Our crop plants evolved about 400 million years ago, when CO2 in the atmosphere was about 5000 parts per million! Our evergreen trees and shrubs evolved about 360 million years ago, with CO2 levels at about 4000 ppm. When our deciduous trees evolved about 160 million years ago, the C02 level was about 2200 ppm—still five times the current level.
Source: When Too Little CO2 Nearly Doomed Humanity – Dennis Avery
