The Precambrian Era began with the formation of the earth and lasted about 3.5 billion years, which accounts for nearly 80 percent of the entire geologic time scale. Evidence of life appears in the early Archean eon, but the fossil record at that time is rare. By the Late Archean, stromatolites (microbial mat-like structures) started to appear in the rock record. Stromatolites are the most common Precambrian fossils.

In Wyoming, stromatolites are found in the Snowy Range (Medicine Bow Mountains) in the Nash Fork Formation of the Libby Creek Group. The stromatolites started out as microbial mats that colonized in a continental shelf environment. They are believed to have formed ~1.7 billion years ago (~1,700 Ma).