Daily History Challenge · No. 81 · Thursday 17 September 2026 · Chain

Puzzle No. 81: Chain

The anchor was End of the African Humid Period and Desertification of the Sahara (3500 BCE). Every challenger is listed below with its year.

The anchor

c. 3500 to 2700 BCEEnd of the African Humid Period and Desertification of the Sahara

Driven by gradual changes in Earth's orbital geometry that weakened the West African and North African monsoons, the 'Green Sahara' of the early-to-mid Holocene gave way to the modern desert. Lakes, rivers and savanna that had supported hippos, elephants, giraffes and herds of cattle dried out, and the region's pastoralist populations were displaced southward and toward the Nile Valley. Many researchers argue the termination was abrupt in places, unfolding over a few centuries (some estimates as short as a century or two), though pollen and lake-sediment records such as those from Lake Yoa in Chad indicate a more gradual decline in some areas; the timing also varied regionally, ending earlier in the north and east. This environmental transformation concentrated human settlement along the Nile and other water sources, a demographic squeeze that scholars link to the consolidation of Nile Valley societies and the long-distance dispersal of cattle-keeping peoples. The vivid rock art of sites such as Tassili n'Ajjer preserves a visual record of the vanished wetter world.

Read the full story · See the world in 3500 BCE