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How Did Man Lose His Penis Bone?
By Suzanne M Harvey, on 26 November 2012
The walrus penis bone, also known as an os penis or baculum, is one of the most popular objects at the Grant Museum. The human penis is haemodynamic, meaning an erection is achieved by blood pressure alone. In animals with an os penis, blood pressure still plays an important role, but the pressure functions to push a bone structure into the penis in order to achieve an erection. This has many benefits over an erection sustained by blood pressure alone, not least in keeping the glans open for sperm to pass through.
While the importance of shaft size and sperm competition has been discussed in my previous blog post, even the largest penis will offer no evolutionary advantage if sperm cannot escape: these much desired qualities will never be passed to offspring. This is not the only benefit. The os penis increases the potential duration of intercourse and also the frequency with which intercourse can take place. For example, a lioness can copulate 100 times per day, sometimes with only four minute intervals, but has only a 38% conception rate1 – males need to keep up if they’re to achieve the best chance of paternity. It comes as a surprise to many people that the os penis exists at all, but in fact humans, woolley monkeys and spider monkeys are the only primates to lack this handy piece of anatomy.
With such benefits to structure, strength, endurance and recovery time, the next question must be: where is man’s penis bone? This is where the story becomes more intriguing…
Was it sacrificed for the sake of woman?
While the lack of a human penis bone may have interested evolutionary scientists for many years, it has recently attracted attention from professors of Theology as well. The debate was reignited in 2001 with the publication of a letter entitled Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. Without wishing to delve into a religion vs. science debate, perhaps the most surprising aspect of this article is that it was published in the American Journal of Medical Genetics, with its central theory that God took Adam’s os penis to create Eve in the Garden of Eden, rather than his rib:
Ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates and most other mammals, did not have one.2
The authors then go on to cite possible mistranslations of the Hebrew word ‘tzela’, which can be translated as ‘rib’ or more generally as a supporting structure such as a baculum. More recently however, a study using the dissection and microscopic analysis of human, dog and rat penises revealed something rather unexpected…
Perhaps it was there all along!
With the comprehensive list of benefits discussed above, the root of this problem seems to be in discovering how on earth humans manage to function without an os penis. As a rule, larger penises in humans can give an evolutionary advantage, but without the toughness of structure provided by a bone, how do they avoid buckling under the pressure? Well, through both microscopic analyses and collagen tissue staining, the elusive evidence has finally been discovered. In the core of the human penis, there is a tunica (or lining) with many elastic fibres acting to keep the penis rigid in much the same way as an os penis. This has been referred to as the distal ligament, and is a structure so robust that even after some venous removal, erection can still be achieved without the need for a fully formed bone structure.4 In somewhat less good news for men, the authors of this study further hypothesise that if damaged, the distal ligament may take as long as a broken bone to heal.
So we are left with the final puzzle of why man lost his penis bone but retained a structure that serves a similar purpose. For the answer to this, we may need to look to women. Sexual selection works on the premise of ‘honest signals’. Put simply, females need to select high quality males to ensure the best genes for their offspring, and a penis that works predominantly through haemodynamics takes a lot of energy to produce. Therefore this is an honest signal of the most healthy males in the same way that the healthiest stags grow the largest antlers. In the end, for all the dissection, microscopic analyses and experiments with cadavers, Adam may have given up his penis bone for Eve after all.
Suzanne Harvey is a PhD student in Biological Anthropology, working on social interactions and communication in wild olive baboons. She is also a teaching assistant on the UCL Arts and Sciences BASc, a new interdisciplinary degree, and can be found on twitter @suzemonkey.
References:
1. Rudnai, J. A. (1973). Reproductive biology of lions (Panthera leo massaica Neumann) in Nairobi National Park. East African Wildlife Journal II: 241 – 53.
2. Gilbert, S. F. & Zevit, Z. (2001). Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. American Journal of Medical Genetics 101(3): 284-5.
3. Hsu, G. L., Lin, C. W., Hsieh, C. H., Hsieh, J. T., Chen, S. C., Kuo, T. F., Ling, P. Y., Huang, H. M., Wang, C. J., & Tseng, G. F. (2005). Distal ligament in human glans: a comparative study of penile architecture.Journal of Andrology 26(5): 624-8.
4. Hsieh, C. H., Liu, S. P., Hsu, G. L., Chen, H. S., Molodysky, E., Chen, Y. H., Yu, H. J. (2012). Advances in understanding of mammalian penile evolution, human penile anatomy and human erection physiology: clinical implications for physicians and surgeons. Medical Science Monitor 18(7): 118-25.
22 Responses to “How Did Man Lose His Penis Bone?”
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Suzanne Harvey wrote on 17 January 2013:
Fascinating! Particularly the insight into the meaning of ‘tsela’. I spoke about this theory at Science Showoff this week, raising the point that spider monkeys and wooly monkeys are the only other primates that don’t have a baculum. I wonder how the Genesis explanation can accommodate the fact humans are not unique? Thanks for the response!
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dannybirchall wrote on 22 December 2012:
RT @ResearchEngager: New Blog Post from @SuzHarvey_UCL ! : How Did Man Lose His Penis Bone?: http://t.co/K9HvPv5O
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10 Human Attributes Explained By Evolution | Brewokr wrote on 13 April 2013:
[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]
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Cal King wrote on 14 March 2014:
We lost our penile bone not because women pick their men based on penis size, but because of male-male competition in the form of a war of the penis. The war had begun in our ape ancestor. The chimp lives in groups with multiple males, who cooperate for group defense. Unlike gorillas, male chimps cannot exclude other males from mating with the females. Simultaneously, female chimps want to have sex with multiple males to reduce the risk of infanticide. If each male thinks that he is the father, he will be less likely to try to kill the baby. Males in turn need each other’s good will to hunt monkeys cooperatively and to defend their territory as a group. Therefore they cannot beat each other up over the ladies.
