The Archean is a geological eon spanning from 4,031 mya to 2,500 mya.
This era is cool because during this period of history, Earth was an ocean world. This is when life started here, according to analysis of stromatolites from 3.48 billion years ago in Australia.
Stromatolites are microbial mats formed in shallow water by cyanobacteria. I guess during this era, oxygen in the atmosphere was really rare, and cyanobacteria helped change that.