The Primitive Sponge was seen in the episode "SB-129" and the end of "Ugh." 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 jellyfish 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 and created by Squidward)
Making Loincloths (possibly; A new one appeared as he gave Squidward the loincloth.)
Destroying sources of annoying/high pitched sound (Case in point. Squidward's clarinet)
He and Primitive Star briefly appear in Ugh when the "When Worlds Collide" song happened.
Primitive Sponge is believed to be an ancestor of SpongeGar and SpongeBob.
Unlike SpongeBob and SpongeGar, Primitive Sponge is not known to have any pets.
The kelp he was eating was possibly an ancestor to SpongeBob's present day Pineapple.
Primitive Sponge seemed to dislike Squidward. This is ironic because the modern day SpongeBob likes Squidward. It can be explained however by the following.
Primitive Sponge could have been angry at Squidward when he played his clarinet, badly.
Primitive Sponge could have just been territorial.
Unlike SpongeBob, Primitive Sponge has no eyelashes and has two saber-tooth tiger teeth instead of two buck-teeth.
Considering the fact that Squidward made the first two Jellyfishing nets, it might be connected to the episode SB-129 when Squidward came back to present time SpongeBob and Patrick said that Squidward invented Jellyfishing.