Conversation so far:
What is Keeper of the Ashmolean?
In 1884, Arthur Evans, at the age of 34, was appointed Keeper of the Ashmolean Museum.
How did Arthur Evans become keeper of the Ashmolean?
Arthur Evans had the right qualifications and took the position of keeper at the Ashmolean when it was offered.

Question: When did he become keeper?
Self-contained rewrite: When did he become keeper?