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: -Nitrogen, carbon dioxide, hydrogen, and water vapor, or water in gas form, were in the first atmosphere. There was very little oxygen in the atmosphere until plants evolved. Plants produce oxygen as a product of photosynthesis. Oxygen allowed animals to evolve and ozone to form.

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: -(4.6 billion years ago) 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.