First Land Animals Evolved From Gogonasus, An Ancient Fish That Breathed Air Through Its Head
The first four-legged vertebrates evolved from a fish that breathed through a hole atop its head, according to a recently published study. The hole, called a spiracle, was present on the 380-million-year-old fish gogonasus and is still present in the fish polypterus. With gogonasus able to breathe air through this hole, or spiracle, it was only a matter of time before air-breathing, land-dwelling vertebrates called tetrapods came into being, the researchers say.
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"For many years, us paleontologists have found fossils of ancient fish that have these peculiarly large holes on the top of their heads," said study co-author John Long, a paleontologist from Flinders University in Australia. "We never knew what they were for so the evidence from our study shows these ancient fossil fishes were using these holes to breathe air."
There are two main evolutionary advantages to spiracles. Because spiracles are on top of its head, a fish can surface to breathe while still keeping an eye on predators in the water. A fish can also breathe in shallow water, where it be difficult to raise its head to breathe. And, as it turns out, with a spiracle, a fish can evolve into a land animal.
In the study, Long and his colleagues observed polypterus, the most primitive actinopterygians, or bony fish, alive today. Watching polypterus in a laboratory tank for 360 hours, the researchers found that the fish breathed through its spiracle between 40 and 93 percent of the time.
The spiracle didn't just lead to the ability to breathe on land--it also allowed tetrapods to hear, according to Long. After tetrapods began to breathe through their mouths and nostrils, there was a vestigial spiracle canal just sitting around. Eventually, tetrapods evolved to make use of it. "Transmitting vibrations through the air to the brain would become the next major use of the spiracle canal," Long writes on Slate. "Hearing in early amphibians developed from adapting the spiracles to become the tympanic membrane for transmitting sound to the brain through the stapes, one of our tiny inner ear bones."
According to Long, scientists in the 18th century "had these wacky ideas that fish just jumped onto the land and started gasping for breath and developing limbs. But our research shows that the transformation actually started happening within the fish themselves while they were still in water."
The study, "Spiracular air breathing in polypterid fishes and its implications for aerial respiration in stem tetrapods," was published last week in the journal Nature Communications.
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