Why do we hiccup?
A hiccup can be described as a sharp breath followed by an overlap of the upper airway with the vocal slit. This results in a "hiccup-like" sound. Inhalation is caused by the contraction of a number of muscles belonging to the diaphragm, neck, throat and the wall of our body. And these muscles, in turn, are controlled by nerves coming from the brain stem. Unlike in fish and amphibians, in us these nerves have to travel a longer way, because their brainstem is adjacent to the pharynx and gills. Along the way, there may be events that lead to nerve spasm and, consequently, to hiccups.
But one "hiccup" is not yet a full-fledged hiccup, it must be cycled for a period of time. That interval could be a minute, an hour, or even 68 years, which is how long Charles Osborne, a record holder of sorts, said he hiccupped. The rhythm of the hiccups is most likely provided by the same part of the brain that governs the rhythm of breathing. It is also located in the brain stem, and if an electrical impulse is applied to it, a hiccup can be induced. This is the legacy of our distant ancestors - amphibians. They don't need such a rhythm generator, just as we do, but amphibian larvae - tadpoles - need it vitally.
The fact is that tadpoles, at a certain stage of development, acquire lungs, but have not yet lost their gills. Air enters the lungs through the pharynx, but so does water. When the tadpole swims on the surface, it passes air directly into the lungs through a special slot. But if it is diving and needs to send air through the gills, it closes this slot as soon as water enters the pharynx. Doesn't it remind you of anything? Very similar to our hiccups, and many researchers believe they are variants of the same reaction.
So the hiccup is a legacy of our distant ancestors, who needed it for survival.