In Mother 3.5, the Cavemen are the guardians of the Seven Needles instead of the Magypsies. The reason for this is explained in the "Seven Needles" article. They were originally the watchmen that protected each individual trigger wired to the Three-Mile Army's last-ditch effort bomb, but after the Three-Mile Army was destroyed, the watchmen couldn't escape their subterranean labyrinths. Instead, they lived for a VERY long time underground, exposed for far too long to the switches they guarded. The genetic exposure tied their existence to the switches, so that after the switch was pulled they would cease to exist. Unfortunately, there were other effects that the switch had on them- they became blind and deaf, and completely silent as well. They could not stand light, and it caused them pain. As such, the "Cavemen" stayed by their switches, waiting for somebody who could save them.
One of the switch-watchers managed to NOT become a Caveman. His story is a strange one, and he called himself Fassad.