In the second century AD, Mark was remembered as a helper and close companion of Peter. He was a relative of Barnabas and a travel companion of both Barnabas and Paul on their first missionary journey (Acts 13-14). Early second century writers, believing Mark not to be an eyewitness of Jesus’ ministry, believed Peter told Mark stories and Mark wrote them down, and that became the Gospel of Mark. Some second century Christians believed Mark wrote after Matthew, copying much of his work.
All of this presumed, of course, that the gospel writers wrote simply to recount the life of Christ without any other motives. The evidence, however, is that all the gospel writers wrote what they did to make specific points to their intended readers. Thus, while they often include the same material, it is presented differently in order to make their God inspired point. Additionally, there is ample evidence that Mark, young as he was, did in fact witness at least some of the ministry of Jesus. His gospel account is full of the kind of details that would be common only to an eyewitness. One Bible scholar has listed 200 details, mentioned only by Mark, in his first 6 chapters that would be the sort of thing only an eyewitness would know. I divide Mark as follows:
1) Introduction – Mark 1:1-15
2) Ministry Outside Jerusalem – Mark 1:16 – 10:31
3) Ministry and Death in Jerusalem – Mark 10:32 – 16:20