Why Is the Penis Shaped Like That?

Some science writers are just awesome when it comes to explaining a fine science point to a lay audience like me. First, consider this sentence; isn't this a wonderful piece of writing?

It takes a special type of psychological scientist to tell the little old lady sitting next to him on a flight to Denver that he studies how people use their penises when she asks what he does for a living.

That sentence came from this essay on "secrets of the phallus" in the Scientific American.

according to evolutionary psychologist Gordon Gallup of the State University of New York at Albany, the human penis is actually an impressive “tool” in the truest sense of the word, one manufactured by nature over hundreds of thousands of years of human evolution. You may be surprised to discover just how highly specialized a tool it is. Furthermore, you’d be amazed at what its appearance can tell us about the nature of our sexuality.

It is fascinating that some of the minor anatomical detail about something I see and use everyday was a result of the system adapting to various situations, and then evolving to the tool that it now is. Apparently, "in the case of the human penis, it appears there’s a genuine adaptive reason that it looks the way it does."

In addition to the human penis outsizing that of any other primate,

It turns out that one of the most significant features of the human penis isn’t so much the glans per se, but rather the coronal ridge it forms underneath. The diameter of the glans where it meets the shaft is wider than the shaft itself. This results in the coronal ridge that runs around the circumference of the shaft—something Gallup, by using the logic of reverse-engineering, believed might be an important evolutionary clue to the origins of the strange sight of the human penis.

So, what is the reverse-engineered evolutionary psychological explanation for this? Hey, don't be lazy; read the essay yourself :-)