According to scientific evidence, earth’s earliest atmosphere lacked oxygen. over time, oxygen was added to the atmosphere. which statements explain how this change occurred and how it affected life on earth? choose all answers that are correct. multiple a) photosynthesis was carried out as cyanobacteria evolved. oxygen is needed to carry out photosynthesis. b) oxygen is needed to carry out many life processes. the addition of oxygen to the earth's atmosphere assisted with the development of eukaryotic organisms. c) oxygen is needed to carry out many life processes. the addition of oxygen to the earth's atmosphere assisted with the development of the first prokaryotic organisms. d) photosynthesis was carried out as cyanobacteria evolved. oxygen is a product of photosynthesis.