Genesis: The Scientific Quest for Life's Origin

by Robert M. Hazen

  Lemke and Ross gingerly placed the bag into a water-filled pressure vessel, slowly filled the gold container with a solution of the amino acid glycine and water, sealed the assembly, and ramped up the pressure and temperature. They ran their samples for weeks, monitoring the solution, watching for the glycine to decompose. Over time, the concentration of glycine steadily declined, but even though they didn't use minerals in their experiments, they observed a much slower rate of breakdown than had been reported in previous studies at lower pressure.

  In addition, they found a surprisingly fast rate of peptide-bond formation—amino acids linking together to form molecular chains. This result was unexpected, because hot water tends to break apart rather than form peptide bonds. (That's one reason boiled foods are so squishy—the sturdy bonds between amino acids that give food texture break down.) Once formed, these peptides decomposed rather quickly, but their formation pointed to more complex behavior than had been expected.

  Lemke and Ross found hints of another potentially important behavior in their gold bag. Single amino acid molecules and small clusters with just two or three molecules linked by peptide bonds readily dissolve in water at room temperature, but longer peptide chains proved much less soluble. Lemke and Ross imagine a scenario in which peptides form rapidly in vents and are then exposed to the cooler seawater. Given a high enough concentration of long amino acid chains, these molecules might separate out as a relatively stable concentrated phase—just the kind of emergent molecular selection and organization that life's origin required. There's a lot more work to be done, but it appears that the book is not yet closed on amino acid stability in hydrothermal systems.

  FIXING CARBON

  The most fundamental biological reaction—and one of our group's primary goals in prebiotic-synthesis experiments—is carbon fixation, the incorporation of more carbon atoms (starting with carbon dioxide) into organic molecules. After all, the first chemical step in the path to life must be to make bigger molecules, like amino acids and sugars, out of smaller ones, like carbon dioxide, ammonia, and water. Such reactions occur rapidly in our experiments, but they follow two rather different paths, depending on the mineral employed.

  Many common minerals, including most oxides and sulfides of iron, copper, and zinc, promote carbon addition by a routine, industrially important process known as the Fischer–Tropsch (F–T) synthesis. In its idealized form, F–T synthesis builds long chainlike organic molecules from carbon dioxide and hydrogen that are exposed to hot, dry metal surfaces. Our gold-tube experiments and studies in several other labs display similar reactions in the presence of wet mineral surfaces at high pressure, though a lot less efficiently than the industrial process.

  Field studies complement these experiments. Recent intriguing analyses of organic molecules emanating from hydrothermal vents reveal similar Fischer–Tropsch-like products, and it now appears that F-T synthesis constantly manufactures larger organic molecules from smaller building blocks in Earth's hydrothermal zones. Many of these molecules are hydrocarbons of the type that form petroleum. (Who knows, maybe Tommy Gold is correct and at least some petroleum forms abiotically at depth.)

  Alternatively, when we use nickel or cobalt sulfides, we observe that carbon addition occurs primarily by the insertion of carbon monoxide, a molecule with one carbon atom and one oxygen atom, which readily attaches itself to nickel or cobalt atoms. By repeating these simple kinds of reactions—add a carbon atom here, an oxygen or hydrogen atom there—over and over again, new and more complex organic molecules emerge.

  One conclusion seems certain: Mineral-rich hydrothermal systems contributed to the early Earth's varied inventory of potential bio-building blocks. With tens of thousands of miles of deep-ocean hydrothermal ridges, billions of cubic kilometers of warm wet crust, and hundreds of millions of years to process the raw materials, organic molecules must have been produced in prodigious amounts. But the Geophysical Lab synthesis experiments have done more than simply add to the catalog of interesting molecules that could have been formed on the early Earth. These experiments are now uncovering something quite new about the possible role of minerals in the origin of life.

