I always thought that Piantissimo, Mario and Luigi weren't completely human. There are just so many additions that would suggest otherwise. There are so many sentient species in the Mushroom Kingdom, why not add another, subhuman species? I could go with the Yoshi theory (See Mariology)But that's almost too great and perfect to actually work. The simpler explanation is that they are simply a unique, distinct race.
The Yoshi theory was so perfect, it was cannon, until I disovered Piantissimo. At first i went on with the yoshi theory, denying his very existance, but I had to take notice.
By the way...i don't belive Mario & Luigi ame from Earth. :( Maybe their ancestors (But that's another thory. ;) )