ENVIRONMENT × PEOPLING

Green Corridors Across the Sahara

~125,000 years ago

Palaeolithic · Middle Stone Age · Eemian

The Sahara has not always been a wall. During warm, wet phases driven by slow wobbles in Earth's orbit, monsoon rains pushed north and the desert filled with linked lakes, rivers and inland deltas. The chemistry of now-buried river channels shows southern waters reaching the Mediterranean during the last interglacial, around 125,000 years ago, and the desert's animal geography records more crossings through its heart than along the Nile.

Why it matters

Every early human departure from Africa needed a door, and these were the doors. The green corridors of the last interglacial line up with the first Homo sapiens in the Levant and deep in Arabia; when the rains withdrew, the door closed behind them. The Sahara breathed, filling with lakes and grazers in wet phases and emptying in dry ones, and human movements followed its rhythm.

Dating & uncertainty

Green phases recurred many times, paced by Earth's orbit; the ~125,000 year date marks the last interglacial episode, whose rivers demonstrably reached the Mediterranean and which coincides with the first Homo sapiens found outside Africa. Sediment records off Libya show further, weaker wet episodes punctuating the last glacial cycle.

Sources

1. Drake et al. (2011), PNAS · Ancient watercourses and biogeography of the Sahara explain the peopling of the desertdoi:10.1073/pnas.1012231108
2. Osborne et al. (2008), PNAS · A humid corridor across the Sahara for the migration of early modern humans out of Africa 120,000 years agodoi:10.1073/pnas.0804472105
3. Blanchet et al. (2021), Nature Geoscience · Drivers of river reactivation in North Africa during the last glacial cycledoi:10.1038/s41561-020-00671-3

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