This is not well known

Just a note that the present is on the left of the chart and the right is 500 million years ago.

This makes sense if you think about it: All the coal in the world was Carbon Dioxide in the air before it became coal. Plants took the CO2 and made themselves out of it. In the right settings the old dead plants accumulated and formed coal. This is part of why there is less CO2 in the air than before. Lately it has been rising in part due to us mining the stuff and burning it.

Getting back to the above chart: It can be seen that for most of the last 500 million years, CO2 existed at higher concentrations in the atmosphere than it does now. In this period the Earth has experienced glaciations (ice ages).