How has endgame theory changed over the centuries ???

We all know opening theory above all is the most changed thing in chess and middlegames are and will forever change by what upcoming master bring to the table and how they approach the game softeware resources etc.... All these things in chess are changing but I never really hear anyone talk about how ENDGAME THEORY has changed if it has aand wanted to know how it has.

Chess Endgame theory has always been changing. However, certain principles never change in chess. 

That's true.

I'm going to disagree with progamer88, as people certainly haven't become more intelligent, but rather due to the advances of critical thinking in chess and positional knowledge, our endgame theory has drastically improved due to the knowledge of HOW to think.