The evidence I will provide is purely speculative and hypothetical in relation to the One Piece story. Since the start of the series, a question that may have sat in the back of our minds would be who was Luffy’s predecessor to the devil fruit Gomu Gomu No Mi?
The solution I have found to work for the question, in which I speculate, would be that the specific devil fruit and its powers were once possessed by Portgas D. Rouge, Ace’s mother.
The purpose behind this would be the nature of Ace’s birth. The average pregnancy lasts around 9 months. The purpose of this would be based on the carrier’s body withstanding the pregnancy as well as the baby remaining healthy if possible. In the manga, it says that by the sheer force and will of Rouge, she was able to hold off Ace’s birth until the 20 month mark. By these standards, that would make Ace as old as an 11 month old child with the addition of the 9 month pregnancy. The average 11 month old baby weighs 21 pounds. The normal female human body would not be able to sustain a baby of that weight without abnormalities or powers.
I think Ace had the extended pregnancy due to his mother’s Gomu Gomu No Mi powers. Her powers allowed her to stretch more than the average body, allowing her baby to have an extended pregnancy albeit, there were still consequences for the rather unnatural birth leading to Rouge’s death.
In a different perspective, this would also explain why Shanks would have the devil fruit. Knowing Rouge would have had some sort of connection to Roger, Shanks would have found the fruit to find a suitable user based on his relationship with his former captain (possibly Ace if Shanks knew about his birth/whereabouts).
As uncertain as we are of Shanks, most of us see him as a man of good intent. Based on this fact, I believe that Shanks had the devil fruit to use based on his good standing with Roger. The entirety of this speculation brings a full circle idea that connects Luffy to Roger’s life in a minuscule manner.
*Theory by Skills D. Kyle