It's a mix of both. Humans don't mutate LIKE the critters mutate, when exposed to the airborne FEV and radiation, but they DO mutate. The Chosen One's regenerating 6th toes is a prime example of this. The differences between "prime" and "mutant" humans is, by comparison of giant and normal-sized ants, miniscule. But there are differences. And that was the basis for the Enclave's drastic global extermination methods in the second game.
Also, while as a result it sorta fell into the "plothole glue" cop-out, Wasteland mutations were the results of both FEV and radiation. By FO2, the concept of the jet stream carrying airborne FEV all over the world had been established to explain why such mutations would be possible thousands of miles away from The Glow, so besides the fact that none of this is actually physically possible, the rest isn't much of a stretch to understand.