Lux Interior, of The Cramps, died a month before Primal started shooting. Director Josh Reed and costume designer Emily Schulz made a "RIP LUX" tee shirt which Warren wears throughout the film in his honor.
Josh Reed made the 12,000 year time-lapse sequence at the beginning of the film by individually photoshopping almost 300 frames. If you go through frame by frame you can see Josh buried to his neck in one frame towards the end, and his daughter, Ch'aska, peeking out from behind a tree in another frame.