Baculum – Penis Bone
In The Selfish Gene, Richard Dawkins[43] proposed honest advertising as the evolutionary explanation for the loss of the baculum. The hypothesis states that if erection failure is a sensitive early warning of ill health (physical or mental), females could have gauged the health of a potential mate based on his ability to achieve erection without the support of a baculum.
https://en.wikipedia.org/wiki/Baculum
The baculum (pl.: bacula), also known as the penis bone, penile bone, os penis, os genitale,[1] or os priapi,[2] is a bone in the penis of many placental mammals. It is not present in humans, but is present in the penises of some primates, such as the gorilla and the chimpanzee.[3][4] The baculum arises from primordial cells in soft tissues of the penis, and its formation is largely influenced by androgens.[5] The bone lies above the urethra,[6] and it aids sexual reproduction by maintaining stiffness during sexual penetration. The homologue to the baculum in female mammals is the baubellum (os clitoridis), a bone in the clitoris.[7][8][9]
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Evidence suggests that the baculum was independently evolved 9 times and lost in 10 separate lineages
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The great apes, despite their size, tend to have very small penis bones, and humans are the only ones to have lost them altogether.[16]
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In The Selfish Gene, Richard Dawkins[43] proposed honest advertising as the evolutionary explanation for the loss of the baculum. The hypothesis states that if erection failure is a sensitive early warning of ill health (physical or mental), females could have gauged the health of a potential mate based on his ability to achieve erection without the support of a baculum.
The tactile stimulation hypothesis proposes that the loss of the baculum in humans is linked to the female choice for tactile stimulation: a boneless penis would be more flexible, facilitating a larger range of copulatory positions and whole body movement, giving females greater general physical stimulation.[44]
The mating system shift hypothesis proposes that the shift towards monogamy as the dominant reproductive strategy may have reduced the intensity of copulatory and post-copulatory sexual selection, and made the baculum obsolete.[45][46]
Humans “evolved a mating system in which the male tended to accompany a particular female all the time to try to ensure paternity of her children”[16][better source needed] which allows for frequent matings of short duration. Observation suggests that primates with a baculum only infrequently encounter females, but engage in longer periods of copulation that the baculum makes possible, thereby maximizing their chances of fathering the female’s offspring. Human females exhibit concealed ovulation, also known as hidden estrus, meaning it is almost impossible to tell when the female is fertile (unless the cervical mucus is examined),[47] so frequent matings would be necessary to ensure paternity.[16][48][49]
Strengths and weaknesses of these hypotheses were revised in a 2021 study, which also proposed an alternative hypothesis: that conspecific aggression, in combination with the development of self-awareness, may have played a role in the loss. If the presence of a baculum exacerbated the prevalence and severity of penile injuries resulting from blunt trauma to a flaccid penis, increasing ability to foresee the consequences of their actions would also enable hominins to realise that these injuries are a useful tool in male-male competition. This behavioural innovation, planned conspecific aggression with the goal of temporary exclusion of competitors from the breeding pool, would create an environment in which a genetic mutation for a penis without a baculum (or with an unossified baculum) would strongly increase the fitness of the mutant phenotype. Along with the hominin propensity for social learning and cultural transmission, this hypothetical scenario may explain why this phenotype became fixed in all human populations.[50]
An alternative view is that its loss in humans is an example of neoteny during human evolution; late-stage fetal chimpanzees lack a baculum.[51]
Cultural significance
It has been argued that the “rib” (Hebrew צֵלׇע ṣēlāʿ, also translated “flank” or “side”) in the story of Adam and Eve is actually a mistranslation of a Biblical Hebrew euphemism for baculum, and that its removal from Adam in the Book of Genesis is a creation story to explain this absence (as well as the presence of the perineal raphe – as a resultant “scar”) in humans.[52]