In a section of ‘True Nature – A Theory Of Human Sexual Evolution’ ( http://humansexualevolution.com/instinct-and-intelligence.htm) it was stated that humans lost their sexual instinct in the course of evolution due to increases in intelligence, and thus gained complete control over reproduction. After the human line had finally figured out why reproducing was beneficial to individual survival, and had also deduced what made reproduction possible, the stage was set for a quick loss of sexual instinct to occur. At this juncture it is proposed that the presence of a nasal plug during a critical stage of fetal development eliminated the sexual instinct, and gave humans an unprecedented reproductive freedom.
A nasal plug appears in human fetal development for roughly 8 weeks, beginning at around the 7th week after conception and lasting until about the 15th week. Structurally, it is a mass of epithelial cells that completely seals the nostril openings. A clue to its function is its location, right within the area of olfaction and pheromone detection. In mammals, a sexual instinct is most often dependent upon detecting pheromones via a specialized organ – the vomeronasal organ (VNO) – and passing signals onto the brain areas responsible for initiating the appropriate instinctive behavioural responses. Significantly, the presence of the short-lived nasal plug is concurrent with the onset of important changes in sexual differentiation of the fetus, which also begins at week 7. The ovaries appear in the embryo by week 7, while is males, the testes begin to differentiate. Until the 9th week both males and females have a creased bump that is the phallus. In the 9th week of development in boys, the testis releases hormones that induces the crease to fuse and disappear, leaving the phallus. In girls, nothing of great significance genitally happens in week 9, but over the next few weeks the crease stays and the phallus retracts to become a clitoris. It is my hypothesis that the differentiating sex organs were the origin of the chemical messengers in the amniotic fluid the nasal plug once served to block. More specifically, a sexual instinct requires that the correct gender-specific form of the sexual instinct be set up at a receptor/neural level, and this would presumably require instructions from chemical signals denoting gender (arising from sexual differentiation of the foetus) to be delivered to the developing pheromonal receptors, and perhaps the developing fetus’s brain.
I believe that this developmental change appeared within the last 200,000 years. Of all hominin species that ever existed, only homo sapiens gained knowledge of the essential role of heterosexual intercourse in reproduction. Hence, all other hominid species necessarily continued to possess a sexual instinct (viz. a functional VNO), irrespective of their brain size. It is most probable that the nasal plug is an evolutionary artefact presently in humans, no longer serving the function it once did. Its absence today would be inconsequential because for thousands of years the sexual instinct has not operated in humans, leaving its associated receptor cells and neural programs outside the controls of natural selection, thus allowing random mutations to accumulate over time and render the sexual instinct permanently destroyed and irrecoverable. Prolonged disuse of receptors and neural programs would have resulted in atrophying of pheromonal receptor areas in the nose. Also, the brain would likely have recruited neural cells from a non-functional sexual instinct network for use in other brain functions. It is therefore very improbable that the nasal plug currently has the functional role outlined above . However, I believe that timing of the nasal plug – during a crucial stage of sexual differentiation of the fetus, as well as the formation of the VNO – points to it being a veritable ‘smoking gun’ of human sexual evolution, highly suggestive on its own that humans lack a sexual instinct.