Anoxic photolysis as a Route to Primary Metabolites on Early Earth

Despite many years of research, results from prebiotic chemistry experiments—where biologically relevant compounds are formed from simple monomers such as cyanate, CO2, or formaldehyde— still yield complex mixtures with only trace amounts of the molecules of interest. The prevailing idea that life originated from a sequence of chemical reactions, starting from very simple active molecules and evolving into progressively more complex systems, might be incorrect for two reasons: First, after 70 years of experiments, we still lack robust experimental demonstration of such networks; second, small reactive molecules typically do not exist in high concentrations and readily react to form tars and other complex mixtures. Here I adopt an alternative approach, demonstrating that numerous primary metabolites can be derived from tar-like mixtures through the Advanced Reduction Process (ARP). Tars are formed by polycyclic aromatic hydrocarbons (PAHs), which are widespread in the Universe and likely were the main source of organic carbon on early Earth. Under conditions thought to exist on the prebiotic Earth—characterized by sunlight, an anoxic sulfur-rich atmosphere, and iron-rich dust - PAH suspensions in water are decomposed to succinic, lactic, malonic, and glycolic acids, all known as primary metabolites in all living systems and main components of ubiquitous citric acid cycle. The sum of reaction yields for these biologically important metabolites reaches up to 40%, with individual product yields ranging from 1-15%. The simplicity of this reaction and the ubiquity of PAHs on prebiotic Earth make this process particularly appealing, suggesting it could have occurred on a vast scale on sunlit planetary surfaces. These findings may reveal an overlooked initial step in the origin of life on Earth, and potentially elsewhere in the Universe.