Charlie plays Weakchin, a prehistoric man who wears a bearskin, a derby hat, and carries a cane. In My Autobiography, Charlie described how he came up with the idea: "I started with one gag, which was my first entrance. I appeared dressed as a prehistoric man wearing a bearskin, and, as I scanned the landscape, I began pulling the hair from the bearskin to fill my pipe. This was enough of an idea to stimulate a prehistoric story, introducing love, rivalry, combat and chase. This was the method by which we all worked at Keystone." Charlie also recalled that it was a "strain" to finish the film because there were so many business propositions requiring his attention. “I suppose that was the most exciting period of my career, for I was on the threshold of something wonderful.”
In 1982, silent film historian, Bo Berglund, identified Charlie's half-brother, Sydney, as the cop in the final scene. Syd had just begun his contract with the Keystone company & His Prehistoric Past was only his second film. As Syd's biographer, Lisa Stein Haven, noted, "It seems significant somehow that the brothers would work together in Charlie's final film for Keystone."