In prehistoric times, specifically during the Paleozoic and Mesozoic eras, the Earth's atmosphere had higher levels of oxygen compared to the present-day atmosphere. There are several factors that contributed to this higher oxygen content:
Abundant plant life: During prehistoric times, there was an abundance of plant life, particularly in the form of large forests and vegetation. Plants release oxygen as a byproduct of photosynthesis, the process by which they convert carbon dioxide and sunlight into energy. With vast areas covered in vegetation, the oxygen production was substantial.
Different climate conditions: The climate during prehistoric times was different from the present, with higher temperatures and different weather patterns. These conditions favored the growth of large forests and an extensive range of plant species, which, as mentioned before, contributed to increased oxygen production.
Reduced oxygen consumption: Prehistoric organisms, including animals, insects, and marine life, had different physiological characteristics compared to modern organisms. Many prehistoric organisms had lower metabolic rates, meaning they required less oxygen for their bodily functions. As a result, oxygen consumption by organisms was relatively lower, allowing oxygen to accumulate in the atmosphere.
Lack of oxygen sinks: In prehistoric times, certain geological and chemical processes that consume oxygen, as we observe today, were less active or absent. For example, the oxidation of organic matter in soils and sediments and the oxidation of minerals were likely less significant, allowing oxygen to persist in the atmosphere for longer periods.
Over time, changes in geological processes, climate, and the evolution of different organisms have led to a decrease in atmospheric oxygen levels. Today, oxygen makes up about 21% of the Earth's atmosphere, a level that is still sufficient to support the diverse life forms we have today.