AP BIOLOGY

Detailed explanation-1: -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-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: -Since its first appearance 2.33 billion years ago, oxygen accumulated in high enough concentrations to have a weathering effect on rocks just 10 million years later. This weathering process, however, would have leached more sulfate and certain metals into waterways and ultimately, the oceans.

Detailed explanation-4: -Jay Kaufman, a geoscientist at the University of Maryland, points to a series of ice ages’-at least three of them’-that occurred between 2.4 billion and 2.2 billion years ago, when the era of oxygen began.

Detailed explanation-5: -All the oxygen, which was there in the atmosphere was present in combination with other elements. Early atmosphere of the earth was a reducing atmosphere because there was no free molecular oxygen then.