On the contrary, she seemed strangely proud of the dark, goatish-looking infant who…. was heard to mutter many curious prophecies about its unusual powers and tremendous future.
–H.P. Lovecraft, THE DUNWICH HORROR
In his classic piece of horror fiction (later made into a mediocre film), THE DUNWICH HORROR (1928) H.P. Lovecraft tells a tale of unspeakable terror set in a small New England town. Dunwich, Massachusetts, is an isolated hamlet which is decaying and filled with a sense of uncleanliness, where the inhabitants have interbred for some time, with one another, and eventually, with something else. Out of another "dimension," analogous to the underworlds or fairylands of old, a new source of genetic materal comes, when Old Man Whateley makes a deal with and underworld entity called Yog-Sothoth, and allows the demonic thing to mate with his daughter, Lavinia, impregnating her with a potential threat to the human species. Her son Wilbur Whateley, tall, hulking, and strange of shape through baggy clothing, is not all that he seems, and eventually his slain form is revealed:
"Above the waist it was semi-anthropomorphic; though its chest, where the dog's rending paws still rested watchfully, had the leathery, reticulated hide of a crocodile or alligator. The back was piebald with yellow and black, and dimly suggested the squamous covering of certain snakes. Below the waist, though, it was the worst…."
The rest of the description of Wilbur Whateley goes beyond vertebrate characteristics, with mollusk and cephalopod features (aquatic or semi-aquatic?), limbs, and other members. The full horror of the description will not be repeated here in the interest of "fair usage," but the story is recommended to any student or researcher of subterranean folklore. Needless to say, the resemblance of Wilbur to the Sumerian underworld demon called 'PAZUZU,' or to a variety of cryptid as well as 'reptoid' forms from Ufology and folklore, is apparent. Also of interest is that the story ends with the revelation that Wilbur had a twin, not quite as capable of masquerading as human as he had been…in fact, he is an invisible elemental giant of greatly destructive power. PAZUZU was of the underworld, yet was also a 'demon of the wind,' in effect paralleling both of the abominable brothers from THE DUNWICH HORROR. As an entertaining fictional read, Lovecraft's work in general is highly recommended.
An interesting piece of fiction, one might say, and indeed it is. But do things like this really happen, in the real world? Do human beings, either willingly or unwillingly, make pacts or genetic links with nonhuman beings from beneath the Earth or elsewhere, contaminating the human gene pool even as they enrich the intrusive, parasitical one? Do history, folklore, and medical science hold clues as to this seeming-fantasy being a reality?
Evidence for such activity would necessarily consist of two types: historical and folk accounts, and real persons of questionable and mysterious genetic heritage, a heritage which may result in unfortunate and sometimes inexplicable medical or physical conditions. As in the Lovecraft story, there is sometimes an element of the human family or gene-pool being interbred to some extent as well, and the extent to which they are interbred may have some affect on the likelihood of the non-human genetic characteristics being dominant upon occasion, even surfacing in future generations. This does not mean that these people are not human beings, although upon occasion this may have been debatable. It simply means that they have retained or inherited genetic characteristics from a contamination in the distant past, which is beyond their control or responsibility. As it turns out, both types of circumstantial evidence do exist, and in fact may indicate that the second type of evidence could be the result of the first, even if such trysts or cross-breeding took place decades or centuries before.
Before going further, it must be observed that unfortunate birth defects and malformations exist which doubtless, do not fit into the evidentiary category described. No person should ever be shunned or otherwise mistreated due to any defect beyond their control. Negative behavior of this type is wrong in all instances, and should never be tolerated or condoned for any reason. Compassion is one of the fundamental aspects of true humanity, and many a true "monster" has held a beautiful or otherwise normal appearance, yet inwardly been a beast of the worst description.
To return to the study at hand, folklore and history are filled with tales of such half-human or genetically-contaminated unfortunates, and often the two anecdotal areas overlap. It must be observed that not all such liaisons have resulted in deformity or an otherwise evil condition or nature. How do such 'alien' genes, possibly originating with pre-human races who shun the modern sun, enter the world of human beings?
There seem to be two methods: through forced interbreeding (abduction, rape, and the like), or else willing pacts and even marriages. Closely-monitored or genetically-related groups seem to be the target of the latter, while the former appears more random at times.