Mark begins with the preaching of John the Baptist, a voice announcing Jesus from the wilderness and proclaiming a baptism of repentance. The truth comes from the edges of society. Jesus’ reality is affirmed and announced on the margins, where people are ready to understand and to ask new questions. The establishment at the center is seldom ready for the truth because it's got too much to protect; it has bought into the system. As Walter Brueggeman says, “the home of hope is hurt.”
Remember that John the Baptist’s message of repentance was an invitation to a turned-around life, letting go—downward mobility, as some call it today. John wore a garment of camel hair, and he lived on locusts and wild honey—he identified with the poor and marginalized, as we see Jesus doing. John is so free from his own agenda, religious and cultural system, and ego. He's able to point beyond himself. He's not trying to gather people to himself—which is why he becomes the proto-evangelist—pointing beyond himself and his own ministry.
One can only conclude that Mark began in this way, not just because it was historically true, but because it mirrored his own journey. Some scholars today, especially with new information from the Gnostic Gospels, think that the anonymous man who “runs away naked” in the Garden of Gethsemane (Mark 14:50-52) is very likely Mark himself. He is quietly admitting that he also “deserted him” (verse 50) and ran from suffering and humiliation. His “nakedness” is not just his but ours too.