I believe Starro the Conqueror was based on Pluto in Superman's first adventure with Captain Carrot and the Amazing Zoo Crew. Anyway, it is rays from Pluto that devolve the humans on Earth-1 and lead to Superman and Capt teaming up.
I recall that story. More specifically, Starro was launching his attack from the Pluto of the Earth-C universe ("Pluto-C", I suppose), with the rays affecting Earths-1 and -C...