Once Upon A Time, Humans Had Gills
Hiccups are sudden contractions of the muscles we use to breathe in. Just after the muscles start to move, the glottis shuts off the windpipe, producing the characteristic “hic” sound. Surprisingly, ultrasound scans reveal that babies in the womb start hiccuping after two months, before any breathing movements appear.
That suggests that hiccups in adults are just the remnant of some primitive reflex, which occur only when this brain circuit is accidentally triggered. Yet the purpose of hiccups during pregnancy remains unclear. One theory is that the movements prepare babies’ respiratory muscles for breathing after birth, another that they prevent amniotic fluid entering the lungs.
None of these theories explains all the features of hiccups. If their purpose is to prevent liquid getting into the lungs, points out Christian Straus at Pitie-Salpetriere Hospital in Paris, you would expect the closure of the glottis to be associated with the contraction of the muscles used for breathing out, as in a cough, not those for breathing in.
But there is one group of animals in which the peculiar combination of the contraction of these muscles and the closure of the glottis does serve a clear purpose: primitive air breathers that still possess gills, such as lungfish, gar and many amphibians. These animals push water across their gills by squeezing their mouth cavity while closing the glottis to stop water getting into the lungs.
In the latest issue of BioEssays (vol 25, p 182), a team led by Straus proposes that the brain circuitry controlling gill ventilation in these early ancestors has persisted into modern mammals.
There are many similarities between hiccuping and gill ventilation in animals like tadpoles, the researchers argue. Both are inhibited when the lungs are inflated, for example, and by high carbon dioxide levels in air or water. But why do we still hiccup 370 million years after our ancestors began hauling themselves onto land?
If the team is right, hiccupping before birth is just an early stage in the development of suckling, a little like learning to crawl before you can walk. Straus thinks the circuitry that controls the movements of the gills and glottis was conserved during evolution because it formed a building block for more complex motor patterns, such as suckling in mammals. “Hiccups may be the price to pay to keep this useful pattern generator,” he says.
He points out that the sequence of movements during suckling is very similar to hiccuping, with the glottis closing to prevent milk entering the lungs.
It is a plausible idea, says Allan Pack, an expert in respiratory neurobiology at the University of Pennsylvania. “But it’s going to be very tough to prove.”
Straus thinks the real test of theory will be to look at the specific neurons that control hiccups and suckling. If the team is right, he says, most of the nerve cells that are active during suckling should also be active when we hiccup.