Endermen are the only mobs that pick up blocks like we do, and the endermen idle noises are recognisable speech, they're idling sounds like "hey, wassup,and look for the eye" and endermen are mostly passive, and you the player is passive too, so that means endermen are our lost ancestors of minecraft that were great builders, so, heres my theory on what happened, one day their were other people in the past who were god at building stuff, zombies,skeletons,creepers,slimes and other monsters of the overworld were bothering them, they decided to make the eye of ender, make a stronghold and jump into the portal, but the portal disappears once you're in the end, so, the ender dragon started to control them, since there was nothing with them, they had to build stuff with the blocks in the end,they had chests with stuff in it, but they turned into endermen and chests wouldn't open for them, the eye of ender became a part of them,and they started to teleport around.