The Primitive Sponge is a character who appears in the episode "SB-129" and "Ugh." He along with the Primitive Star, appear when Squidward ends up going too far back into the past. He is also one of SpongeBob's ancestors.
He looks very similar to SpongeBob, but acts primitive as would be expected. He wears a brown loincloth, and has two sharp fangs. He also has no eyelashes, which is also unlike SpongeBob.
He is revealed when Squidward attempts to find a quiet place to play his clarinet. At first, the meeting between the two is awkward but calm; he even sniffs and plays with Squidward's arm out of curiosity. The Primitve Sponge is also shown at first to be afraid of jellyfish, as does his friend Primitve Star. He is later seen holding a jellyfish and being continuously stung by it, screaming and passing it to the Primitive Star. Squidward becomes annoyed at their screams and tells them they are supposed to catch the jellyfish, not torture themselves with it. Taking a stick and loincloth from both primitives, Squidward crafts two Loincloth jellyfish nets and shows them how it works, inadvertantly creating jellyfishing.
Later, as they are learning how to jellyfish, Squidward's clarinet playing enrages them, causing them to give chase to Squidward where he then seeks solitude in the time machine.
The Primitive Sponge is not very intelligent. He could not talk like the Bikini Bottom citizens of the modern day. Instead, he communicates by growls and whimpers. His high tolerance for pain is what keeps him holding onto the angry jellyfish and repeatedly getting stung. He views Squidward as their leader.