This means one way to detect life's activity on a distant planet is to look for significant levels of oxygen.
Other types of bacterial life break down large molecules for energy and create methane or sulfur as a waste product, so scientists would test the levels of these molecules as well.
Sophisticated plant life also goes through the process of respiration, where oxygen and carbohydrates are converted into carbon dioxide.
One way scientists measure increased activity by the plant kingdom on Earth is to test levels of carbon dioxide in the air.
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