Genesis: The Scientific Quest for Life's Origin

by Robert M. Hazen

  BACK TO THE VENTS

  As with any metabolic strategy, the Iron–Sulfur World model requires a source of energy, a source of molecules, and a self-replicating cycle. Here's how Wächtershäuser does it.

  His model relies on the abundant chemical energy of minerals out of equilibrium with their environment. He begins by suggesting that the common iron sulfide mineral pyrrhotite (FeS), which is deposited in abundance at the mouths of many hydrothermal vents, is unstable with respect to the surrounding seawater—that is, given time, it will spontaneously decompose to other more stable chemicals. As a consequence, iron sulfide combines with the volcanic gas hydrogen sulfide (H2S) to produce the shiny mineral pyrite (FeS2) plus hydrogen gas (H2) and a jolt of energy:

  Given that energetic boost, hydrogen reacts immediately with the carbon dioxide (CO2) in seawater to synthesize organic molecules such as formic acid (HCOOH).

  Wächtershäuser envisions cascades of these reactions coupled to build up essential organic molecules from CO2 and other simple gases.

  When these speculations were first proposed, precious little evidence backed them up, but a flurry of experiments in the past decade and a half has lent support to the Iron–Sulfur World hypothesis. Wächtershäuser instigated the initial tests, joining with a team of German scientists from the University of Regensburg to study the reaction of iron sulfide and hydrogen sulfide. Sure enough, just as predicted, they produced pyrite and hydrogen gas. Their brief 1990 paper, “Pyrite formation linked with hydrogen evolution under anaerobic conditions,” appeared in Nature.

  The Iron–Sulfur World hypothesis also makes the unambiguous prediction that iron sulfide minerals promote a variety of organic reactions. In 1996, the Dutch scientists Wolfgang Heinen and Anne Marie Lauwers simulated Wächtershäuser's Iron–Sulfur World scenario by studying reactions of powdered iron sulfide and hydrogen sulfide in water with an atmosphere of carbon dioxide. As predicted, they found the synthesis of organic compounds.

  Subsequent experiments by Wächtershäuser and colleagues in Germany used iron, nickel, and cobalt sulfides to synthesize acetate, an essential metabolic molecule with two carbon atoms that plays a central role in countless biochemical processes. They expanded on this success by adding amino acids to their experiments and making peptides—yet another essential step to life.

  Our research group in Washington also got into the act. In one set of gold-capsule experiments with iron sulfide at high temperature and pressure, we succeeded in producing a variety of organic molecules, including the key metabolic compound pyruvate. We were especially interested to learn whether other minerals could perform the same task. Hydrothermal zones typically feature a variety of minerals in addition to the common iron sulfides; compounds of nickel, cobalt, zinc, copper, and other metals abound. So our Carnegie Institution team ran more than 300 gold-capsule experiments with a dozen different minerals at high pressure and temperature and found organic synthesis reactions in almost every run.

  These experiments have led to an unambiguous conclusion: Common sulfide minerals can promote a variety of interesting synthesis reactions. That's good news for Wächtershäuser's model, but life requires more than a random assortment of chemical reactions—it requires a metabolic cycle. What self-replicating cycle of reactions led to the Iron–Sulfur World?

  THE REVERSE CITRIC ACID CYCLE

  A centerpiece of the Iron–Sulfur World hypothesis is Wächtershäuser's conviction that life began with a simple, self-replicating cycle of compounds similar to the one that lies at the heart of every modern cell's metabolism—the reverse citric acid cycle. A growing cadre of scientists concurs, but none has been more outspoken or articulate than Harold Morowitz, my colleague at George Mason University.

