How the human penis lost its spines

I wonder if this is the "Just So" story which that Kipling, that ol' Victorian prude, wouldn't tell.

To sum up: Humans lack a switch in the genome that would "turn on" penile spines and sensory whiskers. But our primate relatives, such as chimpanzees, have the switch, and that's why they differ from us in these two ways.
And humans are somewhat exceptional in this regard -- a lot of male primates have bumpy penises; mice, which are rodents, have them, too.