In my question:
An answer was proposed for a kind of "Boom Bust Cycle".
The atmosphere has little CO2 but a lot of O2. When land plants use up all the CO2, O2 levels get so high wildfires start but never go out. This purges the land and releases nutrients and carbon back into the atmosphere, and in turn, into the oceans. This allows sea life to bloom until they die bringing carbon back to the land.
Intelligent life requires harsh changes in conditions, but this is pretty tough not to mention regular, every few decades.
Could a migratory or coastal civilization appear in a world like this?
If not, I could say that this is a recent occurrence, similar to the Carboniferous Period.