The Evolution of Human Sexuality

Happy Valentine’s Day! Let’s talk about sex, baby.

I recently read Jared Diamond’s Why Is Sex Fun? The Evolution of Human Sexuality (in fact, the idea of this post and Monday return date motivated me to finish it) and was really fascinated. In all the weird sex in the animal kingdom, humans are one of the strangest, even by mammalian standards. In our very short evolutionary history, we’ve managed to create a never-before-seen system that is still not fully understood.

I will share some tidbits from Why Is Sex Fun? to prove Diamond’s point that the sex life of a human is unique and wonderfully weird:

Menopause: although non-human animals exhibit decreased fertility towards the end of their life, humans are on of the only animals to experience a regular complete halt in female fertility. The prevailing theory is that women* out of their reproductive prime have more to lose than gain by having their own children but rather are in a place to help raise their grandchildren.

Concealed ovulation: Female women are one of the only species who not only don’t actively advertise their ovulation, but it’s almost impossible to tell exactly when she is most fertile. Some women might even struggle to know when their time of the month is. Concealed ovulation is thought to stem from two very different theories: first, that concealed ovulation promotes monogamy, forcing the man to stay with the woman and copulate constantly to increase the chances of fertilization. The other theory says concealed ovulation evolved to give women the opportunity to copulate with many men, causing confusion of paternity and decreasing the chance of infanticide in traditional societies.

Runaway sexual selection: Whether it’s an aggressive display of dominance or intricate plumage, traits that females favor become increasingly more frequent and exaggerated when they keep becoming selected. This is also seen in humans with the male penis. Our ancestors and close ape relatives have really small penises compared to their bodies. While a gorilla is larger than a man, the gorilla penis is less than two inches while the average male penis is five inches. This is more than big enough to perform its biological function and suggests that the superfluous size is merely ornamental.

Sexual signals: You might be familiar with males of certain species putting on displays for females, seen in examples such as bright colors, song and dance. But do humans send sexual signals? Apparently they do! Some we’re not even aware of. One of the ones I found most interesting was women unknowingly selecting men with complementary immune genes based on smell (pheromones) alone. This is a relic of our evolutionary past when the endgame of sex was largely reproduction and survival of offspring.

I hope you all found this enjoyable! I hope to do more of this so any and all feedback is welcome.

*Although I use gendered language in this article for simplicity sake, I understand there are more than two genders and acknowledge there are transgender, gender fluid, non-binary, intersex people as well as those who have yet to make up their minds.

Photo by Mike Fox.



  1. February 14, 2018 / 10:09 am

    “This is more than big enough to perform its biological function and suggests that the superfluous size is merely ornamental.” ahhhhh I’m dying, the way you worded that is perfect and hilarious!

  2. February 15, 2018 / 8:22 am

    Ah I love this post. We are such animals at heart despite all our intelligence and higher cognitive functions! Some very interesting points here! I’ve also heard that women release certain pheromones when they are most fertile during the month to increase chances of copulation and hence offspring! Fascinating! Sarah Elle x

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