  For the better part of two decades, Morowitz has preached a simple philosophy: If you want to understand the chemical emergence of life, look to life's most basic chemistry—the biochemical pathways shared by all organisms. In the case of metabolism, that core biochemistry is found in the citric acid cycle, also known to generations of high-school biology students as the Krebs cycle or the TCA cycle. This circular sequence of 11 small molecules, each made of carbon, hydrogen, and oxygen, forms the core metabolism in every cell. The cycle starts with the 6-carbon molecule citric acid, which is gradually pared down, a few atoms at a time, to make a succession of smaller molecules—5-carbon malate, 4-carbon oxaloacetate, 3-carbon pyruvate, and so forth—each of which is the starting material for all sorts of other essential biomolecules. Add ammonia to pyruvate and you get the amino acid alanine; add carbon dioxide to acetate and you get the building blocks of lipids. Animals and plants use this cycle as the starting point for synthesizing just about everything biochemical. For many modern organisms, the citric acid cycle is an engine of biosynthesis.

  In the mid-1960s, biologists discovered that some primitive microbes run the citric acid cycle the “wrong way” round—a pathway that can be called the “reverse” citric acid cycle. Starting with the 2-carbon molecule acetate (one of the newsworthy compounds synthesized in Wächtershäuser's experiments), the cell adds a carbon dioxide molecule to make 3-carbon pyruvate. Then add another carbon dioxide molecule to make 4-carbon oxaloacetate, and so on. Around the cycle we go, until we get to 6-carbon citric acid. At this point the cycle does something surprising. Citric acid splits into one acetate molecule and one oxaloacetate molecule. Suddenly we have the building blocks for two cycles, so around we go again. The reverse citric acid cycle is a self-replicating synthesis engine that doubles on every turn. In this way, a microbe can build all of its essential biomolecules from the simplest of building blocks—water and gas.

  The reverse citric acid cycle is an engine of synthesis that may have been life's first metabolic cycle.

  Harold loves to lecture on the foundations of metabolism, and he does so with simplicity and elegance. At a recent seminar, he used no aids but a few hand-scrawled viewgraphs and a big chart of all known metabolic pathways (he calls it “my security blanket”). At the heart of that intricate chart lies the simple citric acid cycle—the core chemistry of life itself. “We're looking for a fundamental theory of biology,” he says. “When no one is listening, we call it the Grand Unified Theory of Biology.”

  Harold lectured to the packed, attentive room sitting back in an easy chair, his arms sweeping in wide circles. He reiterated the basic concept of the reverse citric acid cycle: Start with two small molecules—acetate with two carbon atoms and oxaloacetate with four carbon atoms. Then start tacking on carbon dioxide and water—a carbon atom here, an oxygen atom there, a few more hydrogen atoms—until, after just a few steps, you wind up with two citric acid molecules. These citric acid molecules split to begin new cycles. The various intermediate molecules of the cycle serve as starting points for the synthesis of all other key biomolecules, including amino acids, sugars, and lipids. Today, cycles of synthesis lie within cycles, which in turn lie within more cycles, but at the very core is the reverse citric acid cycle. “It's the way CO2 was incorporated into biology before photosynthesis,” explains Harold.

  It's all well and good to point to today's biochemistry as a clue to life's origin, but there's a key difference between modern cells and the presumed first metabolic cycle. Today, enzymes—protein catalysts of incredible complexity—promote each of the 10 separate chemical reactions in the “simple” reverse citric acid cycle. One of these catalytic enzymes speeds up the split of citric acid into acetate and oxaloacetate. Another facilitates the addition of carbon dioxide to pyruvate to make oxaloacetate. Without these enzymes to increase chemical efficiency, today's cells wouldn't stand a chance in the competitive struggle for resources.

  How did the first metabolic cycle go it alone, without an enzymatic boost? Some of the requisite steps, such as combining carbon dioxide and pyruvate to make oxaloacetate, are energetically unlikely. The trouble is that if one step fails, then the whole cycle fails. One of the clever proposals in Wächtershäuser's model is that sulfide mi
nerals promote simple metabolic reactions. It just so happens that many modern metabolic enzymes have at their core a small cluster of iron or nickel and sulfur atoms—clusters that look exactly like tiny bits of sulfide minerals. Perhaps ancient minerals played the same role as modern enzymes.

