It’s safe to say you don’t remember your first hiccup, since it probably occurred before you were born. It is typical for developing human fetuses to have hiccups in the womb, and yet even though we experience them throughout our lifetimes, the cause of these involuntary actions has defied explanation.
To unravel the mystery of why we hiccup — which serve no obvious useful purpose — scientists are looking into our evolutionary past for clues among our distant relatives. One promising candidate: amphibians, in particular tadpoles.
The mechanics of what happens during a hiccup have fueled this theory. A hiccup, known in medical circles as a singultus, includes a sharp contraction of the muscles used for inhalation — the diaphragm, muscles in the chest wall and neck among others. This is counteracted, at the same time, by the inhibition of muscles used during exhalation.
Here, the back of the tongue and roof of the mouth move upward, followed by the clamping shut of the vocal chords, aka the glottis. This last bit, the closing of the glottis, is the source of the eponymous “hic” sound. And, as you no doubt know from first-hand experience, this process doesn’t just happen once but repeats in a rhythmic fashion.
Tadpoles seem to exhibit a similar physiological behavior.
“Halfway through its development a tadpole has both lungs that breathe air and gills for breathing water,” William A. Whitelaw, a professor at the University of Calgary, wrote in Scientific American. “To breathe water, it fills its mouth with water and then closes the glottis and forces the water out through the gills.” This hiccup-like action is seen in many primitive air-breathers, such as gar, lungfish and other amphibians that have gills.
Another clue linking hiccups in humans to these creatures is the electrical origin of the hiccup trigger in our brain, according to Neil Shubin, a professor of organismal biology and anatomy at the University of Chicago. As related by the Guardian: “Spasms in our diaphragms, hiccups are triggered by electric signals generated in the brain stem. Amphibian brain stems emit similar signals, which control the regular motion of their gills. Our brain stems, inherited from amphibian ancestors, still spurt out odd signals producing hiccups that are, according to Shubin, essentially the same phenomenon as gill breathing.”
If hiccups are a remnant of the genetic code passed down by our amphibian ancestors, can it be true that they perform no beneficial function in humans, despite persisting for the last 370 million years since our ancestors first stepped onto dry land?
Christian Straus, a scientist at Pitie-Saltpetriere Hospital in Paris, has put forth a theory that hiccupping might be a mechanism that helps mammals learn to suck, which involves a series of similar movements. While plausible, this theory will be difficult to prove, Allen Pack, an expert in neurobiology at the University of Pennsylvania, told the BBC.
Until Straus and his colleagues can demonstrate a correlation between the areas of the brain that control suckling and those that trigger hiccups, the purpose of the mysterious singultus will remain just that — a mystery.