Role of CO2 in the Evolution of Intelligence: Solving the Dinosauroid Question

> Hence, it is possible to propose that large brains can be developed by natural selection only for atmospheric compositions that allow a high tissue oxygenation, that is high O2/CO2 ratios or, equivalently, low CO2 concentrations when the O2 level remains approximately constant.

It appears we have a new big filter candidate, fortunately behind us. If atmospheres with molecular oxygen are rare, those rich in oxygen while very low on CO2 must be much rarer yet...

> Afterward, the Earth system will resume the long-term trend of decline in CO2 concentrations resulting from the response of the biosphere to the slow increase of the solar irradiation, about 10% every billion years. Some recent results by Ozaki and Reinhard confirm these early results and show that, while oxygen is expected to remain at the current concentrations for the long term future, carbon dioxide is expected to continue its gradual decline, reaching values of the order of 10 ppm in ca. one billion years.

...except when the biosphere is almost out of the inner bound of the habitability zone and you're doomed anyway.