At first, the earth's atmosphere was a reduced atmosphere, with hydrogen, methane and so on. , but there is no oxygen. Primitive creatures live in primitive oceans and are rich in organic matter. Primitive life obtained nutrition by directly ingesting ready-made organic matter, so all the first creatures on the earth were heterotrophic, until such creatures as cyanobacteria appeared, and the first autotrophs were born. They make their own organic matter and can produce oxygen, thus changing the composition of the atmosphere.