A tadpole hanging in pond water breathes by gulping. Water in through the mouth, out across the gills, and a flap of tissue in the throat snaps shut so the developing lungs stay dry. Take the water away and parts of that sequence look uncannily like something with a different name, one that has been wrecking dinner parties for as long as there have been dinner parties.

Sneezing clears the nose. Coughing clears the airway. Hiccups clear a room, and that is roughly the extent of the agreed benefit.

What the body is actually doing

Mechanically it is quick and graceless. The diaphragm and the muscles between the ribs contract hard and without permission, hauling air in. About 35 milliseconds later the vocal cords slam shut, a detail emergency physician Daniel Howes underlines in his BioEssays paper on the reflex. Airflow stops mid-rush, and the sound is the result.

No agreed useful function follows. No gas is exchanged, no blockage shifted, no threat removed. Writing in the journal Gut, gastroenterologists Peter Kahrilas and Guoxiang Shi noted the oddity of something almost every human experiences repeatedly and almost nobody understands.

At least the trigger has been narrowed. In a small experiment published in the same journal, Ronnie Fass and colleagues inflated a balloon inside the oesophagus of ten volunteers and set off hiccups in four, but only when the stretch was sudden and high up the tube. Stretch receptors in the gullet appear to be one way of pressing the button.

The frog argument

“Archaic motor patterns” is how a 2003 paper in BioEssays described the whole business. The team, led by Christian Straus, laid out a phylogenetic case, meaning an explanation built on inherited ancestry rather than present-day usefulness. Four things line up with gill ventilation in animals that breathe both water and air: the throat shuts while air is being pulled in, hiccups appear in the womb before lung breathing does, raising carbon dioxide in inhaled air suppresses them, and the muscle relaxant baclofen switches them off. Frogs run on similar machinery.

That carbon-dioxide finding may help explain why some changes in breathing can interrupt hiccups.

Straus and his co-authors suggested the circuit survives in mammals as useful scaffolding, old code left in the build because newer functions such as suckling and ordinary breathing were written on top of it.

The milk argument

Howes was unconvinced by the idea of a reflex that does nothing. He proposed instead that hiccups evolved to vent swallowed air from the stomachs of young suckling mammals, leaving room for more milk. The chest expands against a shut airway, pressure inside drops sharply, the valve at the base of the oesophagus relaxes, and the trapped bubble is drawn upward and out. A forced burp, essentially, with a soundtrack.

He also swung at the alternatives. Meconium clearance, a long-standing suggestion, would push the material further down the airway. Respiratory muscle training fails too, since those muscles are built for endurance and a brief twitch would do little for them.

Both cases are arguments from anatomy and timing. Neither has been tested to destruction, and one paper remains one paper.

What happens in a newborn’s brain

So why does a foetus, with lungs it has never used and a stomach with no air in it, hiccup at all?

Hiccupping begins at nine weeks gestation. Premature babies spend around one per cent of their time at it, roughly a quarter of an hour a day, which is a serious investment for a movement with no known purpose. A UCL-led team wired up 13 newborns in a neonatal ward and found that each hiccup triggered a large wave of brain activity.

Senior author Lorenzo Fabrizi suggested the signal may be teaching the infant brain to track its own breathing muscles, so that the diaphragm can eventually be moved deliberately. Lead author Kimberley Whitehead went further, wondering whether the adult version is a remnant of a stage of life when the reflex had work to do.

Thirteen babies is a thin sample, and the researchers framed the finding as a possibility.

Sixty-eight years of it

In 1922 an Iowa farmer bent to weigh a hog, fell, and began hiccupping. He did not stop until a morning in February 1990, which Guinness World Records still lists as the longest attack ever verified.

As documented by Smithsonian magazine, he hiccupped 20 to 40 times a minute for 68 years, an estimated 430 million in total. His doctor believed the fall had destroyed a pinprick of tissue in the brain stem, the spot that normally keeps the reflex suppressed. Nothing cured him. He married twice, raised eight children, taught himself to breathe in the gaps so the noise stopped, and got on with it.

If that diagnosis was right, a speck of damage was enough to let the pattern run unchecked for most of a century, which means it was never deleted from the design, only muted. Whatever hiccups were originally for, gill ventilation or a bubble of air under an infant’s ribs, evolution kept the wiring live and fitted a brake instead.

Somewhere under the floor of the skull, a very old pump is still primed, waiting for a cold drink taken too fast.