Period· The columns of time

Quaternary

2.6 Ma → Today

The Quaternary, the last 2.6 million years, is the period of the ice ages and the only one in which Homo sapiens exists: ice sheets advanced and retreated dozens of times with the Earth's orbit.

Period of the ice ages and the only period in which Homo sapiens exists: ice sheets advance and retreat dozens of times in step with orbital variation.

What the evidence shows

Ice sheets spread over North America and northern Europe and then melt, dozens of times, following slow variations in the Earth's orbit and in the tilt of its axis, which alter the sunlight received at high northern latitudes in summer. The demonstration dates from 1976: the orbital periodicities were recovered from the isotopic record of deep-sea cores, which established that the orbit sets the pace of the glaciations. Sea level follows ice volume by more than a hundred metres, so that the coastline drawn on a map is that of the present interglacial. The period comprises two epochs of very unequal length: the Pleistocene, covering everything but the last eleven millennia, and the Holocene, the current interglacial.

Bounds and conventions

Its base moved from 1.8 to 2.58 million years by a ratification in 2009: a limit is a decision before it is a date. Its top is the present, no limit being required, and whether the last century warrants a division of its own was formally examined and rejected. The orbital pacing is not in question; what the orbit does not explain is the change in that pacing midway through.