Dawkins' "impressive double life of the penis", or how male homo sapiens lost that bone

Jonathan wonders why humans "got the short end of the evolutionary stick" when contemplating the human lack of a baculum as compared with even our closest evolutionary cousins. Dawkins doesn't seem to know either but shares his, as he puts it, "less plausible than pleasing" speculations:

First, a theory about why humans have lost the penis bone. An erect human penis can be so hard and so stiff that that people jokingly express skepticism that there is no bone inside. As a matter of fact lots of mammals do have a stiffening bone, the baculum or os penis, to help the erection along. What's more, it is common among our relatives the primates; even our closest cousin the chimpanzee has one, although admittedly a very tiny one which may be on its evolutionary way out. There seems to have been a tendency to reduce the os penis in the primates; our species, along with a couple of monkey species, has lost it completely. So, we have got rid of the bone that in our ancestors presumably made it easy to have a nice stiff penis. Instead, we rely entirely on a hydraulic pumping system, which one cannot but feel is a costly but roundabout way of doing things. And, notoriously, erection can fail--unfortunate, to say the least, for the genetic success of a male in the wild. What is the obvious remedy? A bone in the penis, of course. So why don't we evolve one? For once, biologist of the genetic constraints' brigade cannot cop out with 'Oh, the necessary variation just couldn't arise.' Until recently our ancestors had precisely such a bone and we have actually gone out of our way to lose it! Why?

Glider (Spaceship) from Conway's Game of Life.

Erection in humans is accomplished purely by pressure of blood. It is unfortunately not plausible to suggest that erection hardness is the equivalent of a doctor's blood pressure meter used by female's to gauge male health. But we are not tied to the metaphor of the blood pressure meter. If, for whatever reason, erection failure is a sensitive early warning of certain kinds of ill health, physical or mental, a version of the theory can work. All that females need is a dependable tool for diagnosis. Doctors don't use an erection test in routine health check-ups--they prefer to ask you to stick out your tongue. But erection failure is a known early warning of diabetes and certain neurological diseases. Far more commonly it results from psychological factors--depression, anxiety, stress, overwork, loss of confidence and all that. (In nature, one might imagine males low in the 'pecking order' being afflicted in this way. Some monkeys use the erect penis as a threat signal.) It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male's health, and the robustness of his ability to cope with stress, from the tone and bearing of his penis. But a bone would get in the way! Anybody can grow a bone in the penis; you don't have to be particularly healthy or tough. So selection pressures from females forced males to lose the os penis, because then only genuinely healthy or strong males could present a really stiff erection and the females could make an unobstructed diagnosis. There is a possible zone of contention here [some might say, "a bone to pick"]. How, it might be said, were the females who imposed the selection supposed to know whether the stiffness that they felt was bone or hydraulic pressure? After all, we began with the observation that a human erection can feel like bone. But I doubt if the females were really that easily fooled. They too were under selection, in their case not to lose bone but but to gain judgement. And don't forget, the female is exposed to the very same penis when it is not erect, and the contrast is extremely striking. Bones cannot detumesce (though admittedly they can be retracted). Perhaps it is the impressive double life of the penis that guarantees the authenticity of the hydraulic advertisement. -Richard Dawkins, The Selfish Gene, pp 307-308

I do appreciate Dawkin's dry wit. And has it occurred to you that Adam's rib may very well have been ... not a rib after all?