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Lamprey larval cough said to be a clue to the evolutionary origin of breathing.
The normal neurological stimulus that regulates how fast you breathe