Converted to O2 by plants on land and phytoplankton in the ocean.
The source of most of the CO2 in the Earth's atmosphere is by volcanic eruptions. Over the billions of years there have been less and less eruptions to the point we are living in the most seismically quiet period in the Earth's history.
If you look at this graph, you will see that over 500 million years ago the CO2 level was nearly 8,000pppm!
This also happens to be the longest inter-glacial period during this Holocene we know of for the past 850,000 we have ice core data for. At any time we could easily go back into the 2.5 million-year ice age at any time.