Eon· The columns of time

Archean

4.03 Ga → 2.5 Ga

The Archean, a billion and a half years from 4.03 to 2.5 billion years ago, is the eon in which the planet acquired its continents, its oceans, its magnetic field and life.

Eon covering a billion and a half years during which the planet acquires its continents, its oceans, its magnetic field and life, and whose documentation is extremely sparse.

What the evidence shows

Life appears early in the record of this eon, and molecular clocks place its origin earlier still, in the Hadean; no evidence of that antiquity has escaped contestation. At 3.43 billion years the case becomes difficult to reject: the Pilbara craton, in Western Australia, carries seven distinct forms of stromatolite, layered rock built by microbial mats, distributed across a carbonate platform that the tide uncovered, on a scale no other deposit of that age attains and that none of the purely mineral origins proposed accounts for. Every living organism descends from organisms of this eon, and none left a body fossil: what survives is what they built. The atmosphere contains no free oxygen and would be lethal to any aerobic organism.

Bounds and conventions

Neither limit rests on an event. The younger one, 2.5 billion years, is a round number adopted by convention, the record signalling no transition at that point and a division nonetheless being required.