Thursday, 18 October 2012

New Research Sheds Light On The Origins Of Vertebrate Air Breathing

380 million years ago, one of the most important events in evolutionary history was being played out along the shorelines of an ancient super-continent. Fish with limbs were making their way across swamps, where they made their homes, crawling through damp leaf litter, slithering across rotting logs. These bizarre creatures were the first ever amphibians on the planet. For the first time in Earth history, vertebrates broke their 150 million year old link with the oceans.

Fish now had the ability to survive in the water and on land. They developed a strong, weight bearing skeleton and limbs. Ammonia was converted to harmless urea to prevent them being poisoned by their own bodily waste. Yet their biggest challenge was finding a way to breathe air as fish absorb oxygen from the water via their gills.

For this process to work, the gills have to remain wet. A land dwelling vertebrate actively draws air into its lungs. The origins of this mechanism have been reconstructed from a variety of sources, including fossil evidence and modern creatures which seem to lie on the boundary between fish and amphibians. Systems of control are required to regulate the exchange of gases and their concentration in the blood, in particular carbon dioxide.

'It's the neural circuitry that allows air-breathing organisms to take in oxygen, which cells need to convert food into energy, and expel the waste carbon dioxide resulting from that process,' said Mark Harris, a neuroscientist from the University of Alaska Fairbanks. 'I'm interested in where that carbon-dioxide-sensitive neural circuit, called a rhythm generator, came from.'

Unfortunately, fossils of early amphibians are rare and often fragmentary, meaning that their origins cannot be studied directly. Harris and his colleagues started by looking at primitive vertebrates which possess similar neural circuitry, but for reasons other than regulating the respiratory system. He believes that such a system existed in the ancestors of the first amphibians and was adapted to the purpose of sensing the concentration of the various gases in the blood and lungs.

To do this, they took a lamprey and examined the way it respired. Lampreys are an ancient group of fish very similar to the first ever vertebrates. While today they are, in my opinion, rather repulsive bottom feeders which live on rotting flesh in vampiric fashion, they carry with them an ancient heritage. They do not have lungs and so do not breathe air. As larvae, they live in burrows in soft mud, pumping water through their bodies and extracting oxygen from it.

A screenshot of the experiment. The lamprey is placed inside
a syringe e to mimic its burrow. The cough occurs around
9 seconds in
When the mouth of their burrow becomes clogged, they use a cough-like action to pump water through the obstruction to clear it. This cough is controlled by a rhythm generator in the brain similar to the breathing rhythm generator found in land dwelling vertebrates.'We thought the lamprey 'cough' closely resembled air breathing in amphibians,' said Harris. 'When we removed the brains from lampreys and measured nerve activity that would normally be associated with breathing, we found patterns that resemble breathing and found that the rhythm generator was sensitive to carbon dioxide.'

It would appear that the development of lung breathing is the re-ordering, or re-routing of the cough mechanism which existed in lungless vertebrates. The origins of the tetrapods are not fully known and fossil evidence is scarce. Yet the use of modern day organisms to reconstruct events which occurred hundreds of millions of years ago, allows us to inch slowly towards their hidden truths.