Period· The columns of time

Orosirian

2.05 Ga → 1.8 Ga

In the Orosirian, from 2.05 to 1.8 billion years ago, continents collided, the first mappable supercontinent formed and one cell acquired another for good.

Period of major continental collisions, in which the first mappable supercontinent forms and one cell acquires another permanently.

What the evidence shows

The name, formed on the Greek oroseira, mountain range, designates the dominant character of the period: continental blocks weld along orogenic belts and gather nearly all emergent land into a single mass, Columbia, the first assembly to leave enough rock to be restored as a coherent map. Two of the largest known impacts occur within it, Vredefort at 2.02 billion years, whose crater simulations place at some 250 kilometres, and Sudbury at 1.85 billion years. The great global period of banded iron ends around that latter date: iron, sulfur and carbon measured in Canadian rocks of that age indicate a deep ocean still without oxygen but now rich in sulfide, which removed dissolved iron from solution. A bacterium also establishes itself permanently within another cell and becomes the mitochondrion; molecular estimates place that association between 2.05 and 1.21 billion years.

Bounds and conventions

The limits are conventional. The events that characterise the period are dated by their rocks and not by it, and the most consequential, the appearance of the eukaryotic cell, is placed between 2.0 and 1.8 billion years by fossils but markedly later by several molecular clocks. The order in which the nucleus and the mitochondrion were acquired remains debated.