The Gospel of Mark was the first draft of a doctoral candidate’s dissertation. He submitted it to his advisor who suggested the need for more background information about Jesus’ birth, maybe some more teaching material, and a stronger ending.According to Jordan Scharf here.
The student rewrote his dissertation and submitted the Gospel of Matthew.
His advisor thought the revision was much stronger but felt that the teaching material should be better integrated into the narrative, thought a story about Jesus’ youth might be helpful, and suggested that the genealogy could be expanded back to Adam, etc.
The PhD candidate did another major revision and produced the Gospel of Luke. Once again the advisor was critical and asked for major revisions.
Frustrated, the student took drugs and wrote the Gospel of John.