Biologists studying plant life in the prehistoric rainforest by utilizing samples of ancient ice and fossilized evidence of biological processes found that most of the plants removed an astonishingly small amount of carbon dioxide from the atmosphere  One biologist found that a certain tree currently found in the rainforest absorbs carbon dioxide from the atmosphere at a greater rate than any tree on the planet
The biologist hypothesized that the prevalence of this tree in the prehistoric rainforest accounts for the other plants' failure to remove larger amounts of carbon dioxide from the atmosphere
The prehistoric humans did not seem to be adversely affected by the amount of carbon dioxide in the atmosphere.