Short history on life on Earth

Life on Earth must have started approximately 4 billion years ago and the oldest fossil remains are those of organisms that lived 1 billion years ago: clearly, then, many years of the history of life on Earth are shrouded in the dark of the past and we only know the most recent part of this history.
This part, whose remains have been preserved and accidentally found in the geological strata, testifies of organisms that were already very complicated and divisible into Phyla. So, an essential part of the history of living beings, the one that concerns their origins and first evolutionary stages, can only be based on assumptions, which are corroborated through experiments. Some scientists have assumed that, when the Earth’s crust cooled over 4 billion years ago, some inorganic substances must have synthesised into complex organic molecules. Then, the passage from these complex molecules to the first veritable living beings was slow and difficult. This passage can be assumed to have been as follows:

  1. formation of a number of organic molecules organised into colloidal systems (the so-called “coacervates”);
  2. some protein complexes might have become able to preserve themselves, multiply, transform themselves and use other organic substances existing in the surrounding environment, getting to form, in the end, the first living beings. These must have been the first primary heterotrophic organisms since they metabolised already organic substances;
  3. through accidental chemical changes (mutations), some of these heterotrophic organisms might have become able to perform a rudimental photosynthesis. These organisms must have been autotrophic (able to feed on inorganic substances by themselves), i.e. the direct ancestors of the vegetal organisms that later on would enrich the atmosphere in oxygen;
  4. the autotrophic organisms might have evolved into secondary heterotrophic organisms, the ancestors of animals. Or part of the primary heterotrophic organisms might have evolved into animals. Whatever may have happened, the living beings differentiated into vegetal and animal beings, i.e. into producers and consumers of organic substances. Fungi, that are heterotrophic, and degrading organisms (such as bacteria), able to turn dead organic substances into inorganic substances available for other living beings, must also have evolved at some time
  5. the development and spreading of vegetal species caused oxygen to build up in the atmosphere; this gas changed the breathing of the primitive organisms. Part of the oxygen collected in the upper atmosphere as ozone. Ultraviolet sunrays were thus filtered out, allowing for the development of some special and delicate organisms which could not otherwise have formed and survived in the presence of large amounts of harmful ultraviolet rays. Then, life would find and go the evolutionary way that let it spread all over the Earth, preserve itself until now and diversify into an indefinite number of animal and vegetal species.

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