  Previous origin-of-life studies, such as those of Günter Wächtershäuser, treat minerals essentially as solid and relatively stable platforms for synthesis and assembly of organic molecules. But our experiments reveal another, more complex behavior that may have important consequences for origin-of-life chemistry. We find that in the presence of high-temperature and high-pressure water, minerals often start to dissolve. In the process, the dissolved atoms and molecules from the minerals themselves become crucial reactants in the prebiotic milieu. Sulfur, dissolved from sulfide minerals, combines with carbon dioxide and water to form thiols and thioesters—reactive molecules that can jump-start new synthetic pathways.

  Even more dramatic is the behavior of iron, which can dissolve in water to form brilliantly colorful organic solutions. After one experimental run, George Cody bounced from office to office on the second floor, showing off a particularly striking orange-red solution he had just extracted from a pressure capsule. The deep color was exciting because it pointed to the formation of iron complexes—iron atoms surrounded by a starburst of organic molecules. Chemists have long known that similar iron complexes promote chemical reactions, so Cody speculated that our cheerful solutions might contain a kind of primitive catalyst that promoted the assembly of more complex molecular structures.

  Such behavior is not entirely unexpected, for hydrothermal fluids are well known to dissolve and concentrate mineral matter. Many of the world's richest ore deposits arise from hydrothermal processes. Similarly, spectacular sulfide pillars tens of meters tall grow rapidly at volcanic vents called black smokers, where rising plumes of hot, mineral-rich solutions contact the frigid water of the deep ocean.

  Yet there's so much we don't know about hydrothermal systems and the chemical processes that might occur in their vicinity. And in spite of their prevalence, the role of these dissolved ingredients has not yet figured significantly in origin scenarios. No one yet knows how this rich mix of organic compounds and dissolved minerals might influence the synthesis and assembly of biomolecules. But we're poised to find out.

  9

  Productive Environments

  The limits of life on this planet have expanded to such a degree that our thoughts of both past and future life have been altered.

  Kenneth Nealson, 1997

  Even as the debate between Miller's advocates and the ventists was heating up, an explosion of new research dramatically changed the research community's view of the emergence of biomolecules. When Miller first reported organic synthesis on a benchtop in 1953, the results seemed almost magical. Fifty years ago, no one could have predicted how easy it would be to make amino acids, sugars, and other key biomolecules from water and gas. But the more scientists study carbon chemistry in a wide range of plausible, energetic prebiotic environments, the more diverse and facile organic synthesis seems to be. It now appears that anywhere energy and simple carbon-rich molecules are found together, a suite of interesting organic molecules is sure to emerge. It's all a matter of environment, and it now appears that the universe boasts an extraordinary range of productive environments.

  MOLECULES FROM DEEP SPACE

  The last place you might think to look for life-forming molecules is the black void of interstellar space, but new research reveals that organic molecules from space must have predated Earth by billions of years. Deep space, we now realize, is home to immense tenuous clouds where carbon, hydrogen, oxygen, and nitrogen combine in complex sequences of reactions.

  A research team at NASA Ames Research Center at Moffett Field, California, led by veteran astrochemist Louis Allamandola, has simulated the ultracold deep-space environments of these so-called dense molecular clouds (though these vast volumes of dust and gas are far less dense than the highest vacuum attainable on Earth). A typical interstellar cloud harbors only a measly
million atoms per cubic inch, at temperatures colder than -100°C. Such high vacuums and frigid temperatures would seem to preclude any sort of chemical reaction, but in these remote regions, minute ice-covered dust particles are subjected to ultraviolet radiation from distant stars. Gradually, as molecules absorb this radiation, they become sufficiently reactive to form larger collections of atoms. Radio astronomers have long recognized the distinctive signatures of numerous organic species in these clouds. Each type of molecule absorbs or emits characteristic wavelengths of light—features that appear as sharp lines on a radio spectrum. The most abundant molecules are the diatomic and triatomic species, such as CO, H2, CO2, and H2O, but more than 140 different compounds are known, including many larger molecules with a dozen atoms or more.

  Theorists easily explain such molecular diversity. They calculate the efficiency with which small cold molecules condense onto tiny dust particles, forming submicroscopic ice coatings. They predict details of how icy particles occasionally absorb ultraviolet radiation, which can shuffle electrons and trigger chemical reactions. They plot reaction cascades by which small groups of atoms clump together and slowly cause new larger molecules to accumulate in the cloud. Eventually, under the pervasive inward pull of gravity, local regions of a molecular cloud can collapse into a new planetary system with a central massive star and an array of planets and moons. As each body forms, a steady rain of organic-rich comets and asteroids contributes to the life-forming inventory. So, the theorists tell us, organic molecules inevitably constitute part of any planet-forming mix.

