In 1988, Günter Wächtershäuser published a remarkable idea that excited tremendous interest, even being featured in a Scientific American article. It ran counter to prevailing ideas about the origin of life, and suggested new experimental approaches involving mineral interfaces. Wächtershäuser is a patent lawyer in Munich, Germany who enjoys fabricating intricate and novel approaches to the origin of life, then challenging others to test them. He is greatly influenced by the philosopher Karl Popper, who made the point that explanations are useless unless they are falsifiable.
In other words, don’t publish an idea (or hypothesis) unless you can think of a critical experiment that might prove it to be wrong.
Wächtershäuser basically turned the entire concept of life’s beginning upside down by proposing that life did not arise by assembling pre-existing organic compounds, no matter what the source. Instead, life began as two-dimensional chemical reactions on a special mineral surface called pyrite, or sometimes fool’s gold, a crystalline mineral composed of iron and sulfur.
According to Wächtershäuser’s idea, pyrite has a positive surface charge, and therefore will adsorb and concentrate anions (negatively charged compounds) in solution. This is important, because many biologically relevant organic compounds have anionic carboxylate or phosphate groups as part of their molecular structure. Furthermore, when hydrogen sulfide reacts with iron in solution to form iron sulfide, the reaction produces electrons that can be donated to the bound compounds and thereby drive a series of energetically uphill chemical reactions that otherwise could not occur in solution.
Wächtershäuser sees these reactions as the beginning of a primitive metabolism that occurs on a two dimensional mineral surface rather than in the volume of a cell. He refers to this stage of life’s history as the “Iron-Sulfur World.” After metabolic processes were initiated in this way, he proposes that the reaction pathways would somehow become encapsulated in membranes to produce the more familiar forms of cellular life.
The logic and novelty of Wächtershäuser’s intellectual construct were impressive. However, an elaborate framework of linked ideas, no matter how elegant, can collapse when a single critical experiment fails. Wächtershäuser was a lawyer, not a laboratory scientist, so he needed to find someone with the time and interest to test the main ideas. Claudia Huber, an organic chemist at the Technical University in Munich, was just such a colleague, and together they assembled a working simulation of the kinds of chemical reactions that might occur in volcanic conditions such as the chimneys of the deep sea hydrothermal vents in which sulfide minerals were exposed to hot gases in solution. Their idea was to heat a dispersion of iron and nickel sulfides together with a source of carbon (carbon monoxide) and see whether anything interesting happened.
Something did happen. There was no evidence for a long string of integrated reactions, which would have been the best possible outcome in support of the iron-sulfur concept, but at least a carbon-carbon bond was produced and acetic acid was synthesized. Their 1997 paper in Science concluded that “The reaction can be considered as the primordial initiation reaction for a chemoautotrophic origin of life.”
Huber and Wächtershäuser later published two follow-up papers in Science, both related to conditions in which amino acids could be linked together through peptide bonds. Again, they simulated a hydrothermal environment by producing iron and nickel sulfides in the presence of carbon monoxide in boiling water. If amino acids were also added to the mix, they found that these conditions chemically activated the amino acids which then went on to form peptide bonds. In their 2003 paper, they conclude that “The results support the theory of a chemoautotrophic origin of life with a CO-driven, (Fe,Ni)S-dependent primordial metabolism.”
So, how do we summarize all these ideas and results? On one hand, and to his credit, Wächtershäuser certainly stirred up the field and challenged other workers to be more critical about their assumptions and open to alternative explanations. And he was courageous enough to follow his own advice, with three papers published in Science to prove it. On the other hand, although the results show how a chemical system can become a little more complex, so do a number of other simulation experiments. Nor is there any indication that a series of linked reactions can occur on pyrite surfaces. But like all good hypotheses, there are other tests to explore, and the next few years will show whether or not the Iron Sulfur world has significant explanatory power.
I agree that volcanic hydrothermal systems are plausible sites for the chemical and physical events leading up to the origin of cellular life. But instead of an interface between water and mineral surfaces, it seems more likely to me that the fluctuating conditions prevailing at the edge of hydrothermal springs will be a more fruitful site to model experimentally. Only under those particular conditions do we have a combination of concentrating effects, mineral surfaces that can act as organizing agents, and free energy available to drive the reactions required to assemble the first protocells.
In next week’s column I will describe field sites in Kamchatka, Russia and Mount Lassen, California, where I am testing some of these ideas in a natural setting.