That means males cannot compete through exclusion, but they can compete in other ways. The way to compete is to have a longer penis so sperm can be placed closer to the uterus. Accordingly, male chimps have much longer penises than male gorillas, and they also have large testicles to store more sperms. Think about it, why would female chimps prefer long penises, but female gorillas don’t? But since chimps retain the penile bone, there is a limit to penis size, so they compensate by having testicles that are larger even then those of humans. Humans, by losing the penile bone, have evolved the largest penis (both in absolute size and in proportion to the body) in order to compete, so we don’t need to store as much sperms as chimps. One may ask why do humans have to compete like male chimps, since modern humans are largely monogamous? The answer is that we first evolved in the open African savanna, in which our ancestors must band together to defend themselves against predators. Therefore our ancestors too cannot afford to beat each other up over the ladies. Instead they engage in a cold war of sorts by evolving ways to make sure their sperms have the best chance of being the one that impregnates the females, who are promiscuous. Ironically, since humans have invented agriculture, we no longer need to travel around in large groups. Men can now keep their women from having sex with other men by inventing and enforcing new laws that prohibit adultery, because men can now settle down with a small plot of land and be safe from predators, while growing enough food to feed a family. Penis size is no longer subject to as much natural selection as it used to be, because, men with a small penis can still leave his share of descendants if he has the means to raise a family.
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Question of the Week: Do boys and girls enjoy different museum exhibits/items? | UCL Researchers in Museums wrote on 26 March 2014:
[…] Teenage boys, including a Scouts troop I engaged with, are particularly fascinated by the baculum — but then, so were a duo of thirty-year-old women. Above all, kids of all genders are […]
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Ken wrote on 8 February 2015:
If the biblical idea for losing the baculum was true, amputees could only have babies born missing the same limb.
Before thinking about how a god might have done this… First you must provide evidence that the god existed, to be able to do the task.
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10 Human Attributes Explained By Evolution – wrote on 20 June 2015:
[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]
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Simon English wrote on 29 July 2015:
For a scientist what an odd piece. There is no proof, although there is interesting conjecture.
The penis is used much more frequently for another function and solely for this up to puberty. I would suggest that the loss of the baculum was more to do with walking on the hind legs and hygiene than reproduction. Try peeing with an erection. a chap can not drain properly. the only solution is to get down on all fours in the bath and pee like a dog. -
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10 Human Attributes Explained By Evolution | ISmartNetwork wrote on 12 October 2015:
[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]
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Percy Ferry wrote on 9 November 2015:
Why cite the theological “reasoning” at all. It explains nothing and gives faint credence to outlandish – and wrong – musings.
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sariah anam wrote on 28 December 2015:
i don’t think so am also a student of m.sc zoology…that adam lose his penis due to the creation of eve beacuse if it is then for female xx is necessary not xy chromosoms…
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Bible Shock: Eve Created From Penis Of Adam, Not Rib, Claims Biblical Scholar In New Study wrote on 29 December 2015:
[…] “baculum” is a bone found in the penis of most species of mammals. Humans are one of the few species of mammal to be missing a baculum, or penis bone — elephants being […]
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Bible Shock: Eve Created From Penis Of Adam, Not Rib, Claims Biblical Scholar In New Study wrote on 29 December 2015:
[…] “baculum” is a bone found in the penis of most species of mammals. Humans are one of the few species of mammal to be missing a baculum, or penis bone — elephants being […]
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How Adam Lost His Baculum | Contra Spem Spero… Et Rideo wrote on 30 December 2015:
[…] Suzanne Harvey in How Did Man Lose His Penis Bone? agrees: […]
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Bible Expert Says Eve Came From Adam's Penis – Bitch Poll wrote on 5 January 2016:
[…] is true that human males are one of the only species of mammal that do not have a "baculum," (or dick bone in layman's terms). In fact, even Chimpanzees, our […]
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How Did T. Rex Have Sex? – Doll Alley wrote on 19 February 2016:
[…] many mammals, dinosaurs did not have bones in their penises. And because soft tissue doesn’t fossilize as […]
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10 Human Attributes Explained By Evolution – PandaNova wrote on 12 March 2016:
[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]
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Penis Size Genetics | wrote on 12 May 2016:
[…] How Did Man Lose His Penis Bone? | UCL Researchers in Museums – We lost our penile bone not because women pick their men based on penis size, but because of male-male competition in the form of a war of the penis. […]
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Quora wrote on 23 October 2016:
Why don’t humans have a baculum?
I read that it was the result of sexual selection. I also read that Eve was created from the baculum of Adam and not from a rib. I suspect walking upright might also have something to do with it. https://blogs.ucl.ac.uk/researchers-in-museums/2012/11/2…
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Marguerite wrote on 28 January 2017:
When they hypothesized that the os penis was taken from Adam to create Eve, the American Journal of Medical Genetics lost their scientific credibility, in my honest opinion.
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10 Human Attributes Explained By Evolution – WOWrly wrote on 22 May 2017:
[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]
@ResearchEngager @SuzHarvey_UCL A response! http://t.co/bC5TFoZ8