Notes from my Origin of Life class:
Metabolism-first hypothesis: The first possibility is that life began with metabolism, and genetic molecules were incorporated later. This metabolism-first scenario states that life began autotrophically. Life began with a self-replicating chemical cycle, such as the reverse citric acid cycle, perhaps on a mineral surface. Variants of this cycle competed for resources, and the system became more efficient and more complex. All subsequent chemical complexities, including genetic mechanisms and encapsulation into a cell-like structure, emerged later by the process of natural selection. Life emerged as an evolving chemical coating on rocks.
RNA World: The second scenario is that life began with a self-replicating strand of some genetic molecule—the RNA World. Life relied on an abundance of biomolecules in the environment. Metabolism was incorporated later. A template, such as a clay mineral or astack of PAHs, helped to assemble information-rich polymers from organic molecules in the prebiotic soup. Ultimately, one of these polymers acquired the ability to self-replicate. Then, as variants of the genetic polymer became more efficient at self-replication, new chemical complexities arose through natural selection. In this genetics-first version, life began as an evolving polymer with a functional genetic sequence.
The model lacks the crucial support of one experiment that demonstrates the prebiotic synthesis of a genetic polymer like RNA. If such an experiment succeeds, most experts will agree that the problem of life’s chemical origins has been more or less solved.
Dual metabolism-genetics hypothesis: In the third scenario, life began as a cooperative chemical phenomenon, arising between metabolism and genetics.
It is possible that neither a primitive metabolic cycle nor a primitive genetic molecule could have progressed far by themselves. Metabolic molecules without enzymes tend to react in uncontrolled ways and may be too unconstrained to sustain advanced cellular life. Genetic molecules are much too unstable, and their individual building blocks—the nucleotides—don’t seem to emerge spontaneously from the soup.
Yet what if crudely self-replicating genetic molecules became attached to a crudely functioning surface-bound metabolic coating? A kind of cooperative chemistry might have occurred.
If experiments establish easy synthetic pathways to both a simple metabolic cycle and an RNA-like genetic polymer, such a symbiosis may be the most likely origins scenario of all.
Source credit: Professor Robert M.Hazen, George Mason University
