Man-Thing debuted in the pages of Marvel's experimental magazine Savage Tales, one of several somewhat provocative series to back up a more adult rendition of the Conan the Barbarian. The story (which you can marvel at here) is written by Gerry Conway and Roy Thomas and drawn magnificently by the late Gray Morrow.
With this issue Steve Gerber gets his mitts on the characters and things would never be the same. The earliest Man-Thing stories have a science-based feel which gets turned over for a more magical background, which opens up story possibilities but sacrifices I feel some of the classic horror power of the character's premise.
More later perhaps.