In the geochronology of the Earth according to the Geologic Time Scale, the Precambrian supereon accounts for 88% of all geologic time. The Precambrian lasted from the Earth’s beginnings, over 4.5 billion years ago, until the start of the Phanerozoic eon, 541 million years ago. During these 4 billion years it covered the Hadean, Archean and Proterozoic eons.
Hadean eon (~4500–4000 million years ago)
The Hadean eon is the first subdivision of the Precambrian supereon and therefore the first geological eon. It began with the formation of the Earth around 4.5 billion years ago, and ended 500 million years later, 4 billion years ago.
The name “Hadean” comes from the Ancient Greek Ἅδης, the ancient Greek god of the underworld, in reference to the hellish conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism.
During this eon, the Earth was bombarded for millions of years on end with meteors from outer space. The valleys of craters left by this onslaught of rocks filled up and became the first oceans. Also, because of the heavy bombardment, pieces of rock broke away from the Earth and formed the basis of the Moon. The Hadean is also the eon in which it is suggested that life on Earth began.
“Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy
Archean eon (4000–2500 million years ago)
The Archean eon lasted from from 4 billion years ago to 2.5 billion years ago. Although, in older literature, the Hadean is included as part of the Archean.
The name comes from the Ancient Greek Αρχή, meaning “beginning, origin” and is a reference to the fact that photosynthesis started to take place in this eon.
Even though it is suggested that life on Earth originated earlier during the Hadean eon, we known for certain that certain species of bacteria thrived during the Archean eon. It is also the eon during which the first continents were formed.
“Evolution did not end with us growing opposable thumbs. You do know that, right?” – Bill Hicks
Proterozoic eon (2500–541 million years ago)
The Proterozoic eon is the longest geological eon to date. It is the eon which precedes the Phanerozoic eon in which we currently find ourselves.
The name comes from the Ancient Greek and means “earlier life”. It represents the time just before the proliferation of complex life on Earth.
One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth’s atmosphere. Also, the first appearance of advanced single-celled, eukaryotes and multi-cellular life, roughly coincides with the start of the accumulation of free oxygen. Finally, towards the end of the Proterozoic, the earliest arthropods, fungi and small shelly fauna appear.
“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” – Charles Darwin, The Origin of Species
See other: History of the Earth