Can't figure out what movies are stuck in my head.?

One movie is about a guy taking a trip to some rain forest and visiting a local tribe. They give him some kind of leaf to eat. Well when i gets back to the states the leaf starts changing him to a monster, so he starts killing people in this museum or some place. Any body know?

My second one is either a movie or tv movie or series. I don't remember much about it, except it's basic plot. I believe something big is about to happen that will wipe out all human existence. But it was discovered that this has happened before. So it basically says that the human race grows up but then get's wiped out completely. but several years later it'll start right back up from cave men grow to present day and get wiped out all over again. So the process just repeats.

Anybody have any ideas?