The Precambrian is the earliest part of the EarthÂ’s history and yet very little is known about this Supereon. This is a brief look at that Supereon.
The Precambrian is often referred to as the Cryptozoic or ‘hidden life’ and the Archeozoic Eon as well. It is not only an Eon but classed as a Supereon, along with the Phanerozoic.
This is the Eon/Supereon between the creation of the Earth to the evolution of the first shelled creatures appearing at the very start of the Cambrian Period, some 540 to 545 Ma ago (Ma = Million years).
The Precambrian can further be divided in Eras, the earliest being the Hadean.
The Hadean is defined as the time of the Earths formation to the oldest rocks we see today and so is from 4600Ma to 3800Ma ago. This Era is classed as pre-biotic (before life). Very, very little is known of this time, as much of the evidence has disappeared over the millennia.
In the Hadean, the Earth was subjected to bombardment from debris in space, and it would have been around the time of the moons formation. Much of this Era would have seen mass magma pools covering much of the Earth’s surface, as things were only starting to cool down.
From the Hadean we move into the Archean, which simply means ancient, or beginning and it is in this Era that the very first life-forms occur – mostly in the form of stromatolites and photosynthetic organisms – the first prokaryotes (single-cell organisms) began to evolve in this timescale and indeed thrived at this time.
There is a current debate as to when diamonds formed within the Earth. Many geologists now believe that diamonds (which need extremely high temperatures to form), first began life in the Hadean Era and so it is unlikely that there will ever be any more diamonds created in the Earth, unless it returns to the very high temperatures of this time (and whereby no life could survive). Please note that I have said in the Earth here – diamonds carried via meteorites is another theory altogether.
This Era is also where the rock record begins and when atmospheric oxygen levels first started to rise. The Archean spans 3800Ma to 2500Ma ago.
Rocks discovered in Greenland, called the Isua Rocks, contained the earliest recorded life-forms, aged approximately 3800Ma (or 3.8 billion years) old and were a form of bacteria. Banded Iron Formations also began around this time.
The final part of the Precambrian is called the Proterozoic which means former life. It is the longest part of the Precambrian, spanning 2500Ma to 540-545Ma ago. This is when everything really started to happen. Stromatolites became abundant and Eukaryotes evolved (multi cellular organisms). Many of these Eukaryotes would have been seaweed-like plants.
The Proterozoic saw two periods of glaciation and this was the first Era to have Periods. The most important period within the Proterozoic is the Vendian and its significance is that it saw the formation of the Ediacaran Fauna (will cover this in a later article), which occurred around 610Ma ago. In fact, the Vendian is occasionally called the Ediacaran Period.
The end of the Precambrian, as mentioned earlier, is around 540 to 545Ma ago. From there we move into the Eon called the Phanerozoic, which is occurring today.
As you will see from the figures above, the Precambrian makes up the majority of the Earth’s history so far and yet very, very little is known of this time, as so much of its record has been wiped away in catastrophes, metamorphism and erosion.
Simplified Precambrian Timescale
||Age in Millions of Years Ma
||540/545 to 495Ma
||2500 to 540/545Ma
||3800 to 2500Ma
||4600 to 3800Ma
History of Life by Richard Cowen