Fig. 1. The geologic record of δ¹³C values has remained essentially constant over ~3.5 billion years.

Geologic carbonate and organic δ¹³C record (left) and modern δ¹³C values of inorganic carbon and biomass from diverse taxa (right). Light gray fields represent the range of geologic δ¹³C measurements from Schidlowski [6]. Dark gray fields represent 95% confidence intervals for smoothing analyses of geologic δ¹³C data from Krissansen-Totton et al. [7] and references therein. Modern δ¹³C values from Schidlowski [6]. Bars are colored as follows: black, geologic reservoirs; dark green, land plants; light green, green algae; teal, cyanobacteria; other taxa, yellow. Phases of atmospheric oxygen are labeled at the bottom, from Lyons et al. [24] (“~ no O₂” indicates <10⁻⁵ present atmospheric level (PAL), “low O₂” indicates ~10⁻¹ to 10⁻⁴ PAL, and “modern O₂” indicates PAL). Qualitative O₂ levels are indicated by shades of blue, with lighter shades indicating lower O₂ levels and darker shades indicating higher O₂ levels (also noted by the scale). The Great Oxidation Event (GOE) and Neoproterozoic Oxidation Event (NOE) are indicated by darker blue bars. The earliest potential appearance of cyanobacteria is interpreted from the oldest known oxidized sediments [24, 59, 60]; of eukaryotic algae, from oldest interpreted algal fossils [139] and molecular clock dating [140, 141]; of land plants, from the oldest interpreted pollen fossils [142] and molecular clock dating [143]. PDB Pee Dee Belemnite δ¹³C standard, Phan. Phanerozoic (color figure online).