What was the only type of life on the planet for hundreds of millions of years

The only type of life on the planet for hundreds of millions of years were the bacteria and the cyanobacteria.

In the timeline of the evolutionary history of earth, the first event that occured supporting the appearence of life on earth was the appearence of the liquid water. About 3900-2500 million years ago, the first life forms of prokaryotes which were chemoautotrophs using carbon dioxide as a carbon source and derived energy from the oxidation of inorganic compounds appeared. By 3000 million ago, the photosynthesizing cyanobacteria appeared which used water as a reducing agent and produced oxygen. Till the first eukaryotes that appeared 1850 million years ago, bacteria and the cyanobacteria were the only form of life on the planet for hundreds of millions of years.