Era· The columns of time

Paleoarchean

3.6 Ga → 3.2 Ga

In the Paleoarchean, from 3.6 to 3.2 billion years ago, life stopped being a contested trace and became a structure that can be seen and touched.

Era of four hundred million years in which life ceases to be a contested trace and becomes a macroscopic structure accessible to direct observation.

What the evidence shows

The Pilbara craton, in Western Australia, carries a stromatolite reef dated to 3.43 billion years, which constitutes the oldest evidence of life on which there is consensus. Stromatolites are layered mounds built by microbial communities trapping sediment, and here they take seven distinct forms, each occupying a determined portion of an ancient tidal platform. This ecological zonation, the ordered distribution of different forms according to depth or exposure, is the decisive criterion: it characterises communities of organisms and not mineral precipitation. The world in which they form has oceans, a limited continental crust and an atmosphere without oxygen.

Bounds and conventions

The limits of the era, 3.6 and 3.2 billion years, are among the round numbers dividing the Archaean into four approximately equal parts. No event marks them, and the content of the era would not be altered by moving either.