I need most of my body parts. I figure I have my various meaty chunks for good evolutionary reasons, and far be it from me to sell any, no matter how often that creepy guy shows up at my door with a cooler of dry ice offering me money. But if I ever were going to unload one of my body parts, I’d pick the most useless one of all, one that is even more useless than the appendix (although even it has recently been suggested to not be so useless, see Your appendix and your eyes).

I would sell my belly button. To illustrate just how useless it is, can you recall when you last used it?   I bet you can’t. It is well known that the more common “innie” variety accumulates lint from your clothes (mostly your underwear) over many months, which could be useful if you’re interested in knowing what the average underwear color of your date is – although if you’re close enough to get a good lint sample, my bet is that there is little mystery about the color of the underwear. 

Is underwear color documentation the only use of the belly button? Could it have some better function? Has anyone done the perfect experiment where one removes the belly button? Maybe we’re dead in 15 minutes without a belly button! That does seem unlikely, to put it mildly. 

At any rate, anyone who knows anything will assure you that there is, of course, absolutely no function for the belly button. It is just the remnant scar from the umbilical cord, the cord which was useful while in the womb. 

But I’m not so sure.   Mind you, unlike much of my research where I put forth and test an actual hypothesis for what some biological feature is good for (e.g., color vision, forward-facing eyes, the shapes of writing, illusions, number of limbs, brain anatomy), for the belly button I don’t have any hypothesis at all. For the navel I’m drawing mostly a blank. Nevertheless, there are several reasons which suggest that the belly button is not simply a developmental leftover.

First, do you have any idea how many strange things happen to our bodies during development?! Our wee bodies undergo massive transformations, with certain anatomical features appearing at one stage (e.g., tail-like appendages), and completely going away later. All this is crucial for building fancy machines like us. Just as buildings have scaffolding during construction, but don’t end up with scaffolding scars left on them, biology can build structures during development that are needed during development, and that completely go away later. All these developmental back-flips occur, and then get “covered over.” Why not the belly button? 

Second, in most mammals the belly button does, in fact, get covered over. Have you ever rubbed the tummy of your dog or cat and felt its belly button? No, you haven’t, because you can’t feel it. And unless you’re a veterinarian or are in desperate need of a hobby, you have never shaved its tummy and visually found it either. Mammal belly buttons tend to be like scars – smoothed over and lintless. Our belly buttons, on the other hand, tend to be much more morphologically well-defined. Easy to see. Easy to feel. Why is ours so conspicuous?

Third, it is generally a bad idea to have skin with an inward divot or outward protuberance. Divots are potential places for infection, and protuberances are at extra risk of abrasion. That’s why the skin over your entire body tends to be divotless and protuberanceless. “Smooth” is good for skin and the bodies they cover, so the fact that belly buttons stick their noses up at this design criterion may suggest they have a reason for existing. That is, it seems to indicate that there is a good reason for belly buttons…a reason good enough to outweigh the disadvantage of having a divot or protuberance.

So, there is a case to be made that morphologically visible belly buttons may be selected for in humans. They could be covered over but are not in us, they in fact are covered over in other animals, and there is an apparent disadvantage to having them. 

But what on Earth could belly buttons be for? I don’t know, although my suspicion is that it is linked in some way to sexual selection, and to the fact that we’re the naked upright ape. Other animals’ belly buttons would not be visible to anyone because of fur or because of its being located on the underside, or both. But in our case we’re naked and standing upright, so our navel is something others can potentially see.

This has no value  to you, right?

I have argued elsewhere that perhaps we’re the naked ape because we are color signaling over our entire bodies, not just the face; could it be that the navel is somehow part of the socio-sexual signaling we engage in? 

It has not passed my notice, in this light, that the belly button is definitely sexy. Not everything that’s visible on our bodies is necessarily considered sexy; e.g., eyebrows, nostrils and elbows get few fetish videos made about them…I am told. But we can be shy about our belly buttons, and perhaps that is a hint as to what they’re doing there.