I don't have a definite answer for you, but there is some general information on mites here
http://www.the-piedpiper.co.uk/th5i.htm. There are many species of "outdoor" mites that are similar to the dust mite. This suggests to me that the dust mite could be a relatively new species or variety of mite - specially adapted to live within people's houses and being spread from home to home on people's clothing. Alternatively, dust mites might naturally feed off the dead skin of other mammals, such as rabbits, perhaps spending most of their lives buried in their fur, like fleas but without the bloodsucking. From this situation, the occasional dust mite would inevitably drift in through a window etc., and multiply inside the house.