<human>:  I have a question about the book The Rise of Microorganisms. When did the first microorganisms appear on Earth?
<bot>: The first microorganisms appeared on Earth about 3.5 billion years ago. They were single-celled organisms that were able to survive in the harsh environment of the early Earth. These organisms were called prokaryotes, and they were the first organisms to use the process of photosynthesis to convert sunlight into energy.