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How did life start? There is no clear and clear answer, because when life began there was no one there to serve as a witness. But logical analysis of the problem can be done.
Astronomers have reached certain conclusions about the general composition of the universe. They have found, for example, that 90% of it is hydrogen and 9% helium. The other 1% consists mainly of oxygen, nitrogen, neon, argon, carbon, sulfur, silicon and iron.
Starting from there and knowing how such elements are likely to be combined, it is logical to conclude that the Earth initially had a very rich atmosphere in certain hydrogen compounds: water vapor, ammonia, methane, hydrogen sulfide, hydrogen cyanide , etc. And there would also be an ocean of liquid water with atmospheric gases dissolved in it.
In order for life to begin in a world like this, it is necessary that the elementary molecules that existed, at the beginning, combine together to form complex molecules. In general, the construction of complicated molecules of many atoms based on elementary molecules of few atoms requires a contribution of energy. Sunlight (especially its ultraviolet content), by impacting the ocean, could provide the energy needed to force small molecules to form larger ones.
But what were those major molecules?
American chemist Stanley L. Miller decided in 1952 to find out. It prepared a mixture of substances similar to the one that, it is believed, existed in the primitive terrestrial atmosphere, and made sure that it was completely sterile. He then exposed it for several weeks to an electric shock that served as a source of energy. In the end he found that the mixture contained somewhat more complicated molecules than those with which he had begun. All of them were molecules of the type found in living tissues and among them were some of the amino acids that are the fundamental blocks of some important compounds: proteins.
Since 1952 there have been many researchers, from various countries, who have repeated the experiment, adding details and refinements. They have built several molecules by very different methods and then used them as a starting point for other constructions.
It has been proven that the substances thus formed point directly towards the complex substances of life: proteins and nucleic acids. No substance has been found that differs radically from those that are characteristic of living tissues.
Nothing has been achieved yet that not even a maximum effort of imagination could be called living, but it must be borne in mind that scientists are working with a few deciliters of liquid, for a few weeks each time. In the origins of the Earth, what was exposed to the Sun was an entire ocean of liquid for billions of years.
Under the scourge of sunlight, ocean molecules became increasingly complex, until ultimately one emerged that was able to induce the organization of elementary molecules in another molecule like her. With this, life began and continued, gradually evolving to the present. The primitive forms of "life" had to be much less complex than the simplest forms of life today, but they were still quite complex anyway. Nowadays scientists try to find out how that unique molecule that we just formed was formed, to mention.
It seems quite certain that life developed, not as a miracle, but because of the combination of molecules according to a path of least resistance. Given the conditions of the early Earth, life had no choice but to form, just as iron has no less to oxidize in humid air. Any other planet that physically and chemically resembles the Earth would inevitably develop life, although not necessarily intelligent or as the one we know.
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