AP BIOLOGY

Detailed explanation-1: -It is generally agreed that the atmosphere contained little or no free oxygen initially and that oxygen concentrations increased markedly near 2.0 billion years ago, but the precise timing of and reasons for its rise remain unexplained.

Detailed explanation-2: -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-3: -When Earth formed 4.6 billion years ago from a hot mix of gases and solids, it had almost no atmosphere. The surface was molten. As Earth cooled, an atmosphere formed mainly from gases spewed from volcanoes. It included hydrogen sulfide, methane, and ten to 200 times as much carbon dioxide as today’s atmosphere.

Detailed explanation-4: -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-5: -Caption: MIT scientists say that the Great Oxygenation Event (GOE), a period that scientists believe marked the beginning of oxygen’s permanent presence in the atmosphere, started as early as 2.33 billion years ago.