We had camels in North America and got an Arctic ice cap due to the fact that Earth got cooler and drier. And the atmosphere had 400 parts per million, about what the northern hemisphere could have next month, according to according to the National Oceanic and Atmospheric Administration's Earth System Research Lab.
Old air preserved as bubbles in the Antarctic ice sheet tell us CO2 levels never exceeded 300 parts per million during the last 800,000 years but they started to rise with the advent of industry. Tuesday of this week the reading was 398.44 ppm as measured at Mauna Loa, Hawaii. It was 316 ppm when Charles Keeling, who the Keeling Curve is named after, began taking measurements there.
Carbon dioxide now at highest level in 5 million years by Doyle Rice, USA Today