Answer:
prokaryotes
Explanation:
It's suposed that in the beginning of life the earth had a primitive soup with tons of biomolecules free in the enviroment, the earth had a reducing atmosphere that would be poisonous for animals like mammals, birds, reptiles, invertebrates...etc. For that reason the prokaryotes were the first bacteria in the planet, they started the colonization of the earth and changed significantly the environment, specially the air.
