Question

Explain why it took almost a billion years for the Earth's atmosphere to become oxygen-rich after the beginning of photosynthesis.

Answer 1

Oxygen is the 3rd most abundent element in the universe. This is because of the reactive charecter of O2. There were no O2 in rhe atmosphere on the 1st half of our planet history. All of the O2 on oureadth comes from photosynthesis. It is the process that sustain complex life on earth. That is earth atmosphere had no free O2 before the photosynthesis. There were 2 types of photosynthesis. Oxygenic and anoxygenic. Oxygenic pbotosynthesis uses light energy to split water molecules releasing O2, electron and proton. Anoxygenic photosynthesis use compound like hydrogen sulphide or minerals like iron instead of water and it doesnt produce O2. It is believed that anoxygenic evolved long before oxygenic photosynthesis. Thats why earth atmosphere didn't contains O2 until about 3 billions of year. Later the origin of oxygenic photosynthesis may have been as much is billion year closes to form complex life.