Burton Mack’s The Lost Gospel of Q deals directly with this question with a hypothesis that is wholly plausible. Since studying up on Christian origins, I have run into ideas of Markan priority and the two-source hypothesis, but never fully understood the rationale behind these ideas. Mack shows the problems in harmonizing the various Gospel sayings, demonstrates the feasibility of Markan priority, then interprets the person of Jesus based on the implications. Mack does this in an intelligent, highly readable and nontechnical manner that requires only familiarity with the synoptic Gospels. It does treat some material a little too lightly for my tastes, but on the whole this was an excellent read.