According to Oparin, which of the following gases was not present in the Earth's primitive atmosphere?

Oparin's theory on the origin of life posits that the Earth's primitive atmosphere was quite different from what we have today. He suggested that early atmospheric conditions lacked free oxygen, which is fundamentally important for understanding the early biochemical processes that could have led to the formation of life.

Free oxygen is a byproduct of photosynthesis and was not prevalent in the primordial atmosphere. Instead, the atmosphere was primarily composed of gases like methane, ammonia, hydrogen, and nitrogen. The absence of oxygen created a reducing environment, which Oparin argued would have been conducive to the synthesis of organic compounds.

The presence of nitrogen, hydrogen, and methane is consistent with Oparin's hypothesis, whereas the lack of free oxygen aligns with the notion that biological material needed to evolve in an environment where oxidative processes were minimal. This understanding is crucial for the context of early Earth’s atmosphere and the development of life.