The paleoclimatic framework of human evolution is central to the study of human origins. Olduvai Gorge, Tanzania, has a rich paleontological record, including four hominin species. We analyzed 177 mudstone samples from a lacustrine section, supported by a new high-resolution geochronology, to track changes in paleolake water chemistry. Six peaks in Al2O3/MgO indicate freshening events occurring over a generally saline and alkaline environmental background. Five of the six events occurred at peak climatic precession, correlating with December insolation at 20°S. The southern monsoon therefore may have been more effective at delivering moisture to East Africa than today. Peak amplitude of geochemical variation preceded the ca. 1.84 Ma eccentricity maximum by ∼20 k.y., suggesting that eccentricity modulation was unexpectedly weak in this interval, or that other factors affected the water balance or geochemical record. This study demonstrates that authigenic clay geochemistry determined through whole-rock analyses can preserve a record of astronomically forced environmental change, and documents paleohydrologic changes in tropical Africa at a time when zonal Walker circulation increasingly affected global climate, new stone technologies emerged, and the genus Homo spread beyond Africa.