William Golding says he wrote Lord of the Flies in "an attempt to trace the defects of society back to the defects of human nature," and the theme of primitivism suits his theme perfectly.
Golding starts with a rather blank slate under the best possible conditions for mankind to be able to prove that it will not, when given the opportunity, revert to savagery; but of course that is exactly what happens here. These English schoolboys are the best examples of proper behavior and following rules; even without an adult or other restraints present, they know how to behave appropriately and lawfully. Unfortunately, they fail to live up to their best selves almost from the beginning.
As the story continues, the deterioration also continues. Soon the rules they establish are broken, and soon after that the rules of human decency and lawfulness are broken, as well. Jack is the character most symbolic of this primitivism, as he spends his days doing nothing but hunting. The final descent into savagery for the entire island begins with one simple act.
When Jack realizes he has not been able to catch a pig because they see him, he decides to paint his face. He looks at his reflection in some water.
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes.
This is a significant incident, for Jack is no longer restrained either by shame or by his own conscience. These are the last two defenses against complete barbarism, and they are gone. It is a quick and noticeable change, as demonstrated by Bill's hasty departure.
In the final two chapters of the novel, Golding rarely refers to Jack or any of his tribe (which is all but a few boys) as anything but "savages." After Simon's and Piggy's murders, as well as the smashed conch, chaos reigns on the island. It has become a primitive place in every way: naked (or nearly naked) boys running through the forest with spears, making ululations as they hunt the greatest prey, another human.
At the end of the novel, after the naval officer arrives, Golding makes his final statement about what has been lost:
[W]ith ﬁlthy body, matted hair, and unwiped nose, Ralph wept for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy.