My interest in prebiotic chemistry is to expand the palette of plausible geochemical energy types on the early Earth that could have facilitated the origins of life. I specifically investigate how ionizing radiation (alpha, beta, gamma and X rays) can open up new chemical possibilities that are not possible through redox- and pH-gradient-driven chemical processes alone.
Regardless of how one defines complexity, an implicit idea of ‘complexification’ is woven into the heart of many origins of life scenarios: compounds and the reactions between them are initially simple, and through some (as yet unknown) set of circumstances, the system gradually becomes complex until life emerges. It finds parallels in the macroevolutionary trajectory of life, where some lineages of organisms indeed complexify over time. This is traceable in a fossil record that shows evidence of simple eukaryotes evolving from prokaryotes, and more complex eukaryotes evolving from simpler eukaryotes. This has led to origins research experiment designs that attempt to recapitulate steps of biosynthesis under abiotic circumstances – circumstances that largely match those in which life thrives.
My colleagues and I developed the concept of subsumed complexity to guide the design of new origins research experiments. From a thermodynamic perspective, the geochemical settings in which life emerged are far more likely to have been as complex as or even more complex than the first, simplest living cell. This likelihood leads to the articulation of a completely novel search strategy for life’s origins: we ought to seek conditions of maximal chemosynthetic complexity, regardless of whether or not these circumstances are habitable to current life.
Complexity as Thermodynamic Depth
The concept of subsumed complexity provides a logical basis for seeking out the most complex, naturally-occurring chemical circumstances, but how can one gauge whether a system is more or less complex? The concept of thermodynamic depth developed by Lloyd and Pagels indicates one way (indeed, probably the simplest way) to generate the most complex conditions of organic synthesis. In this formulation, complexity may be assessed by the difference in fine- and coarse-scale entropy. In other words, if there is a significant difference between the entropy of individual energy input processes and the overall, summed effect of those processes at the macroscopic scale for a whole system, then those processes are likely to be more complex. By this logic, energy input by extremely low entropy energy sources (i.e., particles such as alpha, beta, gamma or X-rays) ought to yield more complex outcomes than energy input by relatively high entropy energy sources such as visible light, heat and even highly-reactive compounds, even in cases where the macroscale entropy production are approximately the same.
The result of irradiation by these kinds of powerful particles is a cascade of energy transduction that starts at the level of inner electron displacement, moves to outer valence shell displacement and rearrangement, and ends with the production of stable bonds and waste heat. Moreover, the products of a given level are constantly perturbed by both transfer of new energy into levels above it and the accumulation of products in levels below. As the number of chemical species in the system diversifies, the system must continually adjust to these perturbations. What this means, in practice, is that a mass-enclosed volume can complexify without need of controlling or ‘tuning’ its composition through selected introduction or removal of compounds, such as in a chemostat.
Why is this significant?
Moving from a prebiotic model that looks like a chemostat to one that looks like a mass-enclosed system is a subtle but incredibly powerful possibility. So long as systemic order is reliant upon external mixing and waste product removal, as in a chemostat, it is difficult to assess the extent to which order arises due to natural emergence or due to artificial ‘programming’ of the system. By contrast, a mass-enclosed system would imply a prebiotic setting in which order can arise and persist de novo without external manipulation, save for the removal of waste heat.
An imperfect physical analogy to the energy cascade described above is one of sand grains shifting down sandpile slopes. In this analogy, the individual sand grains are electrons, and the individual tiles where they can be stacked represent atoms and molecules around which these electrons orbit. Sandpile toy models (and their realized counterparts) can exhibit properties found in automata and self-organizating systems. My current work therefore focuses on whether or not self-organizational properties can be found within actual, laboratory-scale radiolytic systems, and if so, how such properties might be observed and quantified.
My accomplishments so far include:
- Discovering the first known path for prolonged and abundant geochemical production of formamide starting from atmospheric methane and nitrogen (Scientific Reports).
- Demonstrating that radioactive mineral deposits on the young Earth could have created uranium fission zones, which would have been the strongest, most concentrated natural source of far from equilibrium energy in the history on our planet (Astrobiology).
- Demonstrating that uranium fission zones create heating patterns that can promote the formation of polymers and oligomers (Origins of Life and the Evolution of Biospheres).
The Cobalt-60 gamma source at the Tokyo Institute of Technology, accessible through my colleagues at ELSI. The glass vials started out as transparent but became darkened by point-defects in the glass caused by the gamma radiation.
In the coming months, I’ll be testing a new theoretical pathway for the production of polyphosphates, and investigating the properties of novel amides and nitriles created by gamma radiolysis.