The Hill people is the name for the inhabitants of Yuma Flats after the nuclear testing done by the US government in the 50's and 60's. The original inhabitants of the town were in the mines when the testing was being carried out and so the children born in the mines were mutated by the seeping radiation that soon filled the shafts of the mine. The mutants soon left the mines when the testing stopped and they began to kill the normal inhabitants of the neighboring small towns, setting up a clan system of different mutant families in different areas of the desert. They soon started killing and eating those who passed through Yuma flats and the area quickly became known as a region filled with rumors of vanished people. The Hill People stashed the cars of their victims in the craters left by the atomic bombs (of which there were a lot of) and became large enough to present a serious threat to a medium to large group of travelers. The Hill People suffered their possibly first defeat at the hands of the Carter family, who was traveling through the area on the way to Los Angeles. The Hill People used a gas station owner to get the family to drive into the mutants' territory and make them fair game. The Carters' truck was sabotaged and "Big Bob" Carter, as well as Doug Bukowski set out to find help at the gas station. The mutants made their move soon after, attacking and burning Bob Carter alive while assaulting the rest of the family in their camper. Their killing of Big Bob, his wife and their nursing daughter as well as their abduction of Doug's infant child caused Doug to set out to find the mutants' home and destroy them. In the process of his escapade, he killed Cyst, Big Brain (possibly), Pluto and Lizard in his assault. Billy Carter also killed Papa Jupiter who was sent to kill the rest of them. The two remaining Carter family members and Doug escaped the area and told their story to the police who realized that their story explained all of the disappearances in Yuma Flats. A short while after, the Hill People were attacked by the US army who intended to purge the region of the mutants completely. Most of the Hill people were killed, but Papa Hades and his group narrowly survived the purge. Realizing that they were among the few survivors, they began to kidnap and impregnate women in order to create more mutants and save their kind from extinction. Papa Hades' group preyed upon a group of US Marines who were sent on a training exercise. The Hill People had already eliminated the local garrison and were waiting for the soldiers. However, Papa Hades and several of his group were killed. So far, this leads to uncertainty over the fate of the hill people.
The Hill People were mutated in the manner that they were born with severe deformities such as in the case of Big Brain, Letch and most of the seen mutants. Their condition also brought about psychosis and mental deficiency in the case of Cyst and Pluto. This made most of them very violent and extremely dangerous, but they were not all evil by default; one mutant Ruby showed a very kind heart and at the end an act of great self sacrifice. In the second film another good mutant Hansel was also shown to simply have more choice whether to be good or bad. Though mentally defective many show signs of well above average strength and endurance such as Lizard and Stabber surviving gun shots, but this varies from mutant to mutant and most will die if shot. These diseases also could lead to new strengths, such as Chameleon's camouflaged skin that allowed him to blend in with the terrain, Pluto's increased size and strength, Grabber's heightened sense of smell, or Lizard's ability to absorb an inhuman amount of shot gun fire.