Well, hopefully your bosses appreciate your yearnings for scientific knowledge. Because, did you know that scientists have only recently started asking themselves why the penis evolved the way it did? (Which is to say, the average human penis is approximately twice the size of chimps.) Probably because it’s harder for researches to get a grant that has “Objective: Looking at wangs over the last million years” written at the top. But at least one scientist, Gordon Gallup, is looking at the penis for what it truly is: a “tool” that, amazingly, is excellent in both form and function. Scientific American:
Gallup’s approach to studying the design of the human penis is a perfect example of of “reverse-engineering” as it’s used in the field of evolutionary psychology. This is a logico-deductive investigative technique for uncovering the adaptive purpose or function of existing (or “extant”) physical traits, psychological processes, or cognitive biases. That is to say, if you start with what you see today—in this case, the oddly shaped penis, with its bulbous glans (the “head” in common parlance), its long, rigid shaft, and the coronal ridge that forms a sort of umbrella-lip between these two parts—and work your way backward regarding how it came to look like that, the reverse-engineer is able to posit a set of function-based hypotheses derived from evolutionary theory.
And sorry gays, but when it comes to evolution, scientists will be looking at how the penis works with the vagina, not an anus. Despite our lobbying efforts.
Magnetic imaging studies of heterosexual couples having sex reveal that, during coitus, the typical penis completely expands and occupies the vaginal tract, and with full penetration can even reach the woman’s cervix and lift her uterus. This combined with the fact that human ejaculate is expelled with great force and considerable distance (up to two feet if not contained), suggests that men are designed to release sperm into the uppermost portion of the vagina possible. Thus, in a theoretical paper published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture that, “A longer penis would not only have been an advantage for leaving semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity.”
This “semen displacement theory” is the most intriguing part of Gallup’s story. We may prefer to regard our species as being blissfully monogamous, but the truth is that, historically, at least some degree of fooling around has been our modus operandi for at least as long we’ve been on two legs. Since sperm cells can survive in a woman’s cervical mucus for up to several days, this means that if she has more than one male sexual partner over this period of time, say within 48 hours, then the sperm of these two men are competing for reproductive access to her ovum. According to Gallup and Burch, “examples include, group sex, gang rape, promiscuity, prostitution, and resident male insistence on sex in response to suspected infidelity.” The authors also cite the well-documented cases of human heteroparity, where “fraternal twins” are in fact sired by two different fathers who had sex with the mother within close succession to each other, as evidence of such sexual inclinations.
So how did natural selection equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to Gallup, is their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus “scooping out” the semen deposited by a sexual rival.