Okay, here's the frickin' deal.
After being cornered by Admiral Coby for the tenth time,
the three remaining straWhats have the choice to use the time machine that
Franky and Usopp built in a hurry, or face a certain death.
They manage to get it working in the last second, but it malfunctions,
sending Usopp to one time and Luffy+Zoro to another.
- 36 years before the great pirate age. -
Luffy and Zoro land in the middle of the grand line, hitting their heads on a piece of rock.
They both get severe amnesia. Zoro's need for craploads of sleep turns into Narcolepsy.
Since there's no Coby in this era, they eventually manage to end up in Fuschia Village, East Blue.
Zoro hooks up with a chick, has a few kids, but Luffy's blood makes him itch to sail the seas.
He becomes a pirate, going by the name Gol D. Roger.
As there's no one of Coby's caliber around, his infamy skyrockets.
After nearly two decades, Luffy returns home for a short while and knocks up Zoro's 16 year old daughter.
Zoro's pissed.
He decides to join the navy, and using the name of Garp and a piece of a dog to cover most of his face,
he starts hunting "Gol" Down. During this time, Zoro's daughter gives birth to Luffy and Ace's dad.
The baby manages to give her an internal bleeding by kicking her in the womb as it exists. She dies.
After another 15 years or so, the navy manages to catch GoLuffy'DRoger, now known as the pirate king.
ZoroGarp laughs heartily as they shove long, sharp things deep into Luffy's body.
Then he goes home, adopts his grandson as his son, and never mentions Luffyroger being his "son's" true father.
What happened to Usopp, then?
Well, he eventually got his wish and became a captain.
PROOF: