The lithostratigraphy at the Pliocene-Pleistocene boundary exposed in Olduvai Gorge, East Africa, was used to determine the duration and cause of climate variations (wet-dry periods). The area has rich paleontological and archaeological records, and the time interval is a critical period in human evolution. Five episodes of lake expansion and contraction can be interpreted from a stratigraphic interval (defined by tuffs) between ca. 1.85 and 1.74 Ma, suggesting precession-controlled cyclicity. The geochronometry of the sequence was determined using the paleomagnetic record (top of subchron CN2, 1.785 Ma), the ages of two tuffs (Tuff IB and Tuff IF), and sedimentation rates. Insolation (W/m2) variations calculated as a function of eccentricity and precession predict five cycles of varying insolation (±10%) between 2.0 and 1.5 Ma. Rainfall would have increased by one-third between dry and wet portions of each ∼21,000 k.y. cycle. This study represents the first dated terrestrial record at the Pliocene-Pleistocene boundary with sufficient resolution to link lake cycles (a climate proxy) to astronomic forcing. The wet-dry climate pendulum may have been a factor in the natural selection processes of hominin evolution and the first wave of hominin migrations out of Africa (1.8 Ma).