John Grimm was born in 1918 to normal human parents of no particular distinction.  When he was born, the family physician noted what appeared to be signs of intense atavistic traits, Neaderthal or more, in the young babe.  Not wanting to upset the new parents and aware that nothing could be done, the doctor told them that John was a normal human baby.   As he grew older, the atavism intensified and John was ridiculed and rejected by his peers for appearance and his rough behavior.   Becoming isolated, John began to spend most of the time in the forest to avoid "civilization".  Sometime before 1942, John's parents were killed in an automobile accident.  The same doctor who birthed him had to deliver the news from the morgue and the revelation broke John's mind.  He began to rave that off his problems stemmed from "civilization" and that it was civilization that was his true enemy, one he intended to defeat.

Grimm goes on a rampage with a large club, smashing cars and causing drivers to flee.  Stretch Skinner and Ted Grant happen upon the scene and when Stretch runs to investigate, Grant changes into Wildcat.  Turning confront the intruders, Grimm - now referred to as "Caveman" - attacks the duo.  After a brief melee, Stretch manages to get off a shot and wounds the Grimm, who retreats back to Grimm Manor, where he has build a cave as an underground retreat.

Realizing he will accomplish little on his own, Grimm decides to enlist others in his anti-civilization crusade.  He forms a "Back to Nature Club", where he invites wealthy participants to live in a more primitive way.  Over the weeks that follow, he develops a cult following that expands throughout the upper echelons of society.  When a wealthy man's wife is caught up in it, he contacts Skinner Detective Agency to investigate.  Recognizing the Caveman, office clerk Joan Fortune offers to join the cult and get a look at the inside.  When she does, she discovers that the Caveman is preparing to lead them all on a rampage to destroy the nearby electrical plant.  Joan summons Stretch and Grant, who slyly becomes Wildcat, The two converge on the Caveman cult as they wreak havoc at the plant, eventually leading to a confrontation between Wildcat and Grimm on a high catwalk.  A right hook across the Caveman's jaw sense him bursting through the curtain wall of the plant and down to the street below.  Out of sight of Wildcat and his colleagues, a car whisks the unconscious Caveman away. 

It is later revealed that the Grimm family physician, who has felt obligations toward the atavistic son of his patients, had followed the Caveman's plight all along and had nursed him back to health.  He creates a fake grave stone and takes Grimm to a remote island where he can live a primitive lifestyle in peace. (Sensation Comics #10).

Whether he ever left the island is unknown and his future activities are unrecorded.