A New Interpretation of the Rise of Intelligence on Earth

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A New Interpretation of the Rise of Intelligence on Earth<br>Falling CO2 levels was the key factors

Ugo Bardi<br>Jun 18, 2026

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Reposted from “Living Earth” Jun 15, 2026

There is a reason why dinosaurs were big and strong, but not that smart. It has to do with their metabolic system and how it was hampered by the high CO2 levels during the Mesozoic.

It is finally published. A huge work: one year of studying, thinking, reasoning, calculating, taking wrong directions, getting back and restarting. And, finally, I got to the point where I think I can propose this revolutionary idea (or so I believe!)

Putting the data together was a lot of work, but in the end, the idea is simple . High intelligence arose in the biosphere during the past few tens of millions of years as a result of a metabolic boost generated by the decline of CO2 concentrations in the atmosphere.<br>If you studied chemistry, the idea will immediately make sense to you. The metabolic reaction of aerobic organisms (as we are) consumes oxygen and carbohydrates and produces CO2. Now, chemical reactions move at a rate that often depends on the concentration of reactants and products. And one of the factors affecting the rate is the need to remove the products; otherwise, the reaction will slow down. That’s the key hypothesis that I thought one early morning one year ago while waiting for my plane at 5 am at the Tesla airport in Belgrade (maybe it was the ghost of Nikola Tesla who inspired me).<br>In short, I found that several biological parameters implying higher metabolic rates, including the encephalization quotient, were proportional to the inverse of CO2 concentrations over geological times.<br>The story, of course, is much more complicated, and you have to read the paper to understand why I think that this is a better explanation than other ideas put forward earlier on. But if this hypothesis of mine turns out to be real, then the consequences are unsettling — to say the least!<br>It means that Earth spent some 500 million years preparing the conditions that would lead to the appearance of intelligence (also sentience, if you like). High oxygen concentrations, and low CO2 ones. The condition that made possible the appearance of the huge human brains.<br>And now, humans are returning to the conditions of tens of millions of years ago, when CO2 concentrations were too high for intelligence. This is not a hypothesis: experimental tests show that CO2 reduces the performance of the brain. We are “unevolving” ourselves.<br>Humans are smart, but not smart enough. Will global dumbing destroy humankind? We cannot say with certainty, but it is not impossible.<br>This huge work was really too much for a single person; it was possible only with substantial help from Claude Opus 4.8, but even so, the task is huge. If you think it is worth pursuing it more, let me know. Working together, we can do better and try to alert people to the dangers of what we have been doing.<br>Here is the abstract of the paper. If you have time to review and comment, you can do this directly on Qeios.<br>Abstract

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...