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Early minority manuscripts end the gospel at Mark 16:8, however Majority Manuscripts (Byzantine) has 12 more verses and so therefore ends at verse 20. Early tradition suggests the longer ending to be valid and therefore historical because early Church Fathers such as Ireneaus quote from these verses. There is one manuscript called the Freer Logion manuscript (dated fourth-fifth century) which says “And they reported all the instructions briefly to Peter’s companions. Afterwards Jesus himself, through them, sent forth from east to west the scared and imperishable proclamation of eternal salvation amen.” Although this is not considered canonical, it maybe an early Christian tradition of how the author first assembled his companions into preaching. This hints of the authorship of Mark via Peter due to him giving out the commandments of his disciples.

Orthodox Study Bible, Wikipedia, New American Bible