Many manga series get their start as stand-alone one-shot chapters meant to tell a single story, and One Piece is no exception. In 1996, Eiichiro Oda presented a one-shot comic called Romance Dawn, Version 1, a prototype to what would later become the most well-known series in manga history. Debuting in Shonen Jump's Summer Special Issue in 1996, this 50-page story centered on a young Monkey D. Luffy, Red-Haired Shanks, and a pirate named Crescent-Moon Gally.
It starts with Luffy on his skiff, the Märchen, and following the ship of Crescent-Moon Gally to a nearby island. The people there are terrified and arguing about how to get rid of the pirates, with the mayor wanting to just pay them off while a young woman named Silk wants to fight. By the time the ship docks, Luffy has already taken over, with Gally and his men taken out. Gally begins to threaten Luffy, but of course, Luffy has gone to find something to eat.
Silk strikes up a conversation with Luffy, who tells her he wants to be a "Peace Main" like Shanks. It's here that we see a retelling of how Shanks gave Luffy the straw hat. Gally escapes his captivity and attacks Luffy, but is quickly taken out again and his ship is sunk. Later, a stubble-faced Luffy is woken up by what appears to be Silk and Gally.
This story is considered a "Chapter 0" of sorts by fans, though the main story took a different turn from the way that this one-shot was heading. You can see early versions of Yasopp and Lucky Roux among Shanks' crew, and Gally was repurposed in an episode of the anime. This story seemed to have a different system of piracy that the main story did, with adventure pirates being called "Main Peace Pirates" and those who pillage being called "Morgania Pirates." Shanks and Luffy would have fallen into the "Main Peace" category, and Gally in the "Morgania." Luffy also collects the Jolly Rogers of pirate crews he defeats, a trait the current incarnation does not share.
An anime adaptation was made in 2008, but also included the current lineup of Straw Hats and the Thousand Sunny. A second version of the story was also drawn and published, though this one was closer to what would become the first chapter of One Piece as we know it today. The art style is also closer to Oda's current style and is more dynamic than the original draft.
Luffy is more of the goofball he is now in this second version. Oda wanted to keep Luffy's relationship with Shanks a secret, so in this second version of the story, Luffy's grandfather is the one that gives him the straw hat. Silk would also be repurposed into a character named Ann, but would later become Nami. All early versions have the Gomu Gomu No Mi, except it wasn't known as a "Devil Fruit" at this stage.
Oda had been planning his pirate-based story since his junior year of high school, and his dedication shows. It's now become a 1,000 chapter masterpiece that people all over the world enjoy, but it all started with this single stand-alone chapter.