Neil Bowdler at BBC tells us of penile spines that Homo sapiens lost in the shuffle of evolution:
The researchers then focused on two deletions, linking one to penile spines and another to the growth of specific areas of the brain.
They then tested the effects of the deleted sequences in human skin and neural tissue, and found further evidence to support their claims.
So now they’re trying to figure out a theory for why the human penis no longer has spines.
Penile spines are barb-like structures found in many mammals. Their role remains under debate, and they may play different roles in different species.
They may increase stimulation for the male during mating. They might also play a part in inducing female ovulation in a small number of species, but there is evidence that they can cause damage to the female too.
Then there is the suggestion that they might have evolved to remove “mating plugs” – material that some male species deposit in the female genital tract to block other males’ attempts to fertilise the same female.
I am not a biologist, but I would like to suggest that “they can cause damage to the female too” may have been motivator enough for our species to shed this feature. Having enormous brains also means the babies present with especially large heads, which (along with our relatively small hips due to upright posture) means that childbirth for humans is difficult, painful and dangerous in ways that it isn’t for most other mammals. Ergo, could it be possible that giving birth to increasingly large-headed infants made human females predisposed to copulate with males with more pleasure-inducing and less injury-inflicting genitalia? Since the commitment of pregnancy makes females the “bottleneck” of reproduction, we don’t need all men—just a small fraction of them, really—to be available for copulation. I can just picture primitive hominid females looking at their options and saying, “If it’s gonna hurt that much coming out, then dammit, I want it to feel good going in.”