The statement, “Bacteria set the stage for other life forms to evolve on earth” was made probably because Earth was populated exclusively by single-celled organisms (Archaea and Bacteria) at the time that it was oxygenated, and only one of these can perform photosynthesis that separates a water molecule and results in oxygen gas, which is called Cyanobacteria, which has a blue-green color that comes from chlorophyll. Today, Cyanobacteria can practically be found everywhere on Earth, possibly in the soil right outside your door, and still play a vital role in producing the oxygen that we breathe. Another reason could be, billions of years ago, single-celled creatures had the planet to themselves. Then, they established different ways for cells to share and communicate resources. These intricate and complex multicellular creatures were the very first animals, and they were totally a major success. Consequently, animal life evolved about 540 million years ago.
