ENVIRONMENT × PEOPLING
The Green Sahara Corridors
~125,000 years ago
Palaeolithic · Middle Stone Age · Eemian
About 125,000 years ago the Sahara was green, crossed by rivers and lakes that opened corridors out of Africa.
The humid periods of the Sahara, or green Sahara periods, are episodes during which the African monsoon advances northward and the desert fills with lakes, watercourses and connected inland deltas. Their cause is astronomical: the precession of the equinoxes and the obliquity of the Earth's axis vary over tens of thousands of years and alter the sunlight received by the northern hemisphere in summer, a hotter northern summer displacing the rain belt to the north. The chemical composition of now-buried river channels attests that water of southern origin reached the Mediterranean during the last interglacial, about 125,000 years ago, and the distribution of animal species indicates a higher number of crossings through the centre of the desert than along the Nile corridor.
Why it matters
These corridors constitute the available routes out of Africa for departing populations. Those of the last interglacial coincide with an early presence of Homo sapiens in the Levant and as far as the interior of Arabia, and their closure follows the withdrawal of the rains. The alternation of humid and arid phases thus makes the Sahara an intermittent barrier whose rhythm partly governs that of human movement.
Dating & uncertainty
The green phases recurred many times, on the rhythm of the orbital parameters. The date of 125,000 years retained here corresponds to the last interglacial episode, the one whose watercourses reached the Mediterranean, as chemical analysis establishes, and which coincides with the early presence of the species in the Levant. Marine sediments off Libya record further humid episodes of lesser amplitude, distributed through the last glacial cycle.
Our species’ history as one day
14:28:349 h 31 min before midnight
if the 315,000 years of Homo sapiens were compressed into a single day