Eon· The columns of time

Phanerozoic

539 Ma → Today

The Phanerozoic, the eon of visible life, covers the last twelve per cent of the Earth's history, and every organism commonly displayed in collections belongs to it.

Eon of visible life, covering the last twelve per cent of the history of the Earth. Every organism commonly displayed in collections belongs to it, which explains the concentration of entries on this timeline at its right-hand end.

What the evidence shows

Its name means visible life, and it opens with the Cambrian explosion, the rapid increase in animal diversity between about 540 and 520 million years ago, in which most present-day body plans appear. The break is less sharp than it appears: macrofossils extend back into the Ediacaran, to about 571 million years, and what reads as a single event is a succession of radiations. What follows is no more regular. The record is interrupted twice: at 252 million years, by an extinction affecting sea and land simultaneously within less than two hundred thousand years, probably through a massive carbon release linked to Siberian volcanism, and at 66 million years, by the impact that ends the non-avian dinosaurs. Set against the three preceding eons, it occupies only the end of the timeline.

Bounds and conventions

The older limit presents no difficulty; the name does. It states that life becomes visible at that limit, whereas what becomes general is mineralised hard parts: the preceding eons are populated by organisms whose constitution did not predispose them to fossilisation, a bias affecting the record as a whole.