  Regardless of how convincing a theory may sound, experiments carry a lot of weight in science. Allamandola and co-workers' experiments at NASA Ames have exploited an elegant chilled vacuum chamber, about 8 inches in diameter, crafted of shiny stainless steel, and equipped with thick glass observation ports, to produce suites of organic molecules. First, they introduce a fine spray of simple gas molecules, such as water, carbon monoxide, methane, and ammonia, into the chamber, where the gases freeze onto an aluminum disk. Then they bathe the thin ice layer with a beam of ultraviolet radiation, which triggers the formation of larger molecules—compounds that match the distinctive molecular emissions from those distant clouds. [Plate 5]

  The NASA team has used their benchtop apparatus to produce a rich variety of interesting molecules: reactive nitriles, ethers, and alcohols abound, as do ringlike hydrocarbons. One set of experiments yielded nitrogen-bearing precursor molecules to amino acids. Another set generated long chainlike molecules reminiscent of the building blocks of cell membranes.

  Evidence from space amply buttresses these nifty experiments. The Murchison meteorite and many other carbon-rich meteorites are loaded with organic molecules thought to be of extraterrestrial origin. Comets, too, are known to be rich in the molecular precursors of life, as are the microscopic interplanetary dust particles that incessantly drift down to Earth's surface. Armed with their vacuum chamber, the Ames team can reproduce the supposed deep-space synthesis processes in the lab. Theory, observations, and experiments agree: The prebiotic Earth was seeded abundantly with extraterrestrial organics.

  Nevertheless, the Miller crowd is unpersuaded by these studies, too. Says Miller, “Organics from outer space, that's garbage, it really is.” Jeff Bada echoes, “Even if cosmic debris struck the prebiotic Earth at 10,000 times the present levels, the resultant prebiotic soup would still have been much too weak to engender life.”

  MOLECULES FROM GIANT IMPACTS

  Meteorites and comets carry a rich inventory of organic molecules, but can these molecules survive the catastrophic insults of collisions with Earth? Deep-space synthesis, no matter how fecund, would be irrelevant to life's origin if the intense temperatures and pressures of impact disintegrated molecules.

  It's hard to imagine an environment more destructive to life and its molecules than the shattering surface impact of an asteroid or comet. Nevertheless, carbon-rich meteorites like the Murchison contain a significant store of amino acids and other potential biomolecules; evidently impacts don't destroy all organic molecules. In fact, recent experiments suggest quite the opposite. Jennifer Blank and her colleagues at the Lawrence Berkeley National Laboratory, in Berkeley, California, use a giant experimental gas gun that hurls hyperfast chunks of metal at innocent rocks. Their goal is to trace the fates of organic molecules during these violent collisions.

  Blank's experiments begin with a flattened cylindrical stainless-steel capsule about 1 inch in diameter that is filled with a solution of five different amino acids in water. She carefully positions the sealed sample in a metal well—the target at the end of a 40-foot-long gun barrel.

  “Clear the room!” she demands, as they close the gun chamber.

  A technician powers up her weapon. “Three, two, one, fire!” and blam!, a tremendous shock wave shakes the building as a massive metal projectile hurls down the barrel at more than 4,000 miles per hour and squashes her neatly prepared sample like a bug. For a few microseconds, the amino acid solution experiences pressures in excess of 200,000 atmospheres at temperatures approaching 900°C.

  Then the fun begins. Blank pries out her deformed steel capsule and mills down the metal to extract a few drops of liquid. The original clear solution has turned a dark brown color—something interesting has happened to the amino acids. The organic chemists' standard analytical techniques, chromatography and mass spectrometry, tell the story. To be sure, most of the original amino acids are lost in every run. But, remarkably, some of the delicate molecules react with each other to form pairs of amino acids. The formation of these peptide bonds between amino acids is a crucial step in the assembly of proteins.

