The search for subsurface microbes began in earnest in 1987, when the Department of Energy decided to drill several 500-meter-deep boreholes in South Carolina near the Savannah River nuclear processing facility. As cores were brought to the surface, drillers quickly isolated them in a sterile plastic enclosure with an inert atmosphere. Researchers then cut away the outer rind of the drill core to reveal pristine rock samples, which were shipped to analytical facilities across the country. The results were spectacular. The deep South Carolina sediments were loaded with microbes that had never seen the light of day.
Subsequent drilling studies have revealed that microbes live in every imaginable warm, wet, deep environment—in granite, in basalt on land and basalt under the ocean, in all variety of sediments, and also in metamorphic rocks that have been altered by high temperature and pressure. Anywhere you live, drill a hole down a mile and the chances are you'll find an abundance of microscopic life.
MINING FOR MICROBES
Earth's deep mining and tunneling operations provide the new breed of geobiologist with an invaluable complement to drilling. Mine tunnels have the advantage that researchers can visit microbial populations in their native habitat. Earth's deepest mines, the fabled gold mines of South Africa's Witwatersrand District, have thus become the site of the heroic and potentially dangerous efforts of Princeton geologist Tullis Onstott.
The East Driefontein Mine, located about 60 miles southwest of Johannesburg, is a vast network of underground workings, reaching more than 2 miles into the crust. A small army of miners labor around the clock for gold. Despite one of the largest air-conditioning systems in the world, these deep tunnels remain at an oppressive 140°F from the heat of Earth's interior, while air pressure is twice that of the surface. Onstott learned the dangers of the place on his first descent: “It was ‘Don't step there, don't touch that,'” he told a writer for the Princeton Weekly Bulletin. “All I knew is that it was deep and dark and hot.”
Every so often, as miners blast new adits, a small flow of water appears—groundwater that has spent countless thousands of years filtering down from the surface and has accumulated in small cracks and fissures, nurturing a tenuous ecosystem of microbial life. Onstott's team, typically a half-dozen young and hearty students and postdocs, camp at the surface with a functional array of sterile sample-collection hardware at hand. When news of a fresh water flow comes in, they scramble to the site, though the miles of elevators and tunnels can take almost an hour to traverse. They have to work fast, both to avoid disruption of the mining routine and because prolonged exposure to the hellish conditions can kill them.
They photograph the site, record its location and geological setting, and collect as many gallons of water as possible fresh from the point of flow. They benefit from the seep's positive water pressure, which prevents much back contamination from the miners' activities or their own collection efforts. Exhausted and sweating profusely, they lug the heavy water-filled bottles to the surface for further investigation.
Remarkably, every single sample from Earth's deepest mines holds microbes that have never seen light, surviving on a meager supply of underground chemical energy. Such deep life lives at a sluggish pace that defies our experience. Isotopic measurements reveal that a single cell may persist for thousands of years, “doing” almost nothing before dividing into two. Colonies of organisms commonly remain isolated from the surface for millions of years. So tenuous are the chemical resources of these deep rocks that reproduction and growth are luxuries seldom indulged. By the same token, deep rocks provide an unvarying safe and reliable environment: no predators, no surprises—unless of course a miner happens to blast into your rocky home of a million years!
THE DEEP HOT BIOSPHERE
The abundance of subterranean one-celled creatures, thriving far from the light of the Sun, inspires the imagination and hints at novel scenarios for life's origin. Of all the scientists in pursuit of deep life, none displayed greater imagination than the late brilliant and pugnacious iconoclast Thomas Gold.
Austrian-born Tommy Gold began his scientific career as an astrophysicist in Britain, but in 1959 he was lured to Cornell University to head the Center for Radiophysics and Space Research. He would achieve lasting scientific fame with his inspired theory that pulsars, steady pulsating radio sources discovered in 1967, are actually rapidly rotating neutron stars. Many honors, including election to the Royal Society of London and the National Academy of Sciences, soon followed.
