Hamburgers might seem pretty straightforward, but actually they've become vehicles for innovation as chefs around the world have started tinkering with the familiar formula, especially over the last 15 years. It makes sense: The concept—meat patty served on a bun—is simple enough that it serves as a blank canvas for creative food minds, whether they're tinkering with 24K gold dust or unusual ingredients from the Far East. Here's a look at some major milestones in the evolution of the hamburger from drive-thru staple to gourmet favorite.