  Wächtershäuser invokes another chemical trick based on sulfur. In his model, the very first cycle was helped along by substituting hydrogen sulfide for water (H2S for H2O) in several crucial reactions, thus forming sulfur-bearing analogs of citric acid cycle compounds. He predicts that this simple chemical substitution leads to faster, energetically more favorable reactions. What's more, reactions with sulfide naturally couple to the formation of pyrite—the physical foundation of the Iron–Sulfur World. Once a sulfide version of the reverse citric acid cycle was well established on the primitive Earth, it was only a matter of time before it switched from hydrogen sulfide (which is common only at volcanic vents) to water (which is everywhere).

  EXPERIMENTS

  What do experiments have to say about the reverse citric acid cycle? In the late 1990s, our Carnegie Institution group initiated a series of gold-tube experiments to see how citric acid degrades in hot, pressurized water reminiscent of conditions at hydrothermal vents. Ideally, we would have studied the system with hydrogen sulfide gas, to match the details of Wächtershäuser's model, but H2S is nasty stuff, technically difficult to load, and potentially deadly if a gas cylinder leaks. We stuck to the closely related water-based system.

  By this time, we had our protocols down pat. I loaded the capsules with a citric acid solution, Hat Yoder set up the high-pressure runs (typically at 2,000 atmospheres and 200°C), and George Cody analyzed the runs. George soon recognized two distinct reaction pathways. Some citric acid molecules followed what he called the alpha pathway, which begins with citric acid splitting into acetate plus oxaloacetate, just like the reverse citric acid cycle is supposed to work. But there the similarity ended. Oxaloacetate rapidly decomposed to pyruvate plus carbon dioxide, and much of the pyruvate then decomposed to acetate. Rather than build up new useful molecules, we destroyed the ones we had.

  The next logical step was to take pyruvate and add sulfide minerals and carbon dioxide—essential ingredients in the Iron–Sulfur World. But try as we might, no combination of minerals and reactants would promote the essential reaction from pyruvate, a 3-carbon molecule, to oxaloacetate, a 4-carbon molecule. Without that step, we had to conclude that the alpha pathway is a dead end that destroys citric acid. Oxaloacetate is the real stopper. Under no plausible prebiotic conditions have we found a way to make oxaloacetate from pyruvate plus carbon dioxide. Pressure doesn't seem to help, nor does any realistic combination of mineral catalysts. Even at modest hydrothermal temperatures less than 100°C, oxaloacetate breaks down rapidly in water.

  George didn't give up. Other citric acid molecules had followed what he called the beta pathway, which begins in more or less the normal way, with citric acid giving up a water molecule to make aconitate, another 6-carbon compound. Then a series of reactions that release carbon dioxide produced compounds with five, then four, then three carbon atoms in succession.

  George Cody identified two distinct reaction pathways in experiments on citric acid at high temperature and pressure. (A) Some citric acid molecules followed what he called the alpha pathway, which begins with citric acid splitting into acetate plus oxaloacetate, which then fragments in a nonbiological manner. (B) Other citric acid molecules followed the beta pathway, which begins with citric acid giving up a water molecule to make aconitate, another 6-carbon compound. Then a series of reactions that release carbon dioxide produced compounds with five, then four, then three carbon atoms in succession. Cody found evidence that this pathway might be reversed to complete a ?? metabolic loop.

  Is this reaction series another dead end for citric acid? Or might the beta pathway be part of an alternative self-replicating citric acid cycle? In subsequent experiments, Cody found evidence that some steps of the beta pathway can be reversed in the presence of nickel sulfide, a common hydrothermal vent mineral. Perhaps an abiotic route from carbon dioxide to citric acid exists after all. We're still some way from demonstrating a self-replicating citric acid cycle, but we haven't given up yet.

  And perhaps someday some brave soul will try these experiments all over again with hydrogen sulfide—the true test of Wächtershäuser's model.

  FLAT LIFE

  Supposing Wächtershäuser is right, what did the Iron–Sulfur World look like?

  Recall that cellular life requires a membrane—a chemical barrier to separate the organized insides from the chaotic outsides. Accordingly, most origin-of-life researchers accept the idea of a cell-like lipid vesicle that surrounded the first self-replicating metabolic cycle. But an enclosing lipid membrane may not be the only possibility.

