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A lot of people don’t realize that Groot predated the other members of the Guardians of the Galaxy by many, many years. In 1960, Stan Lee and Jack Kirby created the character for a one-off horror story in Tales to Astonish #13. Of course he was very different (he could speak full sentences, e.g.), but it’s the same guy.
Over 15 years later, he was recast in Incredible Hulk Annual #5 as a member of a five-monster posse fighting Hulk.
It wasn’t until 2006 that Keith Giffen turned him into a member of the Guardians of the Galaxy.