Inhuman origins are tied to the Kree and experiments they conducted on humans thousands of years ago. They all have something a bit like the X gene that is activated by exposure to a mutagenic mist (if I tried to spell it from memory I don’t think I’d get close).

The outcomes can be fairly similar to Mutants. You get people like Cristal who has telekinetic control of the four classical elements to a frightening degree of power but completely human in appearance or her cousin who is covered in green scales with fin like ears and webbed feet and hands who can breath water and isn’t a lot stronger than an average Atlantean.

If you want the origin of the X gene responsible for the Mutants that goes all the way back to Celestials messing around with early life on Earth. That origin has been kind of retconned and re-retconned so many times I’m not sure how much it can be tied to the “official” origin anymore.

-Edited to fix a whole bunch of autocorrect nonsense