Well see, I was thinking along the lines that the battle actually created Grand Line. It's also responsible somehow for all the fucked up shit there including Calm Belt, DF etc.
I don't know about the Devil Fruit, but this was largely what I was thinking myself.
And while that's still possible, the one thing that tends to drive me away from that thought is the fact that Mariejoa (in some form) did indeed exist back before then and I'm sure a weapon that tears the shit out the world like a bowling ball wouldn't leave one little place intact (and it would be a pretty weak plot point even with the Oda touch).
Ah, but where does it say that Mariejoa existed prior to the destruction of the Unknown Country as Clover exclusively referred to the [paraphrased] "nations that would become the World Government"? Who's to say that Mariejoa wasn't chosen as the center of the new World Government for the fact that it was centrally located and relatively intact after the realignment of the world?
Of course, it may have existed prior to the destruction and "miraculously" survived, which could be responsible for the designation of Mariejoa as a Holy Land.
I don't buy it…giants, on average, are preetty dumb from what we've seen. Doubtful civilization builders.
Which certainly goes against my theory that most of the more intellectual giant tribes were wiped out… oh, wait. It kind of supports that.
Anyway, they're dumb how exactly?
Dorry and Broggy have been fighting a duel for a hundred years over a matter of honor, but that's a cultural thing. They really don't seem all of that stupid to me.
Oimo and Karsie made a deal with the World Government based on the concept that the World Govenrment wasn't a bunch of malicious bastards looking to screw them over. You shouldn't have to assume that the people representing justice are lying to you, especially considering that this was well before the Golden Age of Piracy.
Outside of that, we have Sauro and Big Bun, who, interestingly enough, both use "ishi ishi ishi" laughs, which might suggest that Sauro is simply a Wotan whose giant ancestry is much more apparent than his fishman ancestry. Big Bun didn't even really talk so there's no way to really say, but I don't think we can judge Sauro an idiot either. He's a bit withdrawn and soft-spoken for a giant, but he is obviously more introspective than people give him credit for.
I think the greater likelyhood IMO is that it was a multi-species society and was torn asunder not necesarily by one race but by all of them; Arlong (and you forgot fish-men in your theory, too) type people could exist of any species. All you need is two Arlong type people of different races in the same place and a war can start pretty quick.
Well, first of all, I didn't say it wasn't a multi-species society, but that it was dominated by giants for logical reasons. Fishmen would largely be in the same boat as the regular humans and even intermarry as well judging by the fact that there's actually a name for a Fishman-Giant hybrid.