So, we know that Devil Fruits apparently (according to legend) came from a 'Demon'.
I think the 'Demon' is Raftel which in times past the World Government probably forbade people to go anywhere near (and probably still do). The WG found the island and its inhabitants, and due to greed wanted all the DFs for themselves for their own purpose of world domination. The WG was probably originally just a handful of explorers.
The Devil Fruits are normal-growing fruits in Raftel which were spread everywhere as a last ditch attempt to prevent the WG from stealing them all and using them for their own purposes.
This has to some extent succeeded - lots of pirates now have DFs; although some of the most powerful DFs also reside with the WG and Marines.
Here then comes what I believe is the reason for everything in the world of One Piece.
The World Government controls the Marines to get them to collect DFs on the pretense of fighting piracy. With each pirate with a DF who is killed, the DF is relinquished from their body and somewhere in Raftel a plant blooms and the same Devil Fruit ripens.
So pretty much the WG - having taken over Raftel long ago during the blank century - harvests these DFs and possibly believes that they can somehow combine all these DFs together into a massive super weapon (but as of yet can't so the DFs are held in storage)(they have already done experiments with pacifistas to 'duplicate' a DF's ability, with inanimate objects eating DFs; and now know that for people like Blackbeard, it is possible to combine more than one DF to a living being).
The idea of Raftel being the source of the DFs would explain why there is a book about the DFs; even on the abilities of DFs that supposedly haven't been eaten yet (like how Sanji read about the DF that makes you invisible, and Blackbeard read about the Darkness-Darkness fruit and knew what it looked like before Thatch ate it).
I think this is explained by those on the island of Raftel who documented each and every DF, and its abilities. This would have been made possible if after dying the DF regrows itself in Raftel.
Roger is the first to possibly get close to the truth having made it (or gotten close) to Raftel; and decided in his dying last words to spread the news of a 'treasure' (in this case an island filled with DFs) to invoke an uprising and surge in the number of pirates out there who will inevitably along the way be fighting the Marines and World Government for their booty. He went as fall as envisaging that those who could make it to Raftel would be strong enough by that point to stand up against the WG.
Or at least that is my theory anyway. It would explain things like the book of devil fruits for fruits not yet eaten; would explain WGs worries of people pursuing One Piece and the likelihood of more people attempting to get to Raftel thus increasing WG's enemies/etc and why they desperately want to keep the blank century a mystery.
Of course my theory still has a ton of holes in it such as why those on the island never bothered to rule the world themselves with their DF abilities - I can only explain that off as to the idea that perhaps they never learnt how to build ships...and were stuck on the island forever with their DF abilities and their weakness to the sea so they just sort of eventually got along with each other - or perhaps the island is inhabited by curious, scientific minds who don't really have any ambitions to dominate each other.
i think the DFs are something else all together besides fruits. its going to be intersting to see what they really are. i personally think they were manufactured by the 20 kingdoms long ago to fight the ancient kingdom since they were so powerful.
this is how i see it, the ancient kingdom is the origin of the D's, and everyone from that kingdom shared that middle initial. if what we know about the D's, they are selfish and hot tempered. they probably held regular royal meetings and in some dispute i sure the king of the ancient kingdom disagreed or didnt want to go along with the other 20 kingdoms.
the 20 kingdoms knew they could not take on the ancient kingdom head up, so they had the fruits developed to help battle them. in a long brutal battle, the ancient kingdom saw their end and had the poneglymphs made, and the remaining D's split up and vowed to live on, and one day when the time was right step to the forefront again. over time there were so many fruit the government couldnt keep up with them and eventually lost most of them
Well, I actually don't think it's the origin of them but at this point every theory is valid.
For me the fact that the fruit doesn't allow you to go to water is something to take in consideration.
Although the relation of D's and Devil Fruits (LOL, Intitals D) makes me wondering if they're not part of the same story.
And let's remember that Vegapunk wrote the manual of DF which makes me think He knows more than we can speculate.
Like LordUriel said, every theory is valid for the time being
Reading the latest chapters, i couldn't help but notice the "Eve Tree" thing... When i think about it, it reminds me the Genesis. "Adam Tree", "Eve Tree" and "Demon Fruit" ! The fruit in the Bible was also a curse in a way and according to the given names, it wouldn't surprise me if these trees were the very first ones of this world !
So, perhaps when the trees "meet" they give birth to these kind of fruits. But i think they're released underwater only, i like the idea of a fruit "falling" upside down: from the depths to the ground (sea level)
@Geez - that would be epic. I can totally imagine Oda doing something like that - he is quite imaginative.
my thought and theory on the origin of devil fruits is just like evrey islands has a magnetic field and each island has it own minerals plus weather and element an example earth has gold and caper and weather pattern
an advance race probably went to each island on the grand line and collected them and somehow put fruits and these properties together this is just a theory
Does anybody know if Devil Fruits are mentioned being had by anyone before Roger became Pirate King? Like did Whitebeard have his DF abilities back in the day or just afterwards? If nobody had fruits before then than maybe we can assume that the DF tree is on Raftel after all.