The Jesus wrote...
I'm still trying to figure out how the skin and muscles were able to grow around the seat.
I think it was caused by the skins adaptation to the surface after a long period of sitting. I once experience something similar, It was near school exams, my fingers kept gripping my pen while studying, everyday studying with my pen. After a while, the skin on my finger reshapes itself like the pen's surface, it stayed that way for a few days, and had "relapses" every time I grip a pen for a long period of time.
You could probably call it a "mini evolution".