Because at the beginning of life, there was no evolutionary autotrophic mechanism until the emergence of cyanobacteria. This kind of organism containing photosynthetic pigment begins photosynthesis, forms autotrophic mechanism and releases oxygen at the same time. Previously, there was almost no oxygen in the earth's atmosphere, and oxygen was a gas that was toxic to organisms at that time. Since then, with the mass production of oxygen, aerobic organisms have become dominant.

Life originated from water, and a large number of water-soluble compounds provide the possibility for the origin of life. At the same time, because there was no protection of the atmosphere at first, ultraviolet rays directly irradiated the surface of the earth, and only in water could they resist ultraviolet rays, so life originated from water. With the evolution, the competition of life in water became more and more fierce, so creatures began to March on land.

Personal understanding, if there are any shortcomings, please add and correct them.