I gave my penis a long, hard look today. ([Beavis voice] Heh heh, heh heh, heh heh[/Beavis voice])
I started wondering … From an evolutionary POV, that thing is literally my family jewels, the reason for my existence. I merely exist to carry the damn thing around in order to stick it into as many females as possible, and so pass on my genes.
Well, if it’s so freaking important, what evolutionary sense does it make to have it sticking out in front, pretty much unprotected, a prisoner of fate and fortune?
On a related note, why the penis at all? It seems to be the reproductive organ of choice for all “higher” vertebrates (reptiles, birds, mammals), but surely it can’t be the only, or even the safest and most effecient, method for a male to convey genetic material to a female.
Any thoughts on the evolution of the penis?
Sua
Now that I think about it, it does seem awfully exposed just dangling there, doesn’t it? One that telescoped all the way into the body seems safer. From an evolutionary stand point, I guess it must be pretty safe, or it wouldn’t be there!
A couple of points.
Evolution doesn’t start out with the concept of’ “What would be best”. Things evolve. Sometimes the first ‘good enough’ mutation corners the market, so to speak. I’m sure you can look at most living organisms and imagine a better solution. But that’s not the way it works. Are 5 digits the BEST number to have? No,. But it’s good enough that a different number won’t weed it out for a while.
Also, even advanced organisms, like you, do not technically require a male/ female interaction. Biologically you COULD produce both the sperm and egg.But (short version) this would keep diversity out of the species leading to, among other problems, a single virus wiping out the species.
Plus you always know where it’;s at 
.
Also note that a penis is not identical across species. Some animals do have fully retracting genitals. Like dolphins.
And the anatomical structure of how the work varies. Some animals actually do have a bone in there.
Besides, would you rather reproduce like earthworms - secrete at one end and absorb at the other, with no protrusions or cavities involved?
If we still walked around on all fours like our early primate/mammaliam ancestors, the penis would be underneath the body, relatively protected from harm and attack (along, it might be noted, with the rest of the abdomen and the vital organs). It would seem that the advantages conferred by upright stature overrode any potential disadvantages from having the abdomen and genitals exposed in front. At least for now, anyway.
So we can have something to hold on to. 
Sorry, I had to do it.
Seriously, though, I’ve wondered about this myself. It would seem to me that to keep us safe, the penis would retract completely into the body. But, as warmgun said, I guess this is good enough. Perhaps the invention of the jock strap/cup(or other historical equivalents) has prevented further evolution.
Perhaps, if all human penises retracted into the body, that would protect them all, and help to assure the survival of the species as a whole. But specifically because they are located in a vulnerable location, this facet of evolution favors the survival of humans who are strong enough and/or smart enough to protect themselves from injury, while the weaker and/or dumber humans would die out.
All this talk about protecting the penis from injury, and no one has mentioned the testicles!! How can you forget the testicles?!
*Orig. posted by pldennison
If we still walked around on all fours like our early primate/mammaliam ancestors, the penis would be underneath the body, relatively protected from harm and attack (along, it might be noted, with the rest of the abdomen and the vital organs). *
Those were pretty much my thoughts exactly.
Good thoughts, everyone. In particular, kudos to pldennison, who gave what I think is a dead-on answer. IIRC, there are other problems humans have because we evolved from four-legged critters (back pain and respiratory infections come to mind.*) These problems, along with the whole “winky in jeopardy”, are outweighed, I guess, by the benefits of the upright posture.
That said, the universality of the penis still befuddles me. Warmgun and Irishman make good points, but I’m not convinced. In particular, Warmgun’s point about “good enough” is inadequate for my tastes. Sure, evolution isn’t (to anthropomorphize a sec.) going for perfection, but if something isn’t “perfect”, it usually doesn’t become universal. I mean, look at all the varied, diverse, and occasionally bizarre evolutionary adaptations of reptiles, mammals, and birds. Hell, reptiles have three chambered hearts and are cold-blooded, a massive difference between them and us, but they still got d**cks! Someone’s got some 'splaining to do.
Sua
If the male genitals were designed to be retractible, then it would have been necessary to design a cooling system as well. OK I guess the penis could have been retractible but the testicles need to be outside the abdomen because the temperature inside the abdomen is too hot for the sperm to survive. That’s why paeds check so carefully for descended testes in young boys. If the testes are up in the abdominal cavity they need to be brought down to ensure normal fertility. I vaguely remember something about undescended testes leading to cancer in later life too.
