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This question has been asked many times now, but what happened in between Adventure and the present that would make Knuckles the last echidna? Adventure establishes that the echidnas got wiped out by Perfect Chaos, but if that were the case, how on earth did they survive long enough to allow Knuckles to exist in the first place? And frankly how is it that he doesn't know about any other echidnas like say his parents if he exists which means that he must have had parents?
There's really just a giant gaping hole in between these times and it's never been answered.
MY own theory is that Knuckles was stuck in some kind of temporal stasis as an egg and thus he never even knew his parents, and thus is technically hundreds of years old, but this runs into the problem of how he can speak perfect english...unless Tikal somehow learned english during that time and transferred the knowledge to him via Master Emerald deus ex machina or something...
I just figure that chaos' attack and the island rising into the sky disrupted the echidna civilization enough that they never really recovered, and they just slowly died out over time until knuckles was the only one left.
also echidnas lay eggs, so he could've been "born" after both his parents were dead. but that still leaves the question of who raised him.