Summary
Highlights
To understand Earth's history, we start from its formation approximately 4.57 billion years ago. Geologists divide this vast timescale into four eons: Hadean, Archean, Proterozoic, and Phanerozoic, to better categorize Earth's evolutionary stages.
From 4.54 to 4 billion years ago, the Hadean eon was a hostile period characterized by intense volcanism, frequent asteroid impacts, and no continents. A significant event was the collision with a Mars-sized protoplanet, Theia, which formed the Moon and created a global magma ocean. Only durable zircon crystals survive from this mysterious eon.
The Archean eon (4 to 2.5 billion years ago) saw the decline of heavy asteroid bombardment and the emergence of life. The oldest fossils, 3.48-billion-year-old stromatolites, indicate flourishing single-celled life. Large continents began to form through the accretion of microcontinents along subduction zones, a process similar to modern plate tectonics.
By the end of the Archean, Earth had oceans and continents but an atmosphere with 100,000 times less oxygen, rich in carbon dioxide and methane. Life remained exclusively unicellular for another billion years.
The Proterozoic eon (2.5 billion to 541 million years ago) began with a gradual increase in atmospheric oxygen, generated by photosynthetic cyanobacteria. This led to the Great Oxidation Event, a mass extinction of anaerobic organisms, and the formation of Banded Iron Formations. This eon also saw the evolution of unicellular eukaryotes and the first multicellular life, including the Ediacara Fauna.
During the Proterozoic, supercontinents like Nuna and Rodinia formed and broke apart. The breakup of Rodinia and the movement of a large chunk (Pannotia) to the South Pole triggered a massive ice age, culminating in a 'Snowball Earth' event where most of the planet was covered in ice.
The Hadean, Archean, and Proterozoic eons are collectively known as the Precambrian. Its end, 541 million years ago, was marked by the Cambrian explosion, an incredible diversification of life. This event led to the appearance of every major animal phylum, with intricate life forms flourishing in the oceans, leading into the Phanerozoic eon.