Japanese researchers have just brought new elements which plead in favor of the hypothesis of the “world with RNA”, which suggests that life would have developed from these molecules.
The origin of life on Earth is one of the questions that has tortured scientists since the dawn of time, and which remains particularly vivid in our time. There are many more or less far-fetched theories, but new work is bringing additional credence to the most promising hypothesis.
If the researchers are still very divided on this question, there is a point which is the subject of a more or less global consensus; if life did appear directly on our planet and did not arrive from elsewhere, this implies at some point in its existence, the Earth must necessarily have housed a cocktail of chemical elements that can serve as a basis for future living beings.
In the scientific literature, the very colorful term of prebiotic soup is used. The challenge is therefore to identify the transition that could have taken place to go from a theoretical culture broth to life as we know it, with its superb diversity of animals, plants, and so on.
But from there, all clues disappear; at the present time, science does not yet have any determining element which would make it possible to affirm with certainty when, and especially how the line between complex chemistry and life was crossed.
Unfortunately, we are talking about an extremely distant period in time; it is therefore almost illusory to hope to find direct proof of this transition. The only approach therefore remains to use your imagination to try to model the conditions of the time. And that’s where the researchers from the team of Ryo Mizuuchi, a biologist at the University of Tokyo, come in.
Simple RNA molecules, but “Darwinian evolution”
In a recent publication, they showed that when all the right conditions are met, a single molecule of RNA is able to begin a process of “Darwinian evolution”, which continually feeds itself through changes based on mutations and natural selection.
This is particularly important, because if we want to summarize the issue in the simplest possible way, life is nothing more than a collection of self-replicating molecules. The diversity of living things comes from the fact that these molecules respond imperfectlywhich tends to generate a virtually unlimited number of new variants over mutations.
To determine the origin of life, researchers are therefore trying toidentify elements that seem able to replicate in this way, and as independently as possible. RNA – an absolutely fundamental element of this cellular machinery – has been among the most promising candidates for ages because of its universality.
Hazardous and difficult conditions to put in place
It combines several important criteria. We know in fact that it is present throughout the biospherethat it can theoretically form spontaneously from a non-biological processand above all that it is a dynamic unit likely tointeract with its environment. In other words: RNA molecules are able to recombine between them to form new molecules which will in turn be used to develop new structures, and so on.
It has been known for several decades that RNA has this feature, and it is also very important for its role in the human body. But we also know that as such, RNA is far too unstable to be able to serve as a support for a complex and lasting life. It is no coincidence that we have inherited a DNA-based system after millennia of natural selection; it is an immensely more stable support and much more suitable for the preservation of the genome.
This does not, however, rule out the trail of RNA as a precursor to life. On the other hand, this means that there was very probably, at some point in the process, interactions between several RNA molecules able to replicate differently; in the context of this work, the researchers then speak of “replicators”.
Theoretically, if there are enough different replicators, then a population is able to sustain itself; we therefore know that the concept holds up on paper. But at this stage, we cannot yet speak of life. For this, simple replication is no longer enough; the population should be able to become more complex over generations. And at this level, research has come to a standstill for a very long time… until the work of Japanese researchers.
An unexpected result
Indeed, Mizuuchi and his team have finally managed to produce a set of replicators that fit this definition. They can “operate collectively” not only to preserve information, but information is also altered over generations. A process that researchers believe is comparable to “Darwinian evolution“, or a “self-sustaining process of continuous change based on mutations and natural selection.”
It’s a really surprising result that the teams didn’t particularly expect. Because in evolutionary biology, the basic postulate stipulates that in the very long term, species competing for the same resource cannot coexist; if given enough time, the one best suited to its environment is supposed to take over. And this is true at all levels, even for simple molecules.
“Quite honestly, we doubted that such varied RNAs could evolve and coexist because of this principle of exclusion by competition.”, explains Mizuuchi. “These are just single molecules, so we wondered if it was possible for non-living compounds to develop such innovations spontaneously.”
And obviously the answer was a resounding “yes”. This is a very impressive proof-of-concept that makes a strong argument for claiming that life arose from RNA precursors, a hypothesis known as the “RNA World”.
An interesting piece of the great puzzle of life
But despite everything, it is not yet indisputable proof, far from it. The only way to do that will be to keep digging. To hope to arrive at a possible indisputable proof, it will still be necessary to relate piles of data from work in geology, biology, but also in astrophysics.
But that does not mean that this beautiful find will fall into oblivion. On the contrary, these RNAs are a superb platform for studying the mechanisms of evolution. They make it possible to take an interest in it on the most basic scale possible, without having to sort through the immense complexity of living things.
“The simplicity of this molecular replication system, compared to other organisms, allows us to examine evolutionary phenomena with unprecedented precision.”, rejoices Mizuuchi. And with a little luck, this approach will make it possible to discover new elements which will make it possible to determine for sure whether, yes or not, life did indeed develop from RNA.
The research paper is available here.