Sediment sequences from eastern African lakes preserve evidence of environmental landscapes occupied by hominins and record changes in climate. Carbon isotope profiles for biomarkers in early Pleistocene lake sediments from Olduvai Gorge document environmental changes associated with alternating wet/dry cycles during a critical interval in hominin evolution. During drier intervals the δ¹³C signatures of algal biomarkers reflect utilization of bicarbonate rather than CO₂ in response to increased lake alkalinity. They demonstrate that less negative δ¹³C_org values for lacustrine deposits may reflect changes in the balance of terrestrial versus aquatic organic matter rasther than an increase in C₄ plants.