There is usually confusion about the difference between evolution as the study for how life forms become more complex, and abiogenesis, which studies the origin of life. People arguing against Evolution sometimes demand an explanation of how life originated. They don't seem to actually know that evolutionary biology, evolutionary genetics, abiogenesis, paleontology, and palaeoanthropology are all different studies, each contributing one piece to the puzzle.

Not that I'm well learned in any of them, so I'll take my amateur biologist hat off and simply say that non of the concepts behind evolution are difficult to understand at all! Anyone who refuses to accept the theory of evolution by natural selection has just never learned about it. Anyone still refusing to accept it after studying the theory has just never been to the Smithsonian museum.