Atmospheric CO₂ as a Biophysical Driver of Mesozoic–Cenozoic Biodiversity

The Phanerozoic rise in biodiversity and the parallel late‐Phanerozoic increase in maximum encephalization are often explained as the autonomous output of a diversifying biosphere under the influence of natural selection. However, biophysical drivers have also been proposed as the causative factors. Three commonly‐proposed drivers — atmospheric O₂, surface temperature, and the increase in productive coastal area following Pangaea fragmentation — are each poorly compatible with the combination of the factors needed to explain the biodiversity increase. Here, I argue that the most plausible candidate biophysical driver is the secular fall of atmospheric CO₂ over the last ∼200 Myr, acting through its effect on the entropy generated by oxidative metabolism in the cells of metazoa[1]. Using the Sepkoski genus‐level diversity curve, the Judd[2] and Lenton[3] CO₂ reconstructions, and the Mills[4] O₂ reconstruction, the comparison shows that marine biodiversity over the Mesozoic–Cenozoic correlates positively with 1/CO₂ (Pearson r = +0.59 to +0.65, p ≤ 10⁻⁴). Adding O₂ to examine the effect of the O₂/CO₂ ratio does not significantly affect the fit, because Phanerozoic CO₂ varies over more than an order of magnitude while O₂ varies only by a factor of two. A second and stronger test on Russell’s[5] compilation of maximum encephalization across 18 vertebrate taxa over 530 Myr gives Pearson r = +0.79 and Spearman ρ = +0.92 against 1/CO₂. I propose that the falling Mesozoic–Cenozoic CO₂ trend expanded the available extent of the fitness landscape of the biosphere, providing the biophysical condition under which the diversification dynamics described by Mussini[6] could escalate. The thermodynamic mechanism developed by Buxton[1], in which the entropy available from oxidative metabolism scales with [O₂]³/[CO₂]³, predicts the asymmetry: above a tissue O₂ threshold, additional O₂ is not a critical factor. This interpretation provides a quantitative answer to the long‐standing ‘dinosauroid question’[7][8] of why no Mesozoic vertebrate ever reached hominin‐grade encephalisation despite ample evolutionary time. The idea that the concentration of CO₂ is a critical factor in the functioning and evolution of the human brain also has important and unsettling consequences for the future of humankind, with concentrations continuing to rise as a consequence of human activities[9].