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Study of African fish helps scientists crack the origin of breathing

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Bridie Smith

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Dr John Long.

Dr John Long.

The overlooked 'nose' of an African fish has forced scientists to look at the fossil record with new eyes.

Often regarded as a living fossil, the polypterus fish has provided researchers with evidence that four-legged land animals first developed the ability to breathe air as ancient fish in water.

Outlined in the journal Nature Communications on Thursday, the discovery is significant as it not only sheds light on the evolution of breathing air in the first amphibians but also the origin of hearing – all the way up the evolutionary tree to humans.

Being water-based creatures, the 'noses' of polypterus fish are located on the top of their heads. Physiologists based at the US Scripps Research Institute spent more than 360 hours observing how the primitive freshwater fish breathe. They found the north African fish, which has four limbs but no digits, was the only fish to still breathe through two valved channels on the top of the head.

"We've solved a 100-year-old mystery about how this thing breathes," said co-author and Flinders University palaeontologist John Long. "It doesn't breathe through its mouth like lung fish and other fish."

Having made the discovery, palaeontologists looked at the fossil record with new eyes. Professor Long said large openings at the top of the head called spiracles were more prevalent than realised among ancient fossil fish.

Among them is the extinct gogonasus fish, named after the place Professor Long found it, at Gogo Station in the Kimberley area of Western Australia.

The 380 million year-old gogonasus fish belongs to a group of fish considered the ancient ancestors of the first land animals. It had large holes on the top of its head, which until now scientists could only speculate were used for breathing.

However the latest findings show that the valved holes on the top of the head were in fact for breathing air.

"It was a revelation," Professor Long said. "It really was a eureka moment because now we have evidence from a living fish as to what this hole in the head was used for so we can apply it to the fossil record."

Professor Long also said once animals left the water for land, they no longer needed to breathe through the top of the head and over time the canals were re-tasked for use as ears.

"We've actually uncovered the origin of breathing and the origin of hearing at the same time," Professor Long said. "It's pretty exciting."

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