How modern is modern art, really?

What exactly is so modern about modern art? This sounds like a silly question. Everyone assumes that the idea of spontaneous expression, free from the fetters religious and political agendas were phenomena that developed at the end of nineteenth and beginning of the twentieth century. I have a deep appreciation of the things that artists like Picasso and Matisse accomplished, but in many respects their creations resembled works found in much earlier ages.
I believe examples of what we could c...