Microbial life may have appeared as early as 3.8 billion years ago, soon after our planet Earth formed out of dust from the young sun. Since then, microbes have diverged into new forms adapted to diverse ways of life, from psychrophiles beneath the ice of Antarctica to anaerobes in the human colon. Descendants of early microbes include all living organisms. How did microbes originate on Earth? How did their metabolism shape the chemistry of Earth’s crust and atmosphere? This chapter explores three main themes of microbial evolution: the origin of life on Earth and the nature of the earliest cells; the divergence of microbes from common ancestors; and gene transfer and symbiosis as agents of evolution. This chapter tackles big questions that should interest all biologists, especially since evolution underpins all of biology.