Who are they in Wrong Turn?

It all started with Maynard Odets:

Maynard worked at the Hope Creek Pulp and Paper Mill for years and in the 70s it was shut down. He and his wife, Deliliah decided to stay in the area although the Toxic Chemicals killed off the wildlife and affected the environment. He and his wife were both exposed to the chemicals, and she would later give birth to Five children, Three Finger, Saw Tooth, One Eye, Ma, and Pa, and decided to become cannibals. They were born deformed due to the chemicals and Maynord and Deliliah both could have already been Inbred, from a continuing family heritage but it's not confirmed.

Later Ma and Pa gave birth to a sister, brother, and Three-toe. They were all deformed cannibals and preferred inbreeding to keep the bloodline pure as shown in sequels.

It was even answered by Maynard in Wrong Turn 2 (transcript from springfieldspringfield):

What are they?

Oh, just like you and me, brother, 'cept uglier...and a lot stronger. Hey, I hit one of them young'uns once in my pickup. He popped up and just kept goin'. It's the chemicals changed 'em. Pulp mill shut down near 30 years ago...and left a mess. And the poison killed off all the game. Most folks 'round these parts left. There's one family that stayed on in this holler...and pretty soon their babies started gushin' out of their mama's womb...just as ugly as them trout I got hangin' out there. And then they grow up, have kids of their own. Turned out even worse.