AP BIOLOGY

Detailed explanation-1: -Earth’s original atmosphere was rich in methane, ammonia, water vapour, and the noble gas neon, but it lacked free oxygen.

Detailed explanation-2: -Oxygen is produced by plants by the process of photosynthesis, earlier there were no plants and vegetation, which’s why no oxygen in the early atmosphere.

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: -Before photosynthesis evolved, Earth’s atmosphere had no free oxygen (O2). Small quantities of oxygen were released by geological and biological processes, but did not build up in the atmosphere due to reactions with reducing minerals.