In Chapter 907 we learned a few things:
-Before Gol D. Roger a pirate captain or organization named Rocks was the top pirate group.
-They were the supreme rulers of the seas. Unlike the Golden Era were we had 3 major players (Gol D. Roger, Whitebeard and Shiki) rule over seas, unlike the Great Era were we have 4 rulers of the seas (Big Mom, Kaido, Blackbeard and Shanks), the Rocks were the ones who ruled over the whole New World!
-Big Mom & Kaido possibly worked under this organization/captain.
Until now, we thought Garp became a Marine Hero because of his fights with Roger, but now we know that it happened before that. Probably when he took down “Rocks” organization or their captain. In fact, Hina revealed that Garp got the distinction of “Hero of the Marines” from dealing with the Rocks Pirates.
So how they defeated the Rocks Pirates? Well, let’s factor in some other point..
-Zephyr became Admiral ad the age of 38. He died at the age of 78. So that happened 40 years before the current timeline.
-Rocks captain/organization was defeated 40+ years prior to current timeline.
40 years ago Garp, Sengoku & Zephyr were the most prominent Vice Admirals of the Marine. They led an Armada against Rocks Pirates to defeat them once & for all.
They also had other strong marines with them like Tsuru, Bogart & others as well who were Rear Admiral at that time.
Big Mom wasn’t directly under Rocks & was just allied captain (like Squardo & Sai etc.). Kaido was the cabin boy like Shanks.
The whole plan was crafted by Sengoku & Tsuru. They timed it just when Big Mom & other allied captains were not present. As a result they only fought the core crew of Rocks Pirates.
Garp beat their captain & became The Hero of Marines. Sengoku & Zephyr beat their Top Commanders & became Admirals.
After Rocks was defeated Big Mom went her own way and started gaining power & eventually became Yonko. Marines didn’t arrest Kaido cause he was a minor & let him go instead.
*Theory by Sherlōck