What Are The Hiccups?

Medically, a hiccup is called a singultus, directly translated from Latin to “sobbing.” Mechanically speaking, it’s an involuntary spasm of the diaphragm that fires in concert with the intercostal muscles of the chest. Almost simultaneously, within about 35 milliseconds, the glottis (the opening between the vocal cords) snaps shut. That abrupt closure is the source of the characteristic sound. The “hic” is not the breath itself, but the sound of a door slamming.

The entire sequence is orchestrated not by any higher cognitive region of the brain but by the brainstem, one of its most ancient structures. This is a reflex, in the truest sense, so it bypasses conscious control entirely. You cannot decide not to hiccup, any more than you can decide not to blink when something flies at your eye.

The neural circuitry involved in a hiccup episode includes the vagus nerve, the phrenic nerve, the medulla oblongata and the hypothalamic reticular formation. This is a constellation of structures that, notably, are all shared with other vertebrates, including some very distant relatives.

Did Fish Give Us The Hiccups?

To understand the hiccup, you first have to understand a peculiar fact about the phrenic nerve. This nerve, which controls the diaphragm, takes a bewilderingly long route to get there: it originates at the base of the skull, snakes through the chest cavity, and only then does it reach the diaphragm it is meant to serve. No engineer who had a say in the design of the human body would allow this. A more sensible arrangement would have the phrenic nerve originate much closer to its destination.

The reason it doesn’t is that we didn’t design ourselves. As Paleontologist Neil Shubin lays out in his 2008 book Your Inner Fish: A journey into the 3.5-billion-year history of the human body, we most likely inherited this layout from our fish ancestors, whose gills sat close to the skull.

The nerve that controlled their gill muscles made reasonable anatomical sense in a fish. But in a land-dwelling mammal with a diaphragm located way below the neck, it makes considerably less sense.

However, evolution will rarely tear something down purely in order to rebuild it entirely; more often, it works with what’s already available. And so we are still living with the plumbing decisions of creatures that have been extinct for hundreds of millions of years.

This architectural eccentricity has real consequences. The phrenic nerve’s long and tortuous path means it is easily irritated by things it passes near — like a distended stomach, an inflamed esophagus or even a tumor in the chest. Irritation anywhere along that nerve can give rise to the hiccups.

What Causes The Hiccups?

If the phrenic nerve’s winding route is our fishy inheritance, the hiccup reflex itself appears to be something we borrowed from amphibians. This is the central argument of a renowned 2003 study published in BioEssays, and it remains the most well-supported hypothesis in the literature.

The authors of the study observed something striking in their research: the pattern of muscle and nerve activity during a human hiccup is remarkably similar to the ventilatory pattern used by tadpoles when breathing through their gills.

When a tadpole pumps water across its gills, it must simultaneously keep that water out of its developing lungs. It does this by sharply contracting the inspiratory muscles — a movement that looks, neurologically, very much like a hiccup — while closing the glottis to seal off the airway. The parallel is precise enough to suggest that it’s not a coincidence.

The researchers also identified a set of pharmacological fingerprints supporting the connection. More specifically, the tadpole’s gill-breathing reflex and the human hiccup are suppressed by elevated concentrations of carbon dioxide — which is why holding your breath, a folk remedy as old as time, occasionally works.

Notably, both hiccups and the gill-breathing reflex are also abolished by baclofen, a drug that acts on GABA-B receptors. These shared inhibitory mechanisms point toward a common origin: the same ancient neural program, running in different bodies across 370 million years of evolution.

What is especially remarkable is that the electric signals that trigger a hiccup originate in the brainstem, which is the same structure that controls gill ventilation in amphibians. This implies that our brainstems were, in an evolutionary sense, once amphibian brainstems. They have been elaborated and modified over hundreds of millions of years, but they have not been replaced. And every so often, they seem to briefly revert to an older subroutine.

The New Explanation For The Hiccups

If you remain skeptical that a reflex so common in healthy adults is purely vestigial, you are in good company. The developmental evidence is worth dwelling on.

Human fetuses begin hiccupping before they develop the neural circuitry for normal breathing. The motor pathways governing hiccups form earlier in fetal development than those governing lung ventilation. Newborns spend roughly 2.5% of their time hiccupping. Premature infants spend even more.

None of these is an incidental finding. It suggests that the hiccup reflex is, in some sense, a developmental precursor — a primitive breathing program that is already running while the more sophisticated one is still under construction.

This observation has given rise to a second hypothesis that competes, in interesting ways, with the amphibian model. Gastroenterologist Daniel Howes proposed in a 2012 study from BioEssays that hiccups in suckling mammals may serve to expel air trapped in the stomach during feeding, almost like a forced burp.

The reflex’s afferent wiring supports this: the stimulus appears to originate in the region of the lower esophagus and stomach, and the lower esophageal sphincter relaxes simultaneously during a hiccup, which would aid in venting gas upward.

The high prevalence of hiccups in newborns, the age group most actively engaged in suckling, lends the theory some credibility. It is possible, of course, that both explanations contain truth: the reflex may be an ancient inheritance that has found, in mammals, a modest secondary utility.

A note of caution here is that, while most people’s hiccups resolve within a few minutes and leave nothing behind but mild annoyance, the reflex also has a darker register.

Episodes persisting beyond 48 hours warrant medical attention, and hiccups lasting more than a month (classified as intractable) are associated with serious underlying pathology: lesions near the diaphragm, infections, metabolic disorders or malignancy in the chest. Men are disproportionately affected by intractable cases.

In rare instances, hiccups have been recorded as the earliest presenting symptom of conditions as serious as myocardial infarction. The reflex that usually exists without fanfare is, in these cases, a signal worth listening to.

Why Do We Still Get The Hiccups?

It is tempting to ask: If hiccups serve no clear purpose in adult humans, why hasn’t evolution gotten rid of them? However, the question misunderstands how natural selection works. Selection acts against traits that reduce survival and reproduction. Neutral traits, which neither help nor hurt, can persist indefinitely, simply because there is no pressure to remove them.

The hiccup reflex is encoded in our neural architecture that also does other genuinely important things. Dismantling it would require precise, costly rewiring of the brainstem, with no obvious payoff. So it remains. It might not be a useful reflex, but it also isn’t a harmful enough reflex to warrant elimination.

This is a pattern visible across biology. The human body is not an optimized machine; it’s painted and repainted over and over, with earlier layers still visible beneath the new ones. The appendix. The goosebump reflex. The tailbone. The wisdom tooth.

Hiccups belong to this family of evolutionary footnotes: relics of a past that still briefly surfaces, every time we eat too fast. The next time your diaphragm fires without permission, consider it a message in a bottle — sent by something that once breathed through gills in the warm Devonian shallows, 375 million years before you were born.

Hiccups, for the most part, remain an evolutionary mystery. If you want to know more cool facts about evolution, you can take the Evolution IQ Test today.