Who Were Mark and Luke?

From [Luke 6:13–16](https://www.biblegateway.com/passage/?search=luke+6%3A13-16&version=NIV) (NIV): > When morning came, he called his disciples to him and chose twelve of > them, whom he also designated apostles: Simon (whom he named Peter), > his brother Andrew, James, **John**, Philip, Bartholomew, **Matthew**, > Thomas, James son of Alphaeus, Simon who was called the Zealot, Judas > son of James, and Judas Iscariot, who became a traitor. John and Matthew were included, but the remaining two authors of the Gospel – Mark and Luke – were not. Why is this, and how did they come to know Jesus so closely as to later write his Gospel?