Imagine the dent the Indigens made in the climate when they crossed the “Bering Land Bridge” into North AMERICA.
56 million rotting dead injuns had to put back some CO2 and methane. Also, lightning caused fires would have burned until the rainy season because no one could put them out.
No mention of volcanic activity which would put more gas in the air than all of man’s activity.
And then there is that decomposing Norwegian Blue Parrot... (/sarc)