Life: Monophyletic and Polyphyletic evolution: an unique event
Three and a half billion years ago ... the earth's crust is now becoming more and more stable and solid, the atmosphere is still unbreathable for organisms such as humans, but it doesn't matter, because at this stage there is no life on the planet. The Moon is still very close, its disk fills a large part of the sky and the tidal effects of its gravitational interaction still upset the primordial oceans, turning the masses of water against the emerged lands with immense waves that tear tons of rocks from the reliefs, mixing everything together with the seabed in a continuous erosion that will eventually make the oceans rich in salts and other substances dissolved in them, an inexhaustible source of nutrients for the life to come.
Thousands of comets and meteorites still continue to bombard the planet, in continuous hisses and deafening roars, followed by lacerating explosions that destroy and reshape the newly formed crust, while the primordial atmosphere is crossed by impetuous winds whose scorching mechanical waves make the entire tour of the planet.
Yet, in these conditions that will remain extreme for three billion years, the primordial molecular elements brought by comets and asteroids together with the billions of tons of water that have stratified, evaporating, above the crust, begin to react with each other, forming more complex structures, according to the natural laws of chemistry, facilitated by the aqueous solvent medium and by the highly reducing conditions of the gases present in the atmosphere, released from volcanic emissions or residue of the constituents of the nebula that originated the solar system, including the Earth, from the remains of a supernova.
This is definitely an unique situation, which will never repeat itself for the entire history of the planet Earth. New organic complexes, such as small peptides and protides, are formed starting from the primary amino acids and amines coming from the stellar nebulae brought by the bolides. Many of them are destroyed in huge explosions that liquefy the crust as soon as solidified. In this continuous molecular mixing, subjected to electrical discharges from the numerous lightning strikes that ionize the atmospheric elements, something molecules manage to organize themself anyway, because these reactions fortunately take place in the primordial ocean on a planetary scale.
And it is thus that, in continuous occasional and fortuitous attempts over billions of years, the first coacervates will be formed and from them the protobionts will gradually acquire more and more complex and functional structures, such as cell walls and other organelles accidentally incorporated. Such structures give rise to the first protovirus and the first archaea. Not just one, that a meteorite could easily destroy, but fortunately countless types of such structures formed independently in different ocean areas. Some of them will inevitably be destroyed, but many others succeed to survive, and from these tenacious and random survivors life evolved as we all know it today.
But if we know well the life present on our planet today, or vaguely the one that preceded humans in all its complexity, unfortunately we do not know at all, nor are we able to imagine in detail, the various passages that, from elementary structures simple of organic and inorganic molecules, gave rise to the first coacervates and from these to the first protobionts. Nor are we yet able to speculate what fantastic reactions between nucleotides and their precursors generated the first simple element of self- replicating nucleic acid. Nor do we know how this element was incorporated and maintained in a coacervate giving rise to the first organelle capable of replicating with all the attached structures, more or less as some crystals do in their simplicity of inorganic structures. We will therefore never be able to classify these hypothetical primordial structures that we have never seen and will never see. We also have to decide whether to define them already living species as they are able to duplicate themselves in copies identical to themselves or not.
It is however logical to suppose that it was initially the nucleic protoacid that replicated, and then a protein ended up binding to it and becoming, together with lipid aggregates, an integral part of an envelope and other structures that went to protect and participate in the very delicate structure of the nucleic acid itself. A subsequent casual incorporation of other organelles with different functions, such as the mitochondria, once free and autonomous, made these protobionts more and more elaborate and complex, until they eventually become what we could undoubtedly define life. But this can no longer be observed, because it has not happened in nature for billions of years, and we are not even able to reproduce it artificially in the laboratory. The atmosphere of the planet Earth has changed radically, the primordial "soup" also no longer exists; all the initial planetary conditions that favored these events now belong to the past, and not knowing the exact combination of parameters, we are unable to recreate in laboratory those events that originated life on Earth.
And let's not forget that it took billions of years of trying to make these first difficult steps, and no laboratory experiment will ever last as long as we may be able to accelerate those biochemical reactions to the nth degree. This is therefore the true miracle of life: not the one we witness every day with the birth of a new organism, the sprouting of a seed or the evolution of new forms from ancestors that have now disappeared. This is trivial, obvious. I would say, having by now become aware of biological and evolutionary mechanisms, but it is also a very fragile mechanism, on the edge of a razor blade. Life, in fact, as we observe it while at the same time being part of it, is nothing but the persistent residue of a chain reaction that does not stop taking place as conditions still allow it. Nothing can stop it abruptly. Life can only replicate itself in all its complexity with difficulty achieved by the pre-existing one and randomly evolve into new forms gradually selected by the changing biological and geological environment. But if a cosmic event interrupted this chain of biochemical events on a planetary level, everything would cease forever without the possibility of replication. Unless the conditions returned to the initial ones, with an atmosphere no longer oxidizing, but again reducing, and with a similar primordial broth where everything can start all over again. But to do this it would take a planetary collision of enormous proportions with a celestial body of the size of the Moon or Ceres, which is able to dissipate the current atmosphere and reshape the earth's crust by releasing again those volcanic gases that favored the miracle of the birth of life from its most elementary precursors.
But all of this has long since ceased, or rather it has been interrupted forever, absurdly. In fact, it is life itself that has evolved on this planet that has decreed the end of the possibility that everything can happen again, thus making itself a unique and unrepeatable event, as if someone had thrown away the mold that "god" created. And this happened when the first autotrophic bacteria able to use stellar radiation to produce the nutrients they needed to grow and reproduce without having to extract or actively search for them in the environmental pool, began to release, as a by-product of these complex photosynthetic reactions, the molecular oxygen in the aqueous solvent that received them.
This at first dissolved in it, thus delaying its release into the atmosphere for billions of years, but then, when the saturation was such that the excess spilled over the ocean surface, the stromatolites and cyanobacteria undertook that terraforming that we know today.
It was only when the earth's atmosphere had a sufficient composition of oxygen and ozone capable of protecting the lands that emerged from the harmful ionizing effects of ultraviolet radiation, that life - that in the meantime continued to evolve and change sheltered under the sea surface - could allow themselves to reach the air and colonize the lands that emerged with the new heterotrophic predatory organisms capable of using for their metabolism that waste product which is the oxygen released by the autotrophic precursors, which slowly and inexorably gave way to them or became theirs prey in the race for evolution. Nowadays there are few bacteria of this type, including the anaerobic sulphobacteria that have remained relegated to extreme environments or to deep ocean in the vicinity of still active volcanic ducts. As for stromatolites, very few living fossils still remain as evidence of something that will probably never happen again. At least on this planet. Elsewhere, in the vast Universe, perhaps something similar has happened, is happening or will happen, but we can only fantasize this at the moment, remembering how the phenomenon of life, although in a different and variegated way it may present itself in dissimilar and multiform planetary situations, is still an event so rare and dependent on so many fortuitous factors, that it is considered to all intents and purposes the most precious event we can ever hope to encounter. Our existence included, we who take everything for granted, as if the resources of this orbiting stone were inexhaustible, destroying and devastating entire ecosystems, leading to extinction every day countless forms of life that Nature will never be able to reproduce again.
