I can't find a thread for this puzzle and I'm surprised no one is looking for help with it.
I have the thematic location and date, the two letters that make up the neat entry in the central cell, and the four-word phrase - all very clever indeed. What I don't have is an explanation of 'half of humanity eventually' (4dn) and the wordplay for 22dn, assuming that the first letter is W.
And I've no idea how you are meant to change the phrase 'in steps'.