Precambrian Time

The Precambrian supereon comprises the materials and processes that have occurred from the formation of the Earth to the biological diversification that gives way to the Phanerozoic, the Cambrian explosion (541 ma).

The Precambrian covers 89% of the Earth's history, and although the remains that have arrived are scarce because they have been subjected to a great variety of geological phenomena, at this time important events took place such as the formation of the Earth, the appearance of of life or the formation of the first continents.

The Precambrian is divided into three eons:

• Hadic Aeon (4,500 to 3,800 ma).
• Archaic Aeon (3,800 to 2,500 ma).
• Proterozoic Aeon (2,500 to 540 ma).

Earth in the Hadic (4,500 to 3,800 ma)

The name Aeon Hadic derives from Hades, guardian of the underworld in Greek mythology. During this time, since the Earth originated, the bombardment of meteorites was continuous, in such a way that its surface did not cool until 3900 my and the formation of the solid crust could begin.

In the Hadic, the Moon was also formed, it is believed that by a collision between the Earth and a protoplanet the size of Mars.

No Hadic rocks have been found. In lunar rocks and meteorites it has been possible to find rocks of ages close to 4500 ma, but on Earth, due to plate tectonics and erosion, there are no rocks older than 3800 my The oldest remains found are zircons of 4100 ma, which would be the oldest minerals on Earth.

The atmosphere in the Hadic was not like it is today. It lacked oxygen and was made up of gases such as hydrogen, methane , ammonia, CO₂, etc.

The main events that took place in the Hadic Aeon were:

• The three solid layers of the Earth were formed: core, mantle and earth's crust.
• The atmosphere was formed, reducing (without oxygen).
• Since the formation of the Earth there has been a great bombardment of meteorites.
• The Moon was formed.
• The primitive oceans were formed.

By Tim Bertelink (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Earth in the Archaic (3,800 to 2,500 ma) and Proterozoic (2,500 to 540 ma)

The Archaic Aeon, formerly known as the Archaeozoic, began 3,800 my ago, the age of the oldest sedimentary rocks known to show that there was biological activity. They are rocks of marine origin, so they are the first evidence of a hydrosphere on Earth.

By Tim Bertelink (Own work) [CC BY-SA 4.0], via Wikimedia Commons

The Proterozic Aeon began when free oxygen in the atmosphere increased and its composition changed, favoring the appearance of eukaryotic cells and multicellular beings.

The main events that occurred in the Archaic and Proterozoic are:

• Cessation of the meteor shower that took place during the Hadic.
• Appearance of the first continents.
• The tectonics starts running.
• Release of oxygen into the atmosphere that goes from being reducing to being oxidizing.
• Although several "Pangeas" probably existed, the last one, Rodinia, was formed around 1,100 million years ago. When Rodinia fragmented 750 million years ago, the continents froze, the coldest period in history. Intense Precambrian glaciations took place, which could cause the first great extinction of living beings.

Life in the Precambrian

The first living beings to appear were anaerobic prokaryotes, of which there is evidence of their existence from about 3,500 my ago. These prokaryotes were heterotrophic, and they fed on organic molecules from archaic seas.

When organic molecules began to be scarce, the first photosynthetic autotrophic prokaryotes, cyanobacteria, emerged 2000 my ago, which due to their photosynthetic activity, reduced the amount of CO₂ and increased the amount of O₂ existing in the atmosphere. During the Proterozoic, colonies of bacteria formed rocks called stromatolites.

The appearance of oxygen in the atmosphere caused the first aerobic respiration organisms to appear.

Then, about 1500 my ago, the first eukaryotic cells appeared, and at the end of the Proterozoic, the first multicellular organisms. They were flat-bodied organisms, such as sponges, coelenterates, etc., which left few fossils. The few fossils that have been found are known as the Ediacara fauna, to be discovered in this Australian locality.

By Daderot (Daderot) [CC0 or CC0], via Wikimedia Commons

The Ediacara fauna represents the first great explosion of life on Earth, made up of marine invertebrates that became extinct at the end of the Proterozoic.

By Ryan Somma (Life in the Ediacaran SeaUploaded by FunkMonk) [CC BY-SA 2.0], via Wikimedia Commons

Images: Precambrian Fossil Gallery.

Interactive activity: Precambrian Events.