Final answer:
Ancient atmospheric levels of gases are determined by studying mineral chemistry, tree rings, plant stomatal indices, and ice cores, but not by recreating the primitive atmosphere.
Step-by-step explanation:
Ancient atmospheric levels of gases cannot be determined by observing the primitive atmosphere, which cannot be recreated due to the oxidizing atmosphere and lack of hydrogen. Unlike the methods of analyzing the chemistry of minerals, examining patterns in trees' annual rings, or the stomatal index in plant epidermal cells, the primitive atmosphere's composition is not directly measurable due to its alteration over geological timescales. Current climatic changes and the composition of Earth's atmosphere can be inferred through these proxies as well as through the study of ice core samples, which trap gases from different periods in Earth's history. The rapid increase in atmospheric carbon dioxide levels in recent centuries, beyond historical maximums, signals a significant influence from modern human industrial activity on these changes.