RNA World Hypothesis

The RNA World Hypothesis Explained

It's widely agreed that the most crucial characteristic of early life forms was the ability to replicate and reproduce. Otherwise, once the life form's time had come, there would be nothing to continue on its legacy, and it certainly couldn't have given rise to the plethora of biodiversity found on earth today.

The RNA World Hypothesis is based on RNA’s ability to self replicate. Could a simple self-replicating strand have been the first life form to emerge? RNA’s ability to replicate without external molecules would suggest so.

Steps within the RNA World Hypothesis

Since all life is descended from one common ancestor, we can presume the same evolutionary steps were taken by all surviving life forms in the early origins of life when the RNA world model applied. This is until the emergence of the last universal common ancestor (LUCA).

Primordial Molecular Soup

Like many other origins of life theories, the RNA world starts in the primordial soup (Fig. 1).

Following spontaneous chemical reactions, the primordial molecular soup became full of free-floating nucleotides, the building blocks which make up DNA and RNA.

These free-floating nucleotides gave rise to the very first RNA strands. However, they were quickly broken down again and then built back up in a repeating cycle. More stable strands of RNA grew longer, binding more nucleotides until they were growing quicker than they were being broken down.

The Emergence of Self-copying RNA

Eventually, self-copying RNA emerged. The structure of the RNA would have influenced how quickly and successfully it could replicate until one self-copying RNA outcompeted the rest through natural selection.

Reproduction, either sexual or self-replication, is a crucial requirement for early life so these self-replicating strands could be considered the very first life form.

The Frst Proteins

The world's first protein emerged with a little help from grabber RNA. Grabber RNA are simple RNAs that literally ‘grab’ onto other RNA strands or molecules to form complexes - changing or enhancing their function in the process. This first protein or compound was likely a primitive ribosome and kickstarted further protein synthesis.

Soon after grabber RNA came the development of spliceosomes.

The ability to produce a variety of proteins from one strand of RNA would have been incredibly beneficial for early organisms, speeding up their evolutionary journey.

The evolution of DNA

Millions of years went by, and more and more complex RNA emerged, eventually giving rise to complex contemporary proteins and later DNA. Then, in the way you might delegate your chores to a younger sibling, proteins and DNA took on the roles filled by early RNA. The proteins became responsible for driving chemical reactions within the cells, whilst DNA, which was more stable than its RNA counterpart, was now responsible for the storage of genetic information.

This is how the RNA World Hypothesis suggests we ended up with DNA as the primary genetic material and proteins as the primary drivers of cellular reactions. RNA formed and eventually gave rise to the predecessors of each of RNA's initial functions, leaving RNA to act as a go-between.

Evidence for the RNA World Hypothesis

The RNA World Hypothesis is the most widely supported theory of our current origin of life on earth hypotheses, as scientists have managed to uncover some pretty convincing evidence to prove their case…

Test tube Earth:

The RNA World Hypothesis is reliant on the existence of free-living nucleotides. Within the primordial soup, RNA was being continuously broken down.

Forming longer and longer strands that grew at a rate faster than they came apart was the only way to escape complete breakdown. Therefore, an abundance of nucleotides would be an advantage, if not a necessity, if RNA strands were to grow into more complex machinery which could underpin life. If RNA really wanted to succeed, it would have to make more nucleotides itself…

Many scientists have recreated the conditions of the early earth and the primordial soup within test tubes. These isolated experiments have eventually evolved complex RNA strands that can drive chemical reactions to produce some nucleotides.

The RNA world hypothesis states nucleotide building RNAs would be required towards the beginning of the origins of life to provide the building blocks for longer and more complex RNA strands. These test tube experiments prove that RNA can drive the required nucleotide-building reactions.

Why RNA not DNA?

RNA can both drive chemical reactions, and carry genetic information. Whereas DNA can only do the latter on its own (Fig. 2).

Figure 2: The difference in structure between RNA and DNA hints at which came first in the evolution of life and genetic material, Source: biologydictionary.net

Not only does the multifunctionality of RNA suggest it was the initial form of genetic information but so does RNAs structure.

One of the building blocks of RNA is ribose, a simple carbohydrate which has been seen produced in 'test tube earth's' replicating primordial conditions.

Conversely, DNA contains deoxyribose, a more stable yet complex sugar which is often produced using ribose as a reactant. In modern cells, this reaction requires protein catalysts, and therefore it is unlikely DNA arose before RNA had a chance to drive protein synthesis. This stands as evidence that RNA is the precursor to DNA.

Signal recognition particles:

The emergence of proteins which convey any kind of benefit to the organism would have been a massive advantage to the early RNA life forms. However, this benefit would only occur if the protein was in the right place at the right time.

As proteins underpin all cellular processes this is the same for modern-day cells. Modern cells contain signal recognition particles (SRPs).

SRPs are constantly scouting for the production of new proteins by ribosomes. When the synthesis of a new protein is registered, SRPs bind to the end of the protein whilst it is still undergoing synthesis, halting the process until the protein is transported to its new home in the cell.

SRPs are made up of proteins and RNA, and this same RNA is found within all modern-day living organisms. This suggests early SRPs evolved near the beginning of the origins of life, and since they are built from RNA this supports the RNA world hypothesis.

Problems with the RNA World Hypothesis

The RNA world hypothesis is by no means perfect, and whilst it is regarded by the majority of scientists as the leading theory, it has no shortage of opposition.

The Absence of Peptides:

Peptides, short chains of the amino acids which make up proteins, facilitate transcription and translation. These are both crucial processes for sustained life.

Some scientists doubt the ability of peptides to have essentially created themselves (as peptides are formed, with the help of enzymes, during translation). They instead suggest the ‘RNA-Peptide World Theory’ where peptides were present in the primordial soup.

However, scientists who support the RNA World Hypothesis have conducted experiments where peptides have formed under conditions similar to primordial earth from the precursors of amino acids.

RNA is a Complex Molecule:

There are some concerns about the complexity of RNA and the likelihood of it arising pre-life. The ‘metabolism-first’ hypothesis proposes that the first living beings were chemical cycles which produced more complex molecules.

Though scientists favoring the RNA World Hypothesis have pointed out these beings would be unable to undergo Darwinian evolution. Therefore, RNA formed after this point would still be classed as the first emergence of life.

You might have noticed earlier that we said only some nucleotides have been produced by RNA in test tube earth experiments. Some scientists believe this isn't enough, and that the remaining nucleotides and corresponding RNA machinery are too complex to form as early as required in the RNA World Hypothesis.

Inaccuracies in RNA copying:

There's a reason why our genetic information is stored within DNA today. It is simply much more reliable, as RNA suffers from frequent copying errors. These errors could be disastrous for early evolution, as when the error rate becomes too high Darwinian evolution losses its advantage.

Summary of the RNA World Hypothesis

In summary, the RNA world hypothesis states the original genetic material must have been RNA as it can perform the functions of both proteins and DNA. Other important characteristics of RNA as the precursor to DNA are RNA's ability to self replicate, RNAs (slightly) simpler molecular makeup, and the structure of ribosomes as evidence of an early need for RNA to drive its own chemical reactions in the absence of DNA and proteins.

The RNA World Hypothesis is the most well regarded and strongly backed theory by scientists for the origin of life on Earth.