72 Origins of Life Chemistries in an RNA World
In the tidal pool scenario, with its feel of ‘best-fit’ with origins of life in a reducing environment, the energy for polymer formation from organic monomers came from an overheated earth environment. In that scenario, we considered the possibility that chains of nucleotides might have been synthesized, and then even replicated to form populations of nucleic acids. But if the prebiotic environment was non-reducing, where would the energy have come from to make any polymers, let alone ones that could replicate themselves? If you guessed that the energy was provided by a proton gradient between biofilm-enclosed acidic proto-cells and an alkaline ocean…, you would have been right! In this case, then polymers would have been synthesized in enclosed spaces, and not in tidal pools only to be dispersed and diluted in the wider oceans. And then, how would replicative, informational and catalytic chemistries have arisen from these organic monomers and polymers? Polypeptides would have formed, but they have no inherent chemical or structural basis for self-replication. Unlike polypeptides, we saw in describing the tidal pool scenario that polynucleotides (nucleic acids) do! In fact, evidence is accumulating to support the increasingly accepted hypothesis that life originated in a RNA world:
- Today’s RNAs include ribozymes that catalyze their own replication (e.g., self-splicing introns).
- Some RNAs are part of ribonucleoproteins with at least co-catalytic activity (recall ribosomes, spliceosomes and the secretory signal recognition particle).
- Retroviruses (e.g., HIV) store their genetic information in RNA genomes that may have been integral to the emergence of cellular life.
Ribozymes, ribonucleoprotein structures and retroviruses may be legacies of a prebiotic RNA world. In fact, in an ‘in vitro evolution study’, self-replicating ribozyme polymerases in a test tube become more efficient at replicating a variety of increasingly longer and more complex RNAs over time.
There are hypothetical RNA world scenarios for the origins of replicating, catalytic polymers, and even a real organic chemical autocatalyst that can catalyze its own synthesis. So, which may have come first? A self-replicating RNA or some other selfreplicating molecule, even a self-replicating organic molecule? Arguably, chemical evolution of an autocatalytic RNA is a stretch, but at least one organic molecule, Amino-Adenosine Triacid-Ester (AATE), is a present-day self-replicating autocatalyst. Could an organic molecule like AATE have been a prebiotic prelude to the RNA world? The structure and replication of AATE are described below (Figure 1).
The replicative reaction proceeds in the following steps:
- The aminoadenosine triacid ester binds another molecule of aminoadenosine.
- The two aminoadenosines, now in opposite orientations, can attract and bind a second ester.
- After bond-rearrangements, the molecule separates into two molecules