Abstract The rise of land plants during the early Palaeozoic had profound effects upon subsequent Earth history and evolution. The sequestration of standing biomass and carbon burial caused a primary shift in the distribution of active carbon within the biosphere and surficial Earth systems. This manifested itself in a dynamic decline in p CO 2 during Silurian–Devonian time, affecting both terrestrial and marine ecosystems. We examined first-order correlations between terrestrialization and p CO 2 by comparing the GEOCARB III data with time-constrained fossil events in the early evolution of land plants. We compared the same GEOCARB III data with the species/genus richness of lower Palaeozoic acritarchs. The correlation between the rise of woody plants and p CO 2 is built into the GEOCARB model for the Late Devonian and later, but p CO 2 begins to decline in the Cambrian long before the origin of woody trees (lignophytes). The influence of early phases in plant evolution may be seen in a two-stage p CO 2 decline corresponding to fossil evidence for the origin of thalloid bryophytes in the Middle Cambrian and the origin of tracheophytes near the Ordovician–Silurian boundary. The decline of the acritarchs shows a highly correlated lag of about 10 Ma with respect to the p CO 2 decline. The relation between p CO 2 and acritarch species richness suggests a tight coupling between the evolution of the marine phytoplankton and atmospheric CO 2 , supporting previous suggestions that p CO 2 was a significant causal factor in the near extinction of acritarchs by the end of the Devonian.