In An RNA World: Why RNA Or An RNA-Like Nucleic Acid Is Such A Popular Candidate For The Origin Of Life

 

A Divided Puzzle

The origin of life remains a puzzle, and may always be that way in the sense that we won't ever completely understand the exact chemistry that gave rise to the first protocell. But the main reason as to why this is the case is the fact that there are many plausible scenarios in which it could have occurred. Never mind the complex chemistry that gave rise to the first building blocks - there is still much division as to what these first building blocks even were. Besides the RNA world, we have metabolism-first, panspermia, iron-sulfur world, clay hypothesis, and more. However, the RNA world remains to be the most plausible explanation for abiogenesis for many scientists. 

The Uniqueness Of RNA 

There are certain features and capabilities of RNA that render it unique and favorable in terms of prebiotic chemistry. For one, it somewhat solves the chicken-and-the-egg paradox in the sense that it functions as a genetic molecule that also catalyzes its own replication. And this replication in turn serves as somewhat of a kickstarter to Darwinian evolution in a molecular genetic sense (opposed to purely chemical systems preceding its emergence). Selection based on the efficiency of RNA replication in which those that display improved replication are selected for seems plausible in and of itself. but another chicken-and-the-egg paradox arises. Some form of evolution is required for the first primitive self-replicating ribozyme to emerge, but without the self-replication feature, there can't be an evolutionary search.  One theory that's been suggested as to the origins of RNA evolution without the assistance of an evolved catalyst is template-directed synthesis, in which some copying occurs preceding the first replicase ribozyme. Basically, RNA can drive chemical reactions as well as store genetic information, making proteins and DNA unnecessary in this origin of life scenario. 

Pre-RNA

While RNA seems like a good candidate for the first self-replicating molecule, it's probably not the very first one. There are a few reasons for this:

(1) It's hard to form long RNA molecules without enzymes.

(2) The building blocks of RNA (ribonucleotides) are difficult to create naturally.

(3) Forming RNA requires a specific type of chemical bond (3' to 5' phosphodiester) to happen repeatedly, but many other reactions could interfere.

Because of these challenges, scientists think the first self-replicating molecules might have been simpler polymers that were similar to RNA. We don't have any traces of these molecules today, but their simpler structure makes them more likely candidates for the first information-storing and catalytic molecules on Earth.

The transition to an RNA world might have happened when these simpler molecules acted as templates and catalysts to create RNA. Lab experiments show that one such simpler molecule (PNA) can indeed act as a template for making RNA.

These pre-RNA molecules likely also helped form the building blocks of RNA from even simpler chemicals. Once RNA appeared, it could have gradually taken over the functions of the pre-RNA molecules, leading to the RNA world.

Emergence Of RNA On The Early Earth 

RNA emerged from nucleotides, likely in warm, shallow ponds on the early Earth. Organic (carbon-based) compounds would have leeched in through drainage and precipitation. Nucleotides likely formed from simpler precursor molecules in this mish mash of organic compounds, aided by wet-dry cycling as well as UV radiation as an energy source. 

The first cells with membranes likely formed when certain molecules that can interact with both water and fats (amphipathic molecules) spontaneously came together to create a barrier. This barrier enclosed a mixture of self-replicating RNA (or its precursor) and other molecules. We're not sure exactly when in the evolution of biological catalysts this happened.

Once RNA molecules were enclosed within these membranes, they could evolve more effectively as carriers of genetic information. This enclosure allowed for selection based on two factors:

(1) The structure of the RNA itself.

(2) How the RNA affected other molecules within the same enclosed space.

This meant that the sequence of nucleotides in the RNA could now influence the characteristics of what we'd consider a basic living cell. The membrane created a distinct unit, separating the internal environment from the external world, which is a fundamental feature of life as we know it.

References:

The Origins of the RNA World

The RNA World and the Origins of Life