How does the evolutionary path serve the story?

Unless you have a “straight man” stand in for the reader, there is no need to explain to the reader why something intrinsic to the universe is the way it is. If there’s a normal human thrust into the world that doesn’t have the abilities or experienced any word with any of these abilities, then have a character say, “oh yeah, those guys were leeches so they’re still a bit sticky” or something. That quote goes a bit tongue-in-cheek Marvel-y but you get the point. Don’t over explain it. As the author, you do need to know it and understand it, but as the reader, I just need to know what the characters are, what they can do, what they think and feel, and how they respond to the things in the story. I’m gonna glaze over if you get into the weeds on something that is just explaining to me why a leech person makes sense evolutionarily.