Sentence:  A few worlds may have made this giant leap, invented sex (or some equivalent), and produced planets with sponge-like organisms to filter-clean their seas as a precedent to the evolution of more complex life. Here and there, and likely very rarely, a few planets may have undergone their own <WSD>Cambrian explosions</WSD>. 
Meaning of the word marked with <WSD> tags:  The relatively rapid appearance, during the Cambrian Period around 541 million years ago, of most major animal phyla, as demonstrated in the fossil record.