino acids. Combining many amino acids into an orderly chainlike protein, however, is best accomplished in a less destructive energy domain. Emergent complexity relies on a flow of energy, to be sure, but not too much energy. Could life have emerged in the harsh glare of daylight, or was there perhaps a different, more benign origin environment?
Faced with such an impasse, a few maverick scientists began to look at other plausible venues for the cradle of life.
7
Heaven or Hell?
It is we who live in the extreme environments.
Thomas Gold, The Deep Hot Biosphere, 1999
For centuries the primary source of life's energy has been as well established as any precept in biology. Every high-school textbook proclaims what we all have accepted as intuitively obvious: All life depends ultimately on the Sun's radiant energy. Nor has there been reason to doubt that claim until recently. But new discoveries of deep life—life-forms at the darkest ocean depths and microbes buried miles beneath Earth's surface in solid rock, forever beyond the Sun's influence—have toppled this comfortable certainty.
If science has taught us anything, it's that cherished notions about our place in the natural world often turn out to be dead wrong. We observe that the Sun rises in the morning and sets at night. An obvious conclusion, reached by almost all observers until relatively recently in human history, is that the Sun circles the Earth. Yet we now know that sunrise and sunset are consequences of Earth's rotation; Earth orbits the Sun, and we are not at the physical center of the universe. We observe mountains and oceans as grand unchanging attributes of the globe—on the scale of a human life, these features are for all intents and purposes permanent. Yet we have learned that through the inexorable processes of plate tectonics, every topographic feature on Earth is transient over geological time and that our war-contested political boundaries are destined eventually to disappear.
The great power of science as a way of knowing is that it leads us to conclusions about the physical universe that are not self-evident. Repeatedly, the history of science has been punctuated by the overthrow of the obvious. Could our intuitive view of life's original energy source be in error as well?
ENERGY
All living cells require a continuous source of energy. Without energy, organisms cannot seek out and consume food, manufacture their cellular structures, or send nerve impulses from one place to another. Lacking energy, they cannot grow, move, or reproduce. Reliable energy input is also essential to maintain the genetic infrastructure of cells, which are constantly subjected to damage by nuclear radiation, toxic chemicals, and other environmental hazards.
Metabolism, the means by which organisms obtain and use energy, is an ancient chemical process that takes place in every living cell, including all of the tens of trillions of cells in our bodies. Until recently, scientists claimed that
the metabolic pathways of virtually all life-forms rely directly or indirectly on photosynthesis. At the base of the food web, we find plants and a host of one-celled organisms that use the Sun's light energy to convert water and carbon dioxide into the chemical energy of sugar molecules (carbohydrates) plus oxygen. Plants manufacture carbohydrates, such as the starch of potatoes and the cellulose of celery, to build leaves, stems, roots, and other physical structures. They also process sugar molecules to provide a source of chemical energy that powers the plant cell's molecular machinery.
While plants synthesize their own carbohydrates, animals and other nonphotosynthetic life higher up the food chain must find another source of sugar. That's why we eat plants, or eat animals that eat plants. Plants synthesize sugar molecules and oxygen from water plus carbon dioxide. Our bodies convert sugar molecules along with the oxygen we breathe to produce water plus the waste gas carbon dioxide. There's an elegant chemical symmetry to this story; the biological world seemed much simpler when the Sun was life's only important energy source.
DEEP ECOSYSTEMS
Our view of life on Earth changed forever in February 1977, when Oregon State University marine geologist Jack Corliss and two crewmates guided the submersible Alvin to the deep volcanic terrain of the East Pacific Rise, 8,000 feet down. This undersea ridge off the Galápagos Islands was known to be a zone of constant volcanic activity associated with the formation of new ocean crust. Oceanographers have documented thousands of miles of similar volcanic ridges, including the sinuous Mid-Atlantic Ridge that bisects the Atlantic Ocean—the longest mountain range on Earth.
On this particular dive, just one of hundreds that Alvin had logged, the scientists hoped to locate and examine a submarine hydrothermal vent, a kind of submarine geyser where hot water jets upward into the cool surrounding ocean. What Corliss and crew discovered was a vibrant and totally unexpected ecosystem with new species of spindly albino crabs, football-sized clams, and bizarre 6-foot tubeworms. One-celled organisms also abounded, coating rock surfaces and clouding the water. These communities, thriving more than a mile and a half beneath the sea, never see the light of the Sun.
In these deep undersea zones, microbes serve as the primary energy producers, playing the same ecological role as plants do on Earth's sunlit surface. These one-celled vent organisms exploit the fact that the cold oxygen-infused ocean water, the hot volcanic water, and the sulfur-rich mineral surfaces over which these mixing fluids flow are not in chemical equilibrium. This situation is similar to the disequilibrium between a piece of coal and the oxygen-rich air. Just as you can heat your house or power machinery by burning coal (thus combining unstable carbon and oxygen to make stable carbon dioxide), so too can these deep microbes obtain energy by the slow alteration of unstable minerals.
The unexpected discovery of this exotic ecosystem was news enough, but Corliss and his Oregon State colleagues soon tried to push the story further. They saw in the vents an ideal environment for the origin of life. Details of this story have become clouded by more than 20 years of sometimes revisionist history. Corliss claims the idea for himself: “I began to wonder what all this might mean, and this sort of naïve idea came to me,” he told an interviewer more than a decade later. “Could the hydrothermal vents be the site of the origin of life?” [Plate 5]
A different history emerges from others close to the story. According to John Baross, a former faculty colleague of Corliss and an expert on microbes in extreme environments, the hydrothermal vent theory of life's origin was first proposed and developed by a perceptive Oregon State graduate student named Sarah Hoffman. She wrote the basic outlines of the hypothesis in 1979, as a project for a biological oceanography seminar taught by Charles Miller, another OSU oceanographer. Hoffman, in frequent consultation with Baross, developed the novel idea as it would appear in print. The two of them claim that the more senior Corliss seized the paper as his own, allowing them, as his coauthors, to expand and polish the prose to conform to the conventions of scientific publishing, after which he submitted the work and placed his name first on the author list. With three coauthors—Corliss, Baross, and Hoffman—the paper would forever be known as “Corliss et al.” Corliss would get the fame, while Hoffman and Baross were effectively relegated to footnote status.
