(ORDO NEWS) — Gills are known to allow many species of animals to breathe underwater, but they also have the additional function of regulating the ionic and pH balance in the blood, just as the kidneys do.
Initially, scientists believed that these functions of the gills arose and developed in parallel, but now they have revised their views.
The worm-like ancestors of all modern vertebrates, from fish to humans, first appeared about 500 million years ago, when all life on Earth was still swimming in the sea.
In these creatures, primitive gills have taken over the function of respiration, which was previously performed by the skin.
Scientists assumed that it was this transition that allowed small creatures to become larger and more active, giving rise to the first fish.
But what if the original function of the gills was not to intensify respiration, but to excrete?
After all, those ancient organisms still did not have kidneys, the water-salt and pH balance of the blood was maintained through the skin, so the appearance of specialized organs could help regulate blood composition more effectively.
To put an end to this issue, the researchers studied the functions of the skin and gills in three animals: lampreys , lancelets , and enteropres . Lampreys are vertebrates, lancelets are chordates (they have an inner core, or notochord , but no true spine), and enteric-breathers are hemichordates .
All these animals reflect different stages of the evolutionary development of vertebrates, which allows them to be used as a simplified model.
The gills perform the respiratory function only in lampreys, but the function of excreting ions is in all three animals, so, most likely, initially they served precisely to maintain the balance of ions in the blood.
It turns out that at first all the gills served our ancestors as an excretory organ. Probably, the gills took over the function of gas exchange as the body size increased and activity increased, that is, with the advent of the closest predecessors of modern fish.
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