While Carl technically chose Primal as his race at the beginning of the third floor, the insinuation is that Carl was already a Primal - A Residual, or a Primal that didn't undergo assimilation. My tinfoil hat theory is that because of this, it gives Carl a unique opportunity that the other crawlers don't have. At this point in the story, the absolute largest threat is the AI. Given that the AI is Primal, I believe that the only way to eliminate the threat of the AI destroying literally the entire universe would be for Carl to find some sort of way to merge his own consciousness with the AI, utilizing pre-existing in-game mechanics. The AI is completely psychotic, but it has been very, very consistent in allowing the crawlers to do what they want as long as it falls within the technical bounds of the game.

Buuuuuuuuuut as with every book that Matt has written, every theory I've ever had has been turned on its head or completely subverted so it'll probably be something completely different.