The Archaean is the earliest period in geological period, extending up to the Lower Silurian, spanning the period between 3.8 and 2.5 billion years ago. It includes an Azoic age, previous to the appearance of life, and an Eozoic age, including the earliest forms of life. This is equivalent to the formerly accepted term Azoic, and to the Eozoic of Dawson. Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3.5 billion years old. Its name means “ancient.
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