The Precambrian eon is vast, almost unimaginably so. In fact, before some fossil discoveries were made late in the 20th century, it was considered unknowable -- evolution's dark ages. The Precambrian encompasses 86 percent of the history of Earth. As its name implies, this includes all of geological time prior to the Cambrian period.

The Precambrian's oldest era, the Hadean, predates most of the geologic record. During the Hadean, the solar system forms out of gas and dust, the sun begins to emit light and heat, and Earth takes shape. Meteors and other galactic debris shower the planet over its first half-billion years, making it entirely uninhabitable.