  Jennifer Blank's highly publicized conclusion: Impacts on the early Earth may have reduced the quantity of organic molecules, but at the same time they increased the diversity of complex prebiotic chemical species.

  MOLECULES FROM HOT ROCKS

  Of all scenarios for the prebiotic production of organic molecules, none is more original (and correspondingly controversial) than the idea of Friedemann Freund, a longtime researcher at the NASA Ames Research Center. He claims that igneous rocks were, and still are, a principal source of Earth's organic molecules. “Maybe,” he remarked to Wes Huntress, the Geophysical Lab's director, “the next chapter in the origin of life is written in the solid state—in the dense, hard, seemingly hostile matrix of crystals.”

  Freund, who is as persistent and unflappable as anyone you're ever likely to meet, smiles and quietly presents his case. Tall, lean, with a shock of graying hair, he speaks gently, with a slight German accent and lots of eye contact. He's always ready to talk about what he's doing and seldom expresses the slightest doubt that he's onto something important.

  Here's how he claims it happens. At high temperatures, every melt contains some dissolved impurities. Molten rocks are no exception; they always incorporate a little bit of water, carbon dioxide, and nitrogen. As the melt cools, minerals begin to crystallize one after another. The first mineral might be rich in magnesium, silicon, and oxygen, but inevitably it will also incorporate a small amount of carbon and nitrogen—elements that don't easily enter the crystal lattice. These residual elements concentrate along crystal defects—zipperlike elongated spaces where the foreign atoms can react and, according to Freund, ultimately form chainlike molecules with a carbon backbone. Freund suspects that every igneous rock has the potential to manufacture such organic molecules. When the rocks weather away, so the story goes, they release vast amounts of organic carbon into the environment.

  Many scientists would say that's a wacky idea. “I am a hundred percent sure that the Freund paper is utter nonsense,” asserts Washington University mineralogist Anne Hofmeister. “Most igneous rocks form from an incandescent melt at temperatures greater than 1,000°C—temperatures at which even the hardiest organic molecule is fragmented into carbon dioxide and water. By contrast, organic contamination is everywhere in our environment.” What causes Freund's observed or
ganics? “It's surface residues,” Hofmeister says, “probably sorbed out of the air.”

  Freund rests his case on two sets of samples he has been studying for almost a quarter century. Two-inch-long synthetic magnesium oxide (MgO) crystals, produced by cooling a white-hot MgO melt from 2,860°C, serve as a simple model system. Pure MgO should be clear and colorless, but Freund's crystals have a cloudy, turbid interior, suggestive of pervasive impurities. Infrared spectra reveal the sharp absorption features of carbon-hydrogen and oxygen-hydrogen bonding, both characteristic of organic molecules. Studies of the crystals' unusually high electrical conductivity and other anomalous properties have further convinced him that the supposed MgO crystals are loaded with excess carbon and hydrogen. The clincher: Subsequent analyses of molecules extracted from crushed MgO crystals reveal the presence of carboxylic acids, which just happen to be essential molecules in the metabolism of all cells.

  Studies of natural gem-quality olivine, an attractive green mineral that is among the commonest constituents of igneous rocks, complement Freund's work on synthetic MgO. Once again, his spectroscopic studies revealed C–H and O–H bonds; once again, he extracted organic molecules from crushed powders. Olivine crystals hold an astonishing 100 parts per million carbon, he claims. Furthermore, much of that carbon occurs in biologically interesting, chainlike organic molecules.

  Others remain unconvinced. Caltech mineralogist George Rossman duplicated some of Freund's olivine results with dirty crystals. “I ran a sample of ours that had been standing around for a while,” he told Anne Hofmeister in 2002. “It had the organic bands. I washed it off with organic solvent and re-ran it. No organic bands.” Organic contamination is everywhere, so any surface—especially any powder—no matter how well cleaned, will quickly become loaded with adsorbed organic molecules. Freund counters that the types of molecules he extracts, carboxylic acids, are not typical of any ordinary environmental contamination. They must have come from inside the mineral.