All life on Earth (bacteria, archaea, eukarya) is thought to have evolved from a common ancestor, or last universal common ancestor (LUCA). What do we know about the characteristics of LUCA based upon extant life?
Carl Woese has much to say on this subject, including:
There is a 2000 Scientific American article by W.F. Doolittle that discusses much of Woese's (and others') work understanding this question.