Think about what is to be accomplished in sexual reproduction: the sperm has to get to the egg. Some animals, such as fish, lay the eggs, then the male comes along and fertilizes them. This is one solution to the problem, and works well if the unfertilized eggs are viable for a length of time outside the female’s body (primarily fish and amphibians)
With many vertebrates, however, being bound to dry land as many are, the eggs would dessicate too quickly if they left the female; probably before the male could get around to fertilizing them. So, an alternate solution evolved: the eggs stay in the female, and the male has to ‘go in and get them,’ so to speak. Thus, the penis.
After fertilization, the egg(s) may remain within the female, or they may become encased in a package which better enables them to withstand the rigors of the environment.
Presumably, this solution evolved early in the history of land-dwellers; since it works, there was no reason for it to be replaced by further evolution, and so was passed on to the various groups such as reptiles, birds and mammals.
Well, I can’t speak for all guys, but when I am flacid there IS some pretty good retraction of the penis, down to maybe an inch or two. As far as why we don’t have a COMPLETELY retractable version, I can only guess it is some kind of a trade off. We all agree the testicles have to be external for cooling and proper sperm development/ lifespan, and perhaps it is a evolutionary ‘package deal’. That is, those animals with fully retractable models also have internal testes, do they not? So maybe you get one with the other. How other warm blooded animals avoid boiling their sperm to death internally, however, is a mystery to me.
Alternative WAG #2 might have to do with the walking upright thing. Perhaps the ability to run on two legs (to chase food, or conversely, to avoid being food for something else) would be greatly hampered in terms of where all the necessary muscles need to attach to the pelvic area if there was a whole retractable penis set-up taking up space in that area that might compete for those same necessary attachment spots.
And of course, lets be sure to throw out the idea that perhaps man was meant to swim and needed a ‘rudder’
Birds don’t have penises. Some mammals do retract their penises. Most notably are aquatic mammals. Reptilian penises are quite different from mammalian penises, and some species don’t have them at all.
There are disadvantages to penises, exposure is probably chief among them. However, there are also huge advantages to having a penis. It’s is excellent at getting sperm as close as possible to the egg in a relatively safe environment, the woman’s body. It is likely the advantages out weigh the disadvantages
The previous posters were right on concerning evolution. Traits that are successful get passed on and they stay around until something better comes along. Imperfect, but sufficent, traits get passed along all the time. What I think you are forgetting is that we have a penis because our gentic forebearers did. All animals that have penises probably evolved from a common ancestor that had one. It became universal from a common ancestor not independent evolution.
A final point, do you know anyone who was rendered infertile due to trauma to his penis. Even if you had, you have to admit that this sort of thing is extremely rare. It looks like the penis is doing all right for itself and you are bad-mouthing it.
To quote George Costanza: “Haven’t you people heard about shrinkage?!?”
Penis evolvement?
Well, we’ve hashed out almost everything else on the SDMB, so why not this?
External male genitalia: Not all that practical for a hunting carnivore such as man. It can get caught on things like thorn bushes when fleeing really pissed off Cro-Magnon animals with lots of teeth and claws and one’s puny spear in it’s butt. However, the penis, due to the foreskin, was probably attached to the underside of man’s belly, but, as he chose to (A) make use of his stereoscopic variable vision, (b) grow a brain that required some elevation above the hot ground to keep cool, © develop hands with opposable thumbs, (d) aquire the ability to reason things out and start using strategic planning, he had to stand up.
That would expose the penis to more harm, so the sheath detached. Human sperm, being more complex than animal – which is why humans may only breed with humans, requires temperature regulation. External testes with automatic temperature control developed. (By then, this condition was more or less selected by mammals due to being the most successful in propagating the species.)
The foreskin is no doubt the remains of the belly sheath, not really needed, like the veriform appendix, adenoids and tonsils.
Now. The positioning and erectile functions are important because evolution designed the human female to carry live young inside. Through a whole mass of special systems, women were specifically adapted to protect and nurture the progeny. She produces specialized milk, high in valuable minerals and vitamins for the developing infant upon birth plus the first milk also contains a massive dosage of natural antibodies to help protect the new born.
