Hydrothermal OOL Chemistry Is Unlikely
A favorite way for theorists to get around the lack of enzymes at the origin of life is shown to be implausible.
Cells generate complex compounds by means of enzymes. These molecular machines can overcome large thermodynamic barriers that would prevent the natural formation of organic compounds that are useful or essential for life. Origin-of-life (OOL) researchers in the metabolism-first camp have looked to hydrothermal vents to provide the energy needed to leap these barriers without the help of enzymes, which could not have been present.
A new paper in Nature‘s open-access journal Scientific Reports pours cold water on the idea. Specifically, molecules involving thioesters cannot be expected to form in any sufficient quantities at hydrothermal vents or anywhere else. Two favored thioesters, TAA and MTA, can be essentially crossed off the list of probable ingredients because their dissolution (hydrolysis) predominates over their formation.
These compounds’ apparent chemical simplicity has made their prebiotic availability assumed, however, although the kinetic behavior and thermochemical properties of TAA and analogous esters have been preliminarily explored in other contexts, the geochemical relevance of these compounds merits further evaluation. Therefore, the chemical behavior of the simplest thiolated acetic acid derivatives, TAA and methylthioacetate (MTA) were explored here. Using laboratory measurements, literature data, and thermochemical models, we examine the plausibility of the accumulation of these compounds in various geological settings. Due to the high free energy change of their hydrolysis and corresponding low equilibrium constants, it is unlikely that these species could have accumulated abiotically to any significant extant.
PhysOrg summarizes the dashed hopes of seeing complex thioesters emerge in a “primordial soup” when chemical kinetics are taken into account. For “different reasons in different contexts,” TAA and MTA are “implausible prebiotic reagants” to act as substitutes for today’s complex enzymes and co-enzymes, such as acetyl co-enzyme A. Being highly subject to temperature and pH, they lack the reactivity and stability needed. Plus, they break down into acids (like acetic acid) and thiols.
The researchers also add that the results suggest “estimates of the ubiquity of suitable environments for the origin of life in and beyond our solar system may be somewhat overestimated, if these compounds are indeed crucial for jumpstarting early metabolism.“
There are other serious problems. One is concentration. There wouldn’t have been enough hydrogen atoms, for one thing, nor carbon, sulfur and iron. Even then, production of desired MTA and TAA is swamped by the reverse reaction, such that concentrations would be “extremely low” and unlikely to participate in OOL pathways. Acetic acid found at vents today is most likely biogenic, “which leaves questions over its existence in the primordial conditions present during the emergence of life.”
Adding insult to injury, the authors point to the fact that “Acidic oceans do not seem to be preponderant on icy worlds in our solar system where measurement has been possible,” such as Mars and Enceladus. The right acidity (pH) is necessary to supply the hydrogen for high-energy molecules envisioned as precursors to biomolecules. Even then, in laboratory conditions, they are only secondary products and are unstable.
Advocates of the hydrothermal vent hypothesis include Michael Russell (JPL), Huber and Wächtershäuser, Christian de Duve and others. The same day the new paper came out (7/21/16), it was listed in Wikipedia‘s article on the origin of life after thiols were mentioned as possible precursors: “There are increasing concerns, however, that prebiotic thiolated (i.e.Thioacetic acid) and Thioester compounds are thermodynamically and kinetically unfavourable to accumulate in presumed prebiotic conditions (i.e Hydrothermal vents).”
By the same logic, the researchers’ conclusions can be extended to any high-energy molecules whose activation energy strongly exceeds the free energy of dissolution. Simply adding heat or changing the pH may be necessary, but insufficient, to overcome the thermodynamic barriers. The enzymes in living cells are amazing molecular machines that routinely overcome these barriers at ambient temperature, often accelerating production by orders of magnitude against strong concentration gradients. They can do this because of their precise shape, coded for by genes in DNA.
The origin of life is the Achille’s heel of scientific materialism. It takes a Mind that knows organic chemistry to overcome the thermodynamic barriers to spontaneous formation of useful complex molecules.
But since the Darwinian Achilles has multiple feet, realistic scientists need to shoot all the heels. One film that does that is “Evolution’s Achilles’ Heels” produced by CMI. Worth watching. Origin of life is one of eight heels they fatally injure.