The spontaneous process at the origins of life. Unknowns and possibilities

When discussing about prebiotic evolution, or how the chemical world on the primordial Earth could lead to the appearance of the first Darwinian replicators and therefore of the first recognizable living organisms, the process under discussion – or rather the various possible processes identified so far to make the journey from the inorganic world to life – must be located within the geological history of our planet. A very young reader of mine, through his mother, asked me for information on the matter, and I will take the opportunity to briefly present to the reader what the current state of our knowledge on the matter is.

Our planet formed about 4.56 billion years ago within the protoplanetary nebula that enveloped our star, the Sun, thus initiating that epoch of about half a billion years called the Hadean. The name recalls Hades, the hell of the ancients: that was an extremely turbulent period for the entire solar system, and our planet was certainly no exception. Even in the presence of a possible ocean and already formed crust, the most accredited hypothesis at the moment is that the planet was devastated by a gigantic impact with a body called Theia, more or less the size of Mars. If the hypothesis is true, this impact, which occurred about 4.5 billion years ago, separated the Moon from the young Earth, which in fact has a chemical composition similar to the Earth’s crust and mantle, and certainly evaporated any ocean that might be present and destroyed every form of prebiotic system that appeared up to that point.

Despite this, in a very short time from a geological point of view, the earth’s crust consolidated again and an ocean enveloped it at least partially: a zircon formed 4.4 billion years ago and analyzed by scientists demonstrates both the presence of solid earth and large extensions of water. Now, the truly amazing thing is that, between 4.28 and 3.77 billion years ago, advanced microbial life was already present on our planet: very ancient sedimentary rocks, recovered in Canada, interpreted as seabed in near hydrothermal vents, they have returned what appear to be microfossils of filamentous bacteria capable of accumulating iron, which from a chemical point of view have an isotopic composition which is the prerogative of living organisms.

think about it: in a time that can be shorter than 300 million years and in any case not more than 800, we have gone from a boiling planet due to the impact with Theia to one with microbial life at least near the hydrothermal vents, supported by microorganisms whose cells , at least from a general point of view, do not appear particularly dissimilar from those of today in the same environments. In this period, the appearance of the first molecular systems capable of self-replicating and mutating must have taken place, which very rapidly then evolved into very complex compartmentalized biochemical systems equipped with metabolism that we know today and which we call microorganisms.

The incredible thing is that, in the last 15-20 years, we have managed to find more different ways in which this could have happened starting from physical conditions and molecules present with high probability in our primordial planet, of which it would be more difficult to explain the absence, given their ubiquity in the cosmos. The very fact that there are many possible alternatives explains why the process that led to life spontaneously took place: every new road that we identify, regardless of whether it is the one actually traveled billions of years ago, increases the overall probability that, given the reconstructed conditions, Darwinian replicators that give rise to complex organisms will evolve (even if not necessarily those we know ).

I will illustrate for my young reader one of the many possibilities, which has the advantage of being supported by an impressive amount of data and scientific publications, all of which show how replicators made of RNA are spontaneously obtained starting from different conditions, all however compatible with our starting point.

RNAs, as we know, are linear polymers (strings) of nucleosides which, by curling up into specific three-dimensional shapes, can perform many different functions. One only 150 nucleosides long, in particular, is able to replicate itself and, if necessary, also other and different RNAs; the interesting thing is that this is formed spontaneously by random assembly of specific fragments, through reactions that occur spontaneously in aqueous solution. Precisely because its formation starts from fragments of smaller dimensions, this particular RNA does not need to be produced “by chance” one nucleoside at a time, which would require infinite time and would have a very low probability; not surprisingly, the scientists who discovered it did not have to wait billions of years for its foundation. The fragments of which it is composed, shorter than 30 nucleosides, can be generated spontaneously under a variety of conditions; on the other hand, starting from random mixtures of small pieces of RNA only 3 nucleosides long (64 types in all, since nucleosides are of 4 different types), RNA molecules even much longer than 150 nucleosides are obtained on volcanic glasses .

The possibility of hierarchical assembly of smaller pieces greatly decreases the time required to obtain a certain sequence of RNA, which is able to catalyze its own replication; on the other hand, since there are many and very different RNA replicators discovered up to now, the probability that in this way one will be obtained in the time and conditions required by what we know of the ancient Earth becomes high (once again, the demonstration lies in the fact that these molecules have been identified by scientists by random spontaneous synthesis under different conditions). Furthermore, once the first replicators have been obtained, in a compartmentalized mixture of RNA and different precursors (for example, in the fissures of a mineral, or in a lipid drop) it has been demonstrated that a tiny ecosystem is formed in a few hundred generations , with different replicators and with parasitic RNAs that exploit them, until everything gradually evolves towards the coordinated replication of several different components – that is, towards a biochemical system with an increasing number of components that cooperate with each other.

Since even very small RNAs, of less than 10 nucleosides, can recruit amino acids and peptides to achieve greater stability, and can transfer these amino acids and peptides to other RNAs, it is easy to imagine how and why, in the first instance, proteins were recruited in this world to RNA; and considering that lipid bubbles were widespread and spontaneously incorporated RNA and other components within them, precisely regulating their distribution in daughter bubbles in correspondence with the fact that they were destabilized when the internal RNAs had increased too much in number due to replication, there is an at least pictorial description that leads from the abiotic world to the first replicators and the first cells, through the combination of spontaneous processes under conditions assumed to exist in the primitive Earth, at a speed that can even be observed during the life of a human being.

Research continues, and the alternatives and details that are emerging are many and fascinating; and thus, abiogenesis appears to be an ever better explainable and probable process, even if perhaps we will never know which precise path was followed in the case of life as we know it.