- 4.6 billion years ago to 541 million years (88% of geologic time)
- Hadean Eon
- 4.6 to 4 billion years ago
- Almost no known rocks of that age
- Archean Eon
- 4 to 2.5 billion years ago
- Rocks difficult to interpret: metamorphosed and deformed, most buried deeply
- Few contain any clear fossils because life was microscopic with no hard parts
- Proterozoic Eon
- 2.5 billion years ago to 539 million years ago
- “snowball earth”
- rise of the Ediacaran fauna (first metazoans?)