AP BIOLOGY

Detailed explanation-1: -Ancient sediments and rocks record past changes in atmospheric composition due to chemical reactions with Earth’s crust and, in particular, to biochemical processes associated with life. Earth’s original atmosphere was rich in methane, ammonia, water vapour, and the noble gas neon, but it lacked free oxygen.

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: -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-4: -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.

Detailed explanation-5: -Oxygen made up 20 percent of the atmosphere-about today’s level-around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years.