Whoever deserves the credit, the hydrothermal-origins thesis is elegantly simple and correspondingly influential. Modern organisms do, in fact, thrive in deep hydrothermal ecosystems. Fossil microbes recovered from 3.5-billion-year-old hydrothermal deposits reinforce this observation. Even without the energy of sunlight, nutrients and chemical energy abound in hydrothermal systems. The OSU scientists saw hydrothermal systems as “ideal reactors for abiotic synthesis,” and they proposed a sequence of chemical steps for the potentially rapid emergence of life.
The controversial manuscript was not eagerly received; it bounced around for the better part of a year. First it was rejected by Nature, then by Science. At the time, Stanley Miller and his protégés dominated the origin-of-life research game, which had seen more than its fair share of quacks and crackpot theories. They were not about to let such unsupported speculation sully their field. Hydrothermal temperatures were much too hot for amino acids and other essential molecules to survive, they said. “The vent hypothesis is a real loser,” Miller complained to a reporter for Discover magazine. “I don't understand why we even have to discuss it.”
Miller's followers found other good reasons to attack the paper. Corliss and co-workers had the ancient ocean chemistry all wrong, they said. Modern hydrothermal ecosystems rely on oxygen-rich ocean water, whose composition is an indirect consequence of plants and photosynthesis. The prebiotic ocean would not have been oxygen-rich, so the proposed life-sustaining chemical reactions would have proceeded slowly, if at all. The bottom line? Decades of Miller-type experiments confirm what is intuitively obvious: Life began at the surface, so why confuse the issue?
Eventually the Corliss, Baross, and Hoffman manuscript was published, in a supplement to the relatively obscure periodical Oceanologica Acta, a journal that not one in a hundred origin-of-life researchers would see. Nevertheless, good ideas have a life of their own, and copies of the paper, entitled “An Hypothesis Concerning the Relationship Between Submarine Hot Springs and the Origin of Life on Earth,” began circulating. I have seen dog-eared underlined photocopies of copies of copies on several colleagues' desks, and I have a pretty battered copy of my own.
New support for the idea gradually consolidated, as hydrothermal ecosystems were found to be abundant along ocean ridges in both the Atlantic and Pacific. It was realized that at a time when Earth's surface was blasted by a continuous meteorite bombardment, deep-ocean ecosystems would have provided a much more benign location than the surface for life's origin and evolution. New discoveries of abundant primitive microbial life in the deep continental crust further underscored the viability of deep, hot environments. By the early 1990s, the deep-origin hypothesis had become widely accepted as a viable, if unsubstantiated, alternative to the Miller surface scenario.
Of the three authors, only John Baross remains active and influential in the field. In 1985, he accepted a professorship at the University of Washington, where he has developed a leading research program on hydrothermal life. His work on deep-sea-vent microbes, often in collaboration with his wife, Jody Deming, who is also a professor of oceanography at the University of Washington, has placed Baross at the forefront of the highly publicized research field of “extremophile” microbes. Sarah Hoffman's graduate work in geochemistry was interrupted by illness, and, after her recovery, she pursued a singing career. As for Corliss, always a bit idiosyncratic, in 1983 he left Oregon for the Central European University in Budapest, where he worked briefly on the deep-origins hypothesis, but soon took up research in the more abstract field of complex systems. After a 3-year stint as director of research at the controversial Biosphere 2 environmental station in Arizona, he returned to Budapest, having abandoned studies of t
he deep ocean.
LIFE IN ROCKS
Following the revolutionary hydrothermal-origins proposal, numerous scientists began the search for life in deep, warm, wet environments. Everywhere they looked, it seems—in deeply buried sediments, in oil wells, even in porous volcanic rocks more than a mile down—microbes abound. Microbes survive under miles of Antarctic ice and deep in dry desert sand. These organisms appear to thrive on mineral surfaces, where interactions between water and chemically unstable rocks provide the chemical energy for life.
One of the most dramatic and difficult pursuits involves deep drilling for life in solid rock. The oil industry has perfected the practice of deep drilling, thanks to decades of experience and vast infusions of cash. They can penetrate several miles into the Earth, drill at angles and around obstacles, and cut through the hardest known rock formations in their quest for black gold. So the problem for geoscientists looking for microbes a mile or more down isn't how to get there, it's how to get there without contaminating the drill hole with hoards of surface bugs. Bacteria are everywhere—in the air, in the water, and in the muck used to lubricate and cool diamond drill bits as they cut through layers of rock. It's relatively easy to bring up rock cores from a couple of miles down, but those slender cylinders of rock will have already been exposed to surface life by the drilling process. What to do?
The commonest retrieval trick is to add a colorful dye or other distinctive chemical tracer to the lubricating fluid. When drillers extract a deep core, it becomes obvious whether or not the rock has been contaminated in the process. Porous sediments or highly fractured formations soak up the dye and thus prove unsuitable for analysis, but many rocks turn out to be impermeable and thus are ideal for recovering deep life.
Genesis: The Scientific Quest for Life's Origin Page 12