Explain how scientist know the oxygen level in the atmosphere has changed because of plants.

Since plants convert carbon dioxide and grow because of the carbon it helps the plants and when that happends where there are plants there is more air produced for us to breath.It can also change the climate because of the levels of air that is produced by plants is in the  atmosthphere.So they know because there is more air and because where there are plants there is more fresh air to breath.Also plants take the carbon dioxide and clean the carbon in the air causing us to breath good air.

i believe its the indians

which time period is this? or is this story fictional, i'm going to need more info to answer this question