The Earth's atmosphere consists of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%), carbon dioxide (0.04%) and other gasses in trace amounts. Most species use oxygen for respiration; bacteria and lightning fix nitrogen to produce ammonia used in the production of nucleotides and amino acids; and plants use carbon dioxide, The atmosphere helps with solar ultraviolet radiation, solar wind, and cosmic rays to protect living organisms from genetic damage. The present composition of the Earth's atmosphere is the result of billions of years of biochemical alteration by living organisms of the paleoatmosphere.