Death Valley Days: Chemists Propose a Seemingly Unlikely Environment for the Origin of Life
Recently the American Chemical Society devoted an entire issue of the journal Accounts in Chemical Research to chemical evolution. The editors introduced the journal and its topic in a somewhat convoluted fashion (see our earlier comments, "Chemists Ponder the Origin of Life," here). Now let us turn to one of the research articles, "Asphalt, Water, and the Prebiotic Synthesis of Ribose, Ribonucleosides, and RNA," by Steve A. Benner, Hyo-Joong Kim, and Matthew A. Carrigan.
Benner et al.'s ambitious task was to address some of the most compelling reasons cited by origin-of-life researchers who reject the RNA-world model. They list the most common critiques of the idea, but the biggest such criticism holds that the RNA world represents a "discontinuous model." In other words, it is a series of individual reactions but not a cohesive synthesis from start to finish, or a "continuous model."
Specifically, Benner and his colleagues consider three major problems with the RNA-world model:
- The "asphalt problem": Organic reactions often produce unreactive byproducts. These byproducts are a mixture of pieces of the product or polymerization of the product, but are chemically insignificant and otherwise unpromising. Hence the metaphor of "asphalt." Typically, avoiding the production of such byproducts requires very specific and controlled conditions, or post-reaction purification steps.
- The "water problem": Many of the bonds in RNA will undergo hydrolysis. This occurs when water reacts with the bond, causing it to break apart. In a lab, the problem is easily addressed by using a different solvent. However, the environment of the early Earth could not draw on the resource of various organic solvents.
- The "impossible bond problem": The authors refer here to the difficulty in forming certain bonds in RNA. Usually this follows from thermodynamic issues that prohibit bonds from spontaneously forming.
They conclude that a solution to the water problem is needed, and so too a synthetic model:
At the very least, a continuous synthesis model must deliver RNA from CO2, H2O, and N2, without human intervention, in steps having plausible geological context. Further, no step may involve introducing materials with "temporal precision."Conspicuously missing from the authors' list of critiques are the "chirality problem" and the "information problem." Later in the paper, however, they concede that their model does not solve the enigma of chirality, and they allude to a potential "fatal flaw" in their proposition, namely that the kinds of RNA molecules that catalyze the destruction of RNA are more likely to emerge than RNA molecules that catalyze the synthesis of RNA.
Thus far, in any event, the critiques they present appear to be valid. There are indeed problems with the formation of chemical byproducts, and water prohibits the formation of many of the bonds in RNA, not least the bond between the ribose and the nucleotide. As skeptics often note, experiments by RNA-world proponents persistently rely on the chemist to set up the right conditions and to add the right chemicals at the right times.
The authors thus propose a continuous-model solution by assuming that life began in a very specific environment, an "intermountain desert valley" similar to Death Valley, California, with formamide serving as the predominant solvent rather than water. Furthermore, the formation of unwelcome byproducts (the "asphalt problem") is remedied by forming a borate intermediate compound.
As a working model, the authors propose the following scenario:
- Temperature range is between -20oC and 60oC with higher temperatures possible due to geothermal effects.
- The valley collects runoff water containing the pertinent atmospheric chemicals for the reaction (HCHO and HCN).
- To incorporate the borate-stabilizing compounds, "The runoff begins as borate-rich aquifers having pH values of 10 to 11. These contain borate-stabilized carbohydrates, formamide, and ammonium formate, representing the inventory of the atmospheric HCHO and HCN that was rained into the watershed."
- The pH of the valley is buffered by CO2, but the pH can occasionally increase when basic compounds are introduced, presumably by the watershed mentioned above.
- The redox potential of the valley needs to be about -150mV so that phosphite and phosphate are in equilibrium. This redox potential comes from particular minerals that must be present.
- The valley undergoes periodic dehydration, allowing for solvents other than water.
As we mentioned earlier, apart from the difficulty represented by chirality, the authors admit that they did not solve the problem of useless RNA degrading the useful RNA. Putting aside these two shortcomings, it seems that the researchers, in trying to avoid intervention by the chemist, have come up with environmental requirements so specific as to undercut the rest of the argument. Tellingly, they confess in a footnote that if a scenario like this is perhaps not feasible on Earth where the prebiotic environment was predominantly water -- not a fierce desert like Death Valley -- then perhaps life started on Mars.
Interestingly, the authors include a quote by Robert Shapiro in reference to another study that seems no less relevant to their hypothetical scenario:
Reviewing Sutherland's proposed route, Shapiro noted that it resembled a golfer, having played an 18 hole course, claiming that he had shown that the golf ball could have, through some combination of wind, rain, heating, cooling, dehydration, and ultraviolet irradiation played itself around the course without the golfer's presence.The real problem with RNA-world theories is that no matter how you tweak it, RNA synthesis depends on a highly specific reaction mechanism. Some scenarios require an improbably intricate sequence of events. Others demand an improbably specific environment. Still others assume the addition of certain compounds at just the right time and harnessing just the right amount of energy.
There comes a point when the level of specificity renders the whole idea totally improbable. Transferring that specificity from one place to another, or from one planet to another, does not solve the chemical evolution puzzle. You are then left to assume that chemistry somehow worked differently than it does today. Either that, or you must accept that something is wrong with your hypothesis -- fatally so.
Image credit: Badwater, Death Valley National Park, CA; o.maloteau/Flickr.