Era· The columns of time
Paleoarchean
3.6 Ga → 3.2 Ga
Four hundred million years in which life stops being a contested trace and becomes a structure you can walk up to and touch.
What the evidence shows
The Pilbara of Western Australia holds a stromatolite reef dated to 3.43 billion years, and it is the oldest evidence of life that convinces nearly everyone. The stromatolites are laminated mounds built by microbial communities trapping sediment, and they take seven distinct shapes, each in its own part of an ancient tidal platform: an arrangement that, in comparable settings ever since, has meant organisms. The world they sit in has oceans, some continental crust, and an atmosphere without oxygen.
Bounds and conventions
The era's bounds, 3.6 and 3.2 billion years, are two of the round numbers that cut the Archean into four roughly equal parts. No event marks either, and the era would hold the same things if either were shifted a little.