It seems all the variations of mutants we see (most famous being Lamias, Bugeyes and Brutes), ultimately come from the same source - the so-called 'Pureblood' humans who used to be the single race inhabiting the Earth. Nowadays, the Pures universally cooperate with the Star Gods to retain their 'purity', amongst other things. But back to the point, since most of the mutant variants can be broken down into several discernible, stable and reproducing 'abhuman' strains, there is only one possible explanation - the Star Gods experimented on humans to custom-tailor slave races for their own ends. What remains on Earth are possibly their failed experiments, or ongoing experiments under observation, or some sort of reserves - the Star Gods have such abundance of resources. Whatever the truth is, the wealth of medical data we have gained could be immensely useful to us.