Why do many scientists believe that bacteria were the first to appear on Earth?

In fact, it is impossible to say for sure that bacteria were the first on earth. It is possible that the first life on Earth is viruses. There can be no actual evidence for this, since viruses cannot turn into fossils. Accordingly, one cannot say when the first viruses appeared. But all cellular organisms are descended from bacteria. Bacteria have a prokaryotic structure that has evolved over time. Eukaryotes, multicellular organisms, were formed from prokaryotes. After the appearance of the first bacteria, evolution began, which lasted for millions of years and continues in our time.