The earth during the last million years traditionally had a CO2 concentration in the atmosphere of around 200 to 300 ppm – parts per million. Humans have used energy for many thousands of years for their comfort, for protection and to prepare food. The fire was a constant companion of humans and they were burning renewable sources of energy, mostly wood. Trees and other plants absorb solar energy by photosynthesis and store this energy carbon-based in wood and other organic material. Whenever man has burned such carrier of stored energy, let’s say the wood of a tree, CO2 was released into the atmosphere. However, the CO2 cycle was closed in the sense that the same amount of CO2, molecule by molecule, was extracted from the atmosphere shortly before, only a few years ago during photosynthesis when the tree was growing.
When you burn the wood of a tree you release CO2 into the environment that has been taken out of it only some years earlier. The tree then absorbed water H2O and carbon dioxide CO2 from its environment plus sunlight energy to form its wood. The cycle to absorb CO2 from and to release it back into the atmosphere closes within a few years.
Therefore there was a balance of extraction and release of CO2 from and into our environment.