It is hypothesized that Earth's atmosphere initially contained much higher % C02.
"Based on today's volcanic evidence, this atmosphere would have contained 80% water vapor, 10% carbon dioxide, 5 to 7% hydrogen-sulfur, and smaller amounts of nitrogen, carbon monoxide, hydrogen, methane and inert gases."
"Based on today's volcanic evidence, this atmosphere would have contained 80% water vapor, 10% carbon dioxide, 5 to 7% hydrogen-sulfur, and smaller amounts of nitrogen, carbon monoxide, hydrogen, methane and inert gases."
http://en.wikipedia.org/wiki/Earth%27s_atmosphere#Earliest_a...