Most scientists would have been content to excel in one chosen area, but Gold throughout his career repeatedly ventured into new and controversial academic domains. In the 1940s, he conducted experiments on hearing and the structure of the mammalian inner ear. Speculative papers on dramatic instabilities of Earth's rotation axis, on steady-state cosmological models of the universe, and on the potential danger to astronauts of deep powdery lunar soils peppered his lengthy curriculum vitae.
In 1977, Gold, by then a safely tenured professor at Cornell, rattled the well-established field of petroleum geology. Geologists had long declared that petroleum is a fossil fuel, formed when huge quantities of decaying cells accumulate over millions of years, to be buried and processed by Earth's heat and pressure. The evidence is overwhelming: Petroleum occurs in sedimentary layers that once held abundant life; petroleum is rich in distinctive biological molecules; petroleum's carbon isotopes also point to a biological source. Armed with these and a dozen other lines of evidence, the case for fossil fuels was open and shut.
Gold disagreed. Petroleum holds lots of distinctive biomolecules, to be sure, but oil fields also contain, along with methane, an abundance of helium gas—a light gas that quickly escapes into space and so could only come from deep within the Earth. How could one reconcile the mixing of deep helium with surface biology? Gold's conclusion: The organic molecules that eventually become petroleum are produced deep underground by purely chemical processes and are then modified by the action of subsurface microbes.
In this scenario, vast sources of primordial hydrocarbons—the major molecular components of oil—exist in Earth's mantle. Because they are lighter than the surrounding rocks, these hydrocarbons slowly but surely rise toward the surface, constantly refilling petroleum reservoirs. In Gold's heretical view, oil is thus a renewable resource instead of a finite one built up over millions of years by the burial and decay of once-living cells. In an extraordinary move, Gold first published this novel idea as an op-ed piece in the June 8, 1977, issue of the Wall Street Journal. An oil-hungry nation in the midst of an energy crisis took considerable notice of the radical hypothesis.
A scientific theory is useful only if it is testable, and Gold soon proposed a test of dramatic proportions. Gold's oil-from-below hypothesis predicts that great oil fields should arise equally in many different types of rock, but all known petroleum has been found in layers of exactly the kind of sedimentary formations that would have collected abundant remnants of past life. Gold countered, logically, that petroleum geologists never look for oil anywhere but in those sedimentary formations. Perhaps, he suggested, immense new oil fields were waiting to be found in igneous and metamorphic rock.
Armed with his provocative theory and a persuasive, dynamic oratorical style, he presented a simple (and expensive) proposal to the Swedish State Power Board in 1983. Drill an oil well in solid granite—the last place on Earth a petroleum geologist would look. He had targeted a unique and tempting granitic mass, the Siljan Ring impact site in central Sweden. This highly fractured granite body, formed 368 million years ago when an asteroid shattered the crust, holds tantalizing hints of petroleum in the form of carbon-filled cracks and flammable methane seeps. Gold's enticing rhetoric, amplified by increasing optimism from Dala Deep Gas, the company formed to do the drilling, lured energy-poor Swedes into spending millions of dollars on exploratory holes.
Seven years and $40 million later, a 6.8-kilometer-deep hole had produced only modest amounts of oil-like hydrocarbons and me
thane gas—a small enough yield for oil experts to say “I told you so,” but large enough to convince Gold that his theory was right. Where else, he asked, could that trace of organic molecules have come from? Nevertheless, most scientists saw the Siljan experiment as a failure, and no one is likely to drill for oil in granite again, at least anytime soon.
The modest production of methane gas and smelly, oily sludge from the Swedish wells inspired Gold to elaborate on his theory. Extrapolating far beyond his peers, he described “the deep hot biosphere” in several articles and a popular 1998 book of that title. The vast, deep hydrocarbon reserves at the heart of Gold's 1977 hypothesis provide a wonderful food source for deep microbes, which coincidently leave their biological overprint on the otherwise abiotic oil. In entertaining prose, he reviewed the growing and accepted body of evidence for deep life in many types of rock—all very reasonable stuff. Gold's conclusion: Deep microbial life, much of it nourished by upwelling hydrocarbons, accounts for fully half of Earth's total biomass. Though living cells represent a tiny fraction of Earth's total rock mass, the volume of rock is so vast—a few billion cubic kilometers—that it shelters astronomical numbers of microbes. Inevitably, our view of life has been skewed because these microscopic life-forms lie completely hidden from everyday view.