  Glasgow-based geoscientists Michael Russell and Allan Hall, both experts in sulfide ore deposits, focus on probable geochemical environments in their clever “iron sulphide bubble” scenario. Unlike Wächtershäuser, they do not rely on the reactive potential of iron sulfides to drive metabolism, nor do their sulfides serve as a solid footing for life. Rather, in their scenario iron sulfide bubbles form spontaneously at hydrothermal vents, where the less acidic vent water contacts the more acidic ocean water. These bubbles form primitive cell-like structures that enclose metabolic chemicals. The bubbles also maintain a strong contrast of acidity between inside and outside—an energetic difference that can promote metabolic reactions.

  By contrast, Wächtershäuser advocates “flat life.” The first self-replicating entity in his proposed Iron–Sulfur World was, as we saw in Chapter 8, a thin layer of chemical reactants on a sulfide mineral surface. The entity grew laterally, spreading from mineral grain to mineral grain as an invisibly thin organic film. Bits of these layers could break off and reattach to other rocks, like cloned colonies. Given time, different minerals and environmental conditions might have induced variations in the film, fostering new “species” of flat life.

  The bold, heretical concept of flat life—a self-replicating chemical layer of molecules built on a solid mineral foundation—raises an intriguing geochemical possibility. A simple layered collection of molecules might be more tolerant of high temperatures and other environmental extremes than life based on nucleic acids, which break down close to 100°C. If so, then colonies of flat life might exist today in deep zones of Earth's crust. Such film-like molecular systems might persist for eons, because they survive at extreme conditions beyond the predation of more efficient cellular life.

  If so, how would we know? Such a layer would be invisible under an ordinary light microscope and would appear as a nondescript film using more powerful atomic microscopes. Flat life would also be undetectable in standard biological assays, which rely on the presence of DNA and proteins. Is it possible that layer life is abundant on Earth today, yet remains overlooked?

  There's much to learn about the emergence of self-replicating chemical systems. Whether they first formed as chainlike peptides or films, self-replicating molecular systems appear to be a necessary antecedent to life. Nevertheless, a self-replicating metabolism by itself is not sufficient for life as we know it, and many scientists still argue that genetics came first.

  16

  The RNA World

  It is generally believed that there was a time in the early history of life on Earth when RNA served as both the genetic material and the agent of catalytic function.

  Gerald Joyce, 1991

  In spite of the elaborate detail of Wächtershäuser's Iron–Sulfur World, most origin experts dismiss the idea of a purely metabolic life-form in favor of a genetics-first scenario. In order to reproduce, even the simplest known cell must pass volumes of information from one generation to the next, and the only known way to store and copy that much information is with a genetic molecule similar to DNA or RNA.

  No one has thought more deeply about genetics and the origins of life than Leslie Orgel
at the Salk Institute for Biological Studies in San Diego. His classic 1968 paper, “Evolution of the genetic apparatus,” has guided generations of researchers, and he continues to exert a tremendous influence on origin theory and experiment. Orgel states that the central dilemma in understanding a genetic origin of life is the identification of a stable, self-replicating genetic molecule—a polymer that simultaneously carries the information to make copies of itself and also catalyzes that replication. Accordingly, he catalogs four broad approaches to the problem of jump-starting such a genetic organism.

  One possibility is the emergence of a self-replicating peptide of the kind made by Reza Ghadiri's group at Scripps, or perhaps a protenoid as championed by Sidney Fox. The idea that proteins emerged first and then “invented” DNA holds some appeal, because amino acids, the constituents of proteins, are thought to have been available in the prebiotic environment. The problem is that the random prebiotic assembly of amino acids would have been a messy business, as Fox's critics were quick to point out. Cells have learned how to form neat, chainlike polymers—the proteins essential to life. But left to their own devices, amino acids link together in irregular, undisciplined clusters—hardly the stuff of genetics.

  The second of Orgel's possibilities, the simultaneous evolution of proteins and DNA, seems even less likely, because it requires the emergence of not one but two improbable macromolecules.

  Graham Cairns-Smith's Clay World scenario provides an intriguing third option, with genetic-like sequences of elements replicating and acting as templates for organic assembly. So far, however, the Clay World scenario is totally unsupported by experimental evidence.