The woman is actually physically more durable than the male, better able to withstand cold, heat, starvation and abuse. The design of her sexual organs encourages the retention of male sperm and extends it’s life span to an extent. Her egg will accept only the strongest sperm, thus decreasing the chances of defectives. The male member has to extend in order to deposit the sperm far enough within her to reach the protected area.
She carries the fetus low in her body, with a wide pelvic region designed not only to hold the developing child, but to actually stretch apart at the birth. Layers of fat develop to insulate the child and her whole system shifts over to functioning primarily to support the baby. Automatically, she will get cravings for minerals and vitamins she needs and our ancient ancestors probably ate a wide variety of odd leafs, fruits and even clay to get what she needed.
The male is designed to hunt, to provide food and shelter for the baby, to fight to protect from animals and other humans, to be stronger for the same reasons and to be more aggressive. Traditionally, he burns himself out and dies off or manages to get himself killed by doing something terribly heroic while the woman goes on.
After birth, the woman becomes the main protector for the child. It is thought that sexual pleasure was evolved to keep the male interested, rather than wandering off after going through rut. Chances are that millions of years ago, the male, after coming out of rut, took off, or if he lingered, it is possible the he would unhesitatingly killed his own offspring if angered.
Sexual pleasure, it also is assumed, was developed within the woman to keep her interested in the male, not only for protection, but to keep popping out children to continue the species. The emotion of love, it is believed, came about later to increase the family bond, which encouraged the propagation of the species and gave the children a greater chance of living until they reached breeding age.
So, clear as mud?
External incubation of young is iffy for a wide ranging, operational omnivore and not particularly safe from predators – as we all know from nature series. There are various species today designed to prey on the eggs of every egg laying creature about. It is notable that very few of those egg layers have much in the way of defense against the egg predators.
The human species with external genitalia might have been a spin-off from other mammals who started out with similar gear and proved it successful.
Somewhere along the line, genetics made the male attracted to the female form and vice versa, which further enhanced the ability of the species to propagate and make sure that many of the viable young survive to breed. Plus I suppose it kept them from wandering off and trying to mate with triceratops, the saber toothed tiger or whatever else happened to catch their interest.
The male, traditionally being short lived, is the most excitable, which seems to indicate that he would impregnate as many females as possible before he got eaten or clubbed to death by some other male. That ensured variation of the species. Sexual pleasure was not necessary on the females part, but made things a whole lot easier if she desired the male for a roll in the hay.
Women back then were liable to kill a rapist, hack him up and have him roasting on the fire by the time ‘hubby’ shambled in from a hard day hunting and trying to avoid being eaten.
Women traditionally outnumber men two or three to one. Perhaps the ratio was greater a million years ago, so the male would be pushed by genetics to make as many with child as possible. Plus, once the woman was pregnant, she would probably fend off his amorous advances with a stone ax, so he would saunter down the valley, to the next cave on the left where a receptive female lived and do the deed there.
Now you probably know more than you wanted to.
Next lecture: Evolution, genetics and the built in urge and reason humans and some animals ‘cheat’ on their mates.
WAG, from an uninformed loser, but I believe that there are quite a few mammals with fully retractable penises, yet external testes. Horses and elephants are two that spreang immediately to mind.
If I might venture another WAG, I would guess that the majority of mammals that have internal testes are aquatic mammals, therefore need to be more worried about keeping the testes warm instead of cool.
You know…shrinkage.
Dr. Lao noted:
Actually, to be nit-picky technical, some do. Ducks, geese, tinamous and ostriches (as well as alligators) possess a vestigial, yet erectile, penis during development and for a few days after hatching (which allows sex to be determined in newly-hatched chicks). This fits well with the idea that the penis in birds was secondarily lost, which in turn fits well with the idea that it has evolved from a common ancestor.
Well then what do birds have? This’s been bothering me for a long time but I’ve never thought to ask.
–Tim
Perhaps the first homo sapiens had one that stuck out quite noticeably, which cannot help but attract the female hominoids. It is said that our only true advantage in the wild, besides our ability to reason, is our sexuality. Something else to consider: the woman is the only animal that is ‘in heat’ all year round.
If the poor penis is so vulnerable and unprotected,we should all be walking around penisless. I think it’s just fine where it is.My wife Lorena and I
are going to bed now,goodnight.