Thomas suggests that Mark's gospel may be Peter's account from prison of Jesus, as told to the writer?
Papias (c 130AD) provides a baseline for Mark.
"The Presbyter said this also: 'Mark, having been the interpreter of Peter, wrote down carefully, though not in order, all that he remembered, both words and deeds of the Lord! For he had neither heard the Lord, nor followed him, but only at a later date, as I have already said, followed Peter. Peter arranged his instructions according to the needs (of his audience) and not as making (a continuous and exhaustive) arrangement of the Lord’s words. So Mark was not wrong to write down some things as he remembered them, for he took care to omit or falsify nothing which he had heard (from Peter)." (The Presbyter in this instance is Presbyter John, probably around 80-90AD.)
Papias is confirmed by the other early Christian writers: Clement of Alexandria (150-215 AD.), Origen (186-254), Tertullian (160-240) and others. The tradition states Mark assisted Peter in his preaching at Rome and that the community requested Mark write down what Peter was preaching. (Papias, 'Exegesis of the Lord’s Oracles', Justin, 'Dialogue with Trypho' and Irenaeus 'Against Heresies'.)
Internal evidence of the Gospel harmonizes with the tradition that places the Gospel in Rome. Mark uses Latin loan words (4,21; 5,9.15; 6,27; 6,37; 7,4; 12,14; 15,39.44; etc.) and adds Latin explanations (Mark 12,42; 15,16; etc.).
The question 'who was Mark' remains something for scholars to pore over. A common thesis is that Mark might well be the John Mark of Acts 12:12 and 15:37, a cousin of Barnabas. Like Barnabas he would have been a Levite. The family home may have been in Cyprus (Acts 4:36ff), and he would speak both Aramaic and Greek. If so, at some point the family must have moved to Jerusalem. it is there that Barnabas becomes a Christian, selling his property and donating the proceeds to the Apostles to distribute among the poor (Acts 4:36-37).
Of Mark we know less, but the thesis holds Mark became a secretary to Peter, who calls him 'my son' (1 Peter 5:13), which in the language of the time indicates that Peter instructed mark in the faith.
The family proved to be very active followers of Christ. Barnabas was a great help to Paul immediately after his vision of Christ on the way to Damascus (Acts 9:27). Mark’s mother Mary offered her house in Jerusalem as a meeting-place for the early Christian community. After his escape St. Peter goes straight to this house, knowing that he will find the others there (Acts 12:12-17).
At some point Mark became Peter’s assistant. In a letter Peter writes: "Your sister Church in Babylon (Rome), also chosen by God, sends you greetings. And so does my son Mark" (1 Peter 5:13).
Around 60AD Paul was taken to Rome under arrest. Mark and Paul are reunited and, having earlier fallen out, repair their friendship. Paul writes: "Aristarchus, who is in prison with me, sends you his greetings and so does Mark, the cousin of Barnabas. You have already received instructions about him, to welcome him, if he comes your way" (Colossians 4:10). "Epaphras, who is in prison with me for the sake of Christ Jesus, sends you his greetings, and so do my fellow workers Mark, Aristarchus, Demas and Luke" (Philemon 24).
According to Clement of Alexandria:
"This was the occasion of Mark’s Gospel. When Peter had publicly preached the word in Rome, and had taught the Gospel in the Spirit, his numerous hearers are supposed to have asked Mark to write down the things which Peter preached. For he had accompanied Peter for a long time and remembered his words. Mark is said to have agreed to their request, and to have given them the Gospel. When Peter learned of it he neither forbade it, nor encouraged it."
By the mid 60s, Peter was dead, killed under the persecution of Nero, and Paul was imprisoned for the second time. Mark was in Asia Minor, and Paul writes to Timothy (Bishop of Ephesus): "Only Luke is with me. Get Mark and bring him with you, for he can help me in the work" (2 Timothy 4:11).