>>538288705
>Humans on South Island before Eggman,
I wonder if the OVA creators referenced that manga. Seira is clearly a cat/human hybrid. Planet Freedom is post-apocalyptic Earth (much like Archie Mobius but less spelled out), many of the humans have mingled with the rise of other sapient lifeforms. These ideas may have been close to becoming game-canon before Sonic Adventure.
>>538289151
Also note Sonic & Tails' base in the OVA being the ruins of a present-day aircraft.