Carbon isotopes through 6 km of fully cored drill holes in 1.7 to 1.5 Ga carbonates of the Mount Isa and McArthur basins, Australia (which host the earliest known eukaryote biomarkers) provide the most comprehensive and best-dated δ13C stratigraphy yet obtained from such ancient rocks. Both basins reveal remarkably stable temporal δ13C trends (mean of −0.6‰ ± 2‰ PDB [Peedee belemnite]) and confirm the impression of δ13C stasis between 2.0 and 1.0 Ga, which, together with other evidence, suggest a prolonged period of stability in crustal dynamics, redox state of surface environments, and planetary climate. This δ13C stasis is consistent with great stability in the carbon cycle controlled, we suggest, by P limitation of primary productivity. Recent evidence shows that P depletion is a major factor in obligate associations between photosymbionts and host cells. We argue that a billion years of stability in the carbon and nutrient cycles may have been the driving force that propelled prokaryotes toward photosymbiosis and the emergence of the autotrophic eukaryote cell.