First: When shown images like the one above and asked to choose which men they found more attractive, women cared significantly more about body shape than penis size. (Also, it's worth noting that the image above is meant to show you an average, actual human guy in the middle and the extremes of body shape and penis size that were shown to the women on either side of him. In reality, the women were shown a full spectrum of images mixing different body types and penis sizes).
Second: The hypothesis (that human women sexually selected human males to have the larger-than-other-primates junk they are blessed with today) comes with some big questions, including the obvious — flaccid penis size does not correlate well to erect penis size.
Third: As Faye Flam points out at The Knight Science Journalism Tracker, the journal Proceedings of the National Academy of Sciences might need a new acronym (or, at least, different subject matter).
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.