Why an atmosphere rich in oxygen was important for the evolution of life?

An atmosphere rich in oxygen was crucial for the evolution of life because it enabled the development of aerobic respiration, a highly efficient way for organisms to generate energy. This process allowed for the growth of complex multicellular life forms, as energy availability increased. Additionally, the presence of oxygen led to the formation of the ozone layer, which protects living organisms from harmful ultraviolet radiation, further facilitating the diversification and evolution of life on land.