Era· The columns of time

Eoarchean

4.03 Ga → 3.6 Ga

The Eoarchean, from 4.03 to 3.6 billion years ago, covers the first four hundred million years from which any rock survives, and the oldest traces of life so far proposed.

Era of the dawn, covering the first four hundred million years from which any rock survives. Its documentation is sparse and contested, and concerns the oldest traces of life so far proposed.

What the evidence shows

Its entire record consists of a handful of outcrops in Greenland, Canada and Australia, metamorphosed to the point where reading them is difficult. They contain the oldest proposed traces of life: stromatolites one to four centimetres high at Isua, small layered mounds attributed to microbes, dated to 3.7 billion years, and graphite in a Labrador rock 3.95 billion years old whose carbon isotope composition shows the fractionation characteristic of metabolism, enzymes taking up the light isotope preferentially. Both are contested for the same reason: at this degree of alteration, the structures left by life and those produced by the deformation of rock are difficult to distinguish.

Bounds and conventions

The era begins with what has been preserved and not with what occurred, and ends, at 3.6 billion years, on a round number to which no event corresponds. The results it carries are at once the most consequential and the least secure on the chart.