The time from 3,800 million years to 2,500 million years ago: Archaeozoic is the era when the Earth's crust formed and unicellular organisms were the earliest forms of life.

In geology, regarding the earlier half of the Precambrian Era, from about 5 billion to 2.5 billion years ago: During the Archaeozoic period, the earliest datable rocks were formed, and from which the oldest known fossil forms, blue-green algae and bacteria, were recovered.