Long before the first nascent cells of primordial sludge appeared on Earth, it was filled with bubbling organic soup that stood on the cusp of something deep.
This fine line between complex chemistry and the evolution of life represents a turning point in the emergence of biology. Unfortunately, for all its importance, we know very few details about exactly how it happened.
An experiment conducted by scientists at the University of Tokyo reinforced the notion that RNA’s unique talents help explain how life began billions of years ago, supporting the so-called “RNA world” hypothesis.
But the study also shows that things may not have gone exactly as we thought.
Their work shows how a molecule, which is still critical to the survival and reproduction of every living being, can work its way into an evolving system if it works as a team.
“We found that one kind of RNA evolved into a complex replication system: a network of replicators made up of five types of RNA with a variety of interactions, which confirms the plausibility of a long-hyped evolutionary transition scenario,” says evolutionary biologist Ryo Mizuuchi.
Life is made up of molecules that can make imperfect copies of themselves, creating a virtually limitless population of variants that may (or may not) stick together long enough to make copies of themselves.
The search for the origin of life has essentially been a hunt for candidates who can perform this task of replication without the help of highly specialized organic materials such as DNA or proteins.
RNA has long been a leader in this search. It is ubiquitous in the modern biosphere, could exist on the ancient Earth as a result of non-biological processes, is capable of storing a large amount of information and acting as a dynamic physical unit.
This means that it can potentially create structures that can physically build new molecules, which in turn can create new structures. If this process is not perfect, then some “replicator” structures will do their job faster or more efficiently than others, becoming the dominant form of RNA…at least until something even better comes along.
As tempting as this idea is, we have known for decades that the self-organizing units of individual RNA molecules are too simple and unstable for such a scenario. Even its deoxygenated sibling, DNA, is not strong enough to hold itself together long enough for natural selection to do its work.
This does not mean that multiple threads acting as a single team cannot do the job. Having several different replicative units operating at the population level can easily solve this information problem.
Various replicators have been developed from RNA, DNA, and even proteins to show how this might work, with researchers working hard to build in the functionality to allow molecular structures to cooperate and make copies at the right speed.
Although they can support replication, so far none of them have become more complex over time, which leaves open the question of whether RNA is able to evolve.
Mizuuchi’s team has chosen the right design of RNA molecules to create individual replicator molecules that can act collectively, not only storing information and changing over time, but doing so in such a way that the decision becomes more complex with each successive generation.
The experiment used cloned RNA segments in water droplets suspended in oil, which went through more than a hundred rounds of replication, with each round being checked and analyzed.
“To be honest, we initially doubted that such diverse RNAs could evolve and coexist,” says Mizuuchi.
In evolutionary biology, the “principle of competitive exclusion” states that multiple species cannot coexist if they compete for the same resources.
This means that in order to diversify sustainably, molecules must find a way to use different resources one after the other. They are just molecules, so we wondered if it was possible for non-living chemical species to spontaneously develop such innovations.”
The proof of concept shows that this is possible, if only the RNAs do not compete with each other for resources, but rely on each other, like a host and a parasite. If even one RNA replicator is removed, the rest will die out.
While we may be more confident in the plausibility of the “RNA world” scenario, it fails to prove that this is how life flourished on Earth billions of years ago. For this, we will need a lot of evidence, from geology to astrophysics, to create a conclusive proof.
Nevertheless, it is a major step forward in the search for chemical models of evolution that can transform the primordial slurry into a dazzling biodiversity that continues to grow in complexity to this day.
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