#72. The Restricted Weathering Theory of Abiogenesis [chemistry, evolution]

EV   CH

Red, theory; black, fact

Urban lichen in the original ecological niche

Starting at the Start

When the early Earth, which was initially molten, had cooled sufficiently to acquire a solid crust and allow liquid water to accumulate on the surface, the formation of the oceans presumably began.

The Original Energy Source: Weathering

Seawater forms from steam outgassing from volcanic vents, which simultaneously emit acid gasses such as hydrochloric acid. Therefore, the first rain would have been highly acidic. In the normal course of events, volcanic rain falls on surface rocks that contain sufficient alkalinity to completely neutralize the acid, contributing cations such as sodium in the process and producing salt water. 

Key Original Difference

However, shortly after the formation of the Earth’s crust, the surface rocks may have been mostly encapsulated in carbonaceous pyrolysis residues originating from carbon-bearing gasses in the atmosphere. How is the acid rain supposed to get to the rocks?

The Germ of an Idea

I postulate that sometimes it did, and sometimes it didn’t, leading to a scenario of nascent crust covered in interconnected puddles in which a broad range of pHs were simultaneously represented. At the connecting points, a pH gradient would have existed, which recalls the pH gradient across the mitochondrial membrane that powers ATP synthesis. So, that's basically my angle.

Second-iteration Theory

More simply, a lump of lava coated in a capsule of pyrolysis residue and immersed in acid rainwater will have a proton gradient across the capsule with the correct direction to model the inner mitochondrial membrane with its enclosed matrix.

The carbon-bearing gasses in the atmosphere would have been methane, carbon monoxide, and the result of their combination with water (formaldehyde),  nitrogen (cyanide and cyanogen), and sulfur (DMSO, only plausible), all triggered by solar ultraviolet. This suggests that the pyrolysis residues will contain sulfur, oxygen, and nitrogen heteroatoms, as does coal. An imaginary pore through the capsule will be lined with such heteroatoms, which are candidates for playing the role of the arginine, lysine, aspartate, and glutamate residues in the ATP synthase catalytic site. Protonation-deprotonation reactions would be available for powering the formation of polyphosphate (a plausible ATP precursor) from orthophosphate. The conformational changes so important in the modern ATP synthase do not appear to be available in this primordial system, so we need to demonstrate the presence of an equivalent. Conceivably, the phosphorus-rich chemicals diffuse up and down in the pore, producing proton transport as they do so that is linked to phosphate condensation reactions. Judging by the modern ATP synthase, coordination of phosphate to magnesium ions may also be part of the mechanism. Mafic rocks such as basalt, a likely early surface rock, are rich in magnesium.

Third-iteration Theory

The flux of acid through the pore will dissolve orthophosphate out of some minerals in the rock such as apatite. If we suppose that the pore is lined with carboxylic acid groups modelling glutamate and aspartate side chains, then at some depth in the pore the pH in the pH gradient will equal the pKa of the acid, and the acid groups will spend half their time protonated and half ionized, resulting in general acid-base catalysis in a narrow zone in the pore. Magnesium-complexed orthophosphate will be catalytically converted to an equilibrium mixture containing some pyrophosphate in this zone and then proceed to diffuse out the exterior opening of the pore before it can be converted back. As a result, the condensing agent pyrophosphate will be available in the early oceans for catalyzing the formation of organic macromolecules such as early proteins and nucleic acids, which are forerunners of important building blocks of modern life forms. 

The efficiency of the pyrophosphate synthesis would be enhanced by a high phosphate concentration, which would be due to the restricted, under-film spaces in which the weathering processes were occurring.

Fourth-iteration Theory

The gradual expansion of the under-film weathered pockets eventually undermines the local pyrolysis film, causing a flake to detach. The remaining rock surface will be largely coated in the first organic polymers created by condensing agents at low temperature. The process then repeats, leading to successive generations of biofilm creation and detachment. At this point, an evolution-like biofilm selection process can be postulated. Polymer chain elongation from outer layer to inner layer would be likely. The outermost sub-layer will be at acidic pHs, which will cleave the outermost polymer into fragments. Some of these fragments will diffuse inward to the polymerization zone and influence events there, leading to a crude form of heredity. The programmed insertion of abscission points would have been an early development, and these may have prefigured the base pairing of modern polynucleotides. The sand produced as a byproduct of rock weathering will end up enmeshed in the polymer and will come off at abscission. 

Fifth-iteration Theory

Could all this happen inside narrow fissures in the rock? Not likely, because the pH gradient would be present only at the opening, a much smaller niche than the area under a surface film. However, the in-fissure microenvironment would be at alkaline pHs, where alkali-requiring reactions would be possible. An example would be the formose synthesis of C5 and C6 sugars from formaldehyde. A C5 sugar, ribose, is an essential ingredient in RNA synthesis. Fissures opening into the under-film spaces could supply sugars to the polymerization zone.