AP BIOLOGY

Detailed explanation-1: -Oxygen gas was not present in the early earth atmosphere because in primitive earth the atmosphere was highly reduced.

Detailed explanation-2: -Earth’s original atmosphere was rich in methane, ammonia, water vapour, and the noble gas neon, but it lacked free oxygen.

Detailed explanation-3: -Scientists broadly agree that Earth’s early atmosphere and oceans were all but devoid of oxygen gas. But there are signs that there was some oxygen around. Geochemists, for example, have found mineral deposits dated to about 3 billion years ago that they argue could only have formed in the presence of oxygen.

Detailed explanation-4: -Oxygen, although always present in compounds in Earth’s interior, atmosphere, and oceans, did not begin to accumulate in the atmosphere as oxygen gas (O2) until well into the planet’s history.