The Precambrian is a very long period in the evolution of the Earth that covers from the formation of the planet, about 4,560 million years ago, until about 541 million years ago
In the table on the previous page the Precambrian appears as Eon formed by three ages. Some people classify it as supereón and divide it into 3 eons: Hedic, Archaic and Proteozoic. The first has no ages, the second has 4 and the third 3, each divided in turn into several periods. Here, to simplify, we make only the three main divisions.
The complete information on the divisions of this supereón can be consulted in the geological time scale from Wikipedia.
The Precambrian lasted more than 4,000 million years. It occupies 88% of the history of planet Earth. Two things are clear: that it is the longest geological period and that, on it, our planet stabilized and the first living organisms appeared, very simple.
Eon Hádico or Azoic era
This period in which the Earth was transforming from an incandescent ball to a planet with crust, is also known as Hadeic or Hadeano. At the beginning, about 4.530 million years ago, the Earth received the impact of a protoplanet the size of Mars, called Torch. The torn material formed the Moon, according to the big impact theory.
The Solar System was still in the process of formation. The earth's crust solidified, although it underwent many changes, due to the continuous volcanic eruptions. The oldest rocks that have been found on Earth are about 4,400 million years old.
The Hádico is usually terminated about 4,000 million years ago, when the Earth was relatively stabilized, although with a thinner and more fragile crust than the current one
Aric Eon or Archeozoic era
During the Archaic era there was a late intense bombardment, which affected the inner planets of the Solar System. The inner core of the Earth formed, which generated a magnetic field. The internal structure of the planet began to resemble the current one, although hotter. The crust was divided into blocks that began to move, starting a Tectonic plates.
It is believed that the Sun at this time had not yet reached its maximum brightness, so the temperature dropped to almost current levels despite the fact that the atmosphere, still without oxygen and with many greenhouse gases, retained heat more than In no other time.
The crust was also cooling and the first igneous and metamorphic rocks formed. The abundant rains generated the oceans and seas, while the temperature at surface level continued to decrease.
The first signs of life appeared in these warm seas about 3.7 billion years ago from simple molecules capable of replicating. 3.5 billion years ago cyanobacteria already existed that made photosynthesis, consuming carbon dioxide, although they still did not release oxygen into the atmosphere.
These bacteria fixed the carbon that formed the first stromatolites. About 2800 million years ago, the first organisms capable of releasing oxygen into the atmosphere appeared, which began to change.
Life was surely present throughout the Archaic, although limited to organisms prokaryotes. And there would be no turning back, despite the numerous cataclysms that, since then, have caused mass extinctions. But life always comes back and reorganizes itself.
Proterozoic Eon or Proterozoic era
About 2.5 billion years ago, the Proterozoic began, a word that means "initial life time." It is the longest division in the geological time scale of the Earth, since it lasts about 1.960 million years.
At this time the continental platforms were formed and large glaciations occurred. The first known dates from about 2.3 billion years ago; the most intense of this eon occurred towards the end, in the Cryogenic Period, clear and evocative name. It is called Earth snowball and was about to end life on the planet.
The oceans stabilized, relatively, and the atmosphere began a slow but constant transformation, increasing the proportion of oxygen and reducing that of methane and carbon dioxide.
As for the evolution of life, in the previous eon some complex molecules got together, in a warm and humid environment, to form the first unicellular organisms. They needed almost 1 billion years to get organized in more complex ways. They did it in the last third of the Proterozoic era with the appearance of eukaryotic cells.
About 541 million years ago multicellular organisms appeared. From here the Proterozoic is terminated and, with it, the Precambrian.
Throughout the dark Precambrian, a good part of the material base that constitutes the Earth's crust was formed, in which we live and where the geological phenomena that most affect us occur.
• The origin of photosynthesis and aerobic respiration
• The formation of the Earth's oceans
• What was the Late Intense Bombing?
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