In April 1998, I invited Gold to visit the Geophysical Lab and present his ideas at our regular Monday morning seminar. Seemingly unfazed by more than two decades of impassioned objections to his views, he delivered a polished and forceful account of many lines of evidence that petroleum is abiotic and rises from the depths. Geology, biology, thermodynamics, experiments on organic molecules, carbon isotopes, observations of diamonds, and, of course, the chemical properties of petroleum itself all came into play. Knowing our special interest in life's origins, he underscored the possible role of this deep hydrocarbon source in supplying critical molecules for prebiotic processes. Perhaps, he posited, life arose from those deep sources of organic molecules. Throughout the entertaining lecture, he bolstered his controversial conclusions with rhetorical flourishes more suited to a courtroom than a scientific seminar (“the only possible explanation,” “no question remains,” the evidence “persuades one completely,” and the like).
I doubt that anyone was persuaded completely, but we had to admire his creativity and conviction. And Tommy Gold helped to remind us all, once again, how much we don't know about the interior of our planet just a few miles beneath our feet.
HEAVEN VERSUS HELL
If so many organisms exist beyond the Sun's radiant reach, then geothermal energy, and the abundant chemically active mineral surfaces that are synthesized in geothermal domains, must be considered as a possible triggering power source for life. To be sure, sunlight remains the leading contender for life's original energy source. The vast majority of known life-forms do rely, directly or indirectly, on photosynthesis. In many scientific circles, a surface origin of life in a nutrient-rich ocean, under a bright Sun, remains the seemingly unassailable conventional wisdom.
But that nagging problem of macromolecular formation remains. Most known living species depend on the Sun directly or indirectly, but the Sun's harsh ultraviolet radiation inhibits the emergence of the larger multimolecular structures on which all organisms depend. Furthermore, if the earliest life of almost 4 billion years ago was confined to the sunlit surface, how did it escape the brutal, sterilizing final stages of bombardment by asteroids and comets? As Gold said over and over again, Earth's surface, bathed in solar radiation and blasted by lightning, is the truly extreme environment.
And there's another reason to look closely at the possibility of hydrothermal origins. If life is constrained to form in a sun-drenched pond or ocean surface, then Earth, and perhaps ancient Mars or Venus, are the only possible places where life could have begun in our solar system. If, however, living cells can emerge from deeply buried wet zones, then life may be much more widespread than previously imagined. The possibility of deep origins raises the stakes in our exploration of other planets and moons.
Jupiter's fourth largest moon, Europa, presents a particularly promising target for exploration. According to recent observations, Europa is covered with a veneer of ice about 10 kilometers thick, covering a deep ocean of liquid water. Hydrothermal activity on the floor of that ocean might be an ideal environment for life-forming chemistry. Saturn's largest moon, Titan, is another intriguing world. Though much colder than Earth, Titan has an organic-rich atmosphere slightly denser than Earth's and, like all large bodies, its own sources of internal heat energy.
The idea that life may have arisen in a deep, dark zone of volcanic heat and sulfurous minerals flies in the face of deeply ingrained religious metaphors. To many people, the Sun represents the life-giving warmth of heaven, while sulfurous volcanoes are the closest terrestrial analog to hell. How could life have come from such a dark, hostile environment?
Nature is not governed by our metaphors, however cherished they may be. Life as we know it demands carbon-based chemicals, a water-rich environment, and energy with which to assemble those ingredients into a self-replicating entity. Ongoing laboratory experiments that simulate deep conditions as well as those on the surface—coupled with observations of environments elsewhere in the solar system—will be the ultimate arbiters of truth.
