Monkey RP: They were an attempt by the Nomads to grow their own home grown humans to make infiltration on the houses easier, unfortunately they missed a few million years of evolution in the DNA splicing and they dumped the failed test subjects onto the planet, where they grew and multiplied.
There.
EDIT@above: The original developers had nothing to with Digital Anvil or even Microsoft, since human cargo isn't vanilla, its a shortcoming on the disco devs part.