The Primitive Sponge was seen in the episode SB-129, and he is SpongeBob SquarePants' ancestor. He had no common sense and lived 546 million years ago in the Cambrian Era. He was revealed when Squidward attempted to find a quiet place to play his clarinet. At first the meeting between the two was awkward yet calm; he even sniffed and played with Squidward's arm out of curiosity.
When a Jellyfish swam by he became afraid and ran away screaming with Primitive Star. He is later seen holding a jelly and being continuously stung by it, screaming and passing it to Primitive Star. Squidward became annoyed at their screams and told them they were supposed to Catch the jelly, not torture themselves with it. Taking a stick and loin cloth from both primitives, Squidward crafted two Jellyfish nets and showed them how it worked. Both primitives took the nets and ran after the jelly while Squidward went back to his clarinet playing. This, however, would prove to be a bad decision, as neither primitive found the sound very pleasant and they angrily pounded their chest and chased Squidward back to the Time machine.
The Primitive Sponge was not very smart. He could not talk as Bikini Bottom citizens of today. Instead he communicated by growls and whimpers. His high tolerance for pain is what kept him holding onto the angry jelly and repeatedly getting stung. He views Squidward as their leader. He is not to be confused with SpongeGar SquarePants, a slightly more intelligent sponge. He looked very crazy when noticing Squidward also.
The Primitive Sponge has a large sloping forehead and a large ape-like mouth with a notable overbite, his teeth are large and thick. He wears a loin cloth that can be replaced upon tearing. And like SpongeBob and SpongeGar, he is square and yellow, but looks quite different from both sponges.
Willingly getting stung by a jellyfish (witnessed by Squidward)
Jellyfishing (caused by Squidward)
Making Loincloths (possibly, A new one appeared as He gave Squidward the loincloth)
Destroying sources of annoying sound (Case in point. Squidward's clarinet)