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Under Pressure
Where is this original homestead of life? … a place where hot volcanic exhalations clash with a circulating hydrothermal water flow; a place deep down where a pyrite-forming autocatalyst once gave, and is still giving, birth to life.
Günter Wächtershäuser, 1988
The first emergent step in life's chemical origin, wherever it may have occurred, must have been the synthesis of organic molecules. Stanley Miller's experiments amply demonstrated one potential ancient energy source with which to effect this synthesis—lightning near the ocean–atmosphere interface. Subsequent workers, notably Thomas Gold, postulated deeper origins and chemically derived energy sources. Any plausible source of carbon-based molecules should be fair game for origin researchers. Every plausible source deserves rigorous study.
In 1996, when George Cody, Hat Yoder, and I began our research on hydrothermal chemistry and the origins of life, we were novices in the origins game, too naïve to suspect that we were wasting our time. We thought that our experimental studies of organic synthesis in hot, high-pressure water provided compelling evidence that hydrothermal zones could have played a role in the prebiotic production of life's molecules. Stanley Miller and his students, most notably chemist Jeffrey Bada at the Scripps Institution of Oceanography, would have been happy to set us straight, but we had yet to cross paths with them. Harold Morowitz, whose ideas instigated the research, had warned us that we might meet with some criticism. He was right.
It must have seemed odd to the scientific community that a group of geologists and geophysicists was studying the origin of life. For almost a century, our home base, the Carnegie Institution's Geophysical Laboratory, had supported research on the physics and chemistry of Earth materials at extreme temperatures and pressures. Carnegie scientists had melted granite, synthesized basalt, and heated and squeezed just about every known rock-forming mineral to understand how our dynamic planet works. They routinely achieved temperatures of thousands of degrees and pressures of millions of atmospheres—extreme environments that would seem to have little to do with life's delicate chemistry. Nevertheless, the lab's arsenal of furnaces and pressure vessels stood ready to heat and squeeze almost any sort of sample, and experiments on carbon compounds weren't intrinsically different from any other chemical we might have chosen to study.
EXPERIMENTS
Now, more than 2,000 experiments later, our hydrothermal organic-synthesis program is expanded, well funded by NASA, and going strong. Our simple strategy, much in the spirit of Stanley Miller's original experiments, has been to subject plausible prebiotic chemical mixes of water, carbon dioxide, and minerals to a contro
lled environment—in our case, a high-pressure and high-temperature environment typical of deep-ocean hydrothermal systems—still often employing the classic gold-tube protocol described in the Prologue.
Our first experiments (the ones with pyruvate and carbon dioxide) were exhilarating. In that series we synthesized sugars, alcohols, and a host of larger molecules, many incorporating dozens of carbon atoms. Under extreme hydrothermal conditions, the pyruvate molecules polymerize; in some cases they form rings and branching structures reminiscent of molecules found in modern cells.
In a later set of experiments, we sealed water, nitrogen gas, and powdered minerals into the gold tubes—a reasonable proxy for primitive conditions at some hydrothermal vents. In these experiments, we produced ammonia, an essential starting material for synthesizing amino acids and other biomolecules. Our work suggests that hydrothermal vents may have been one of the principal sources of the early Earth's ammonia. Carrying on with this line of experiments, we put ammonia into a capsule with pyruvate and found that these two compounds react to form the amino acid alanine. By 1999, we had established one plausible chemical pathway leading from simple carbon compounds, nitrogen gas, and common minerals to key biomolecules.
In 1997, shortly after our initial exciting results, Cody, Yoder, and I submitted our first papers for peer review. We believed optimistically that our demonstrations of facile hydrothermal organic synthesis would be welcomed as a logical extension of the Miller–Urey approach. After all, we, like Miller a half-century before, had subjected a closed vessel containing water and simple carbon-containing molecules to heat. We had followed Miller's lead in attempting to simulate a plausible prebiotic environment. And we, too, had found an abundance of synthetic organic molecules. We thought the Miller camp might actually welcome our contribution. Boy, were we wrong!
Genesis: The Scientific Quest for Life's Origin Page 13