Precambrian period

The Precambrian was the first supereon in Earth's history, lasting from the formation of the Earth to the Cambrian period. During this time, the Earth's atmosphere was high in sulphur and carbon dioxide, consisting of a green, fog-like sulphuric gas, and Precambrian Worms thrived.

Real life
The Precambrian lasted from 4600 to 541 million years ago; beginning with the formation of the Earth, and ending with the Oxygen Catastrophe. Very little is known about the Precambrian, although it is believed that its atmosphere was high in CO2 and nitrogen and had no oxygen content. It is also believed that there were a number of glacial periods during the Precambrian, and possibly even a 'Snowball Earth.' It is widely thought that there were small proto-continents on Earth, which collected into a single supercontinent 1 billion years ago, before breaking up 600 million years ago. Bacterial and protozoan life thrived in the Precambrian, but there is believed to have been no complex, multicellular life, such as plants or animals.

Trivia
The Precambrian is the furthest back in time that any known Anomaly has ever opened to.