"There's a group of human's that can breath fire like a dragon"

Just using some basic knowledge of evolution and genetics, that seems pretty unlikely. What would the evolutionary pressure be ? Would there be enough time for that development? No parallel's in other mammals etc.

You can do the same to see if something is plausible. For this discussion specifically. We know there are physical difference's between populations (Height, drug tolerance, skin colour)

These observable differences are caused by different genes. What are the chances that none of these differences in genes are in the brain ? Basically zero.

So it's very plausible you'd find some differences. Just using some simple reasoning. (Reading a little bit about it, Sam Harris basically makes this exact point. It really is basic logic)