If the odd course of our nerves is a product of our fishy past, the hiccup itself is likely the product of our history as amphibians. Hiccups seem to be controlled by their own pattern generator in the brain stem: stimulate it, and you stimulate hiccups. It turns out that this pattern generator is virtually identical to one found in amphibians—and not in just any amphibians, but specifically in tadpoles. Tadpoles use both lungs and gills to breathe, and this pattern generator is active when they breathe with gills. In that circumstance, a tadpole needs to pump water into its mouth and throat and then out across the gills, but it must also prevent the water from entering its lungs. To keep out the water, it closes the glottis, the flap that can seal off the breathing tube. The central pattern generator in the tadpole brain stem ensures that an inspiration is followed immediately by a closing glottis. They can breathe with their gills thanks to an extended form of hiccup.
There are additional parallels between gill breathing in tadpoles and our hiccups. Gill breathing can be blocked by carbon dioxide, just as hiccups can (one home remedy, breathing into a paper bag, helps concentrate carbon dioxide). Experimentally stretching the wall of a tadpole’s chest is another way to block gill breathing, just as inhaling deeply and holding one’s breath can stop hiccups. Perhaps we could even block gill breathing in tadpoles by having them drink a glass of water upside down.
The hazards of taking a fish body and morphing it into a mammal show up in other specific ways. One is our propensity for hernias, at least for hernias near the groin.
If you were to slit the belly of a shark from mouth to tail, the first thing you’d see is liver, a lot of it. The liver of a shark is gigantic. Some zoologists believe that a large liver contributes to the buoyancy of the shark. Move the liver away, and you’ll find the gonads extending up into the “chest” area, near the heart. This arrangement is typical of most fish: the gonads lie toward the front of the body. In mammals, the location of the gonads is quite different, and therein lies the problem.
Now, it is a very good thing that our gonads are not near our hearts (although it might make reciting the Pledge of Allegiance a different experience). If our gonads were in our chests, we wouldn’t be able to have babies. For males in particular, it would be a disaster. Males continuously produce sperm throughout their lives. Sperm are finicky little cells that need exactly the right range of temperatures to develop correctly for the three months they live. Too hot, too cold, and they die or become malformed. Male mammals have a neat little device for controlling the temperature of the sperm-making apparatus: the scrotum.
As we know, the male gonads sit in a sac. Inside the skin of the sac are muscles that can expand and contract as the temperature changes. Hence the cold shower effect: the scrotum will tuck close to the body when exposed to cold. The whole package rises and falls with temperature. This is all a way to optimize the production of healthy sperm. The dangling scrotum also serves as a sexual signal in many mammals. The problem with this arrangement is that the plumbing that carries sperm to the penis is circuitous. Sperm travel from each testis through a spermatic cord. The cord leaves the scrotum, travels up toward the waist in front of the pelvis, loops rearward over the pubic bone, and doubles back and down through the pelvis to reach the ejaculatory duct, which empties into the urethra. The reason for this absurd roundabout route lies in our developmental and evolutionary history.
Our gonads begin their development in much the same place as a shark’s: up near the liver. As they grow and develop, our gonads descend. In females, the ovaries descend from the midsection to lie near the uterus and fallopian tubes. This ensures that the egg does not have far to travel to be fertilized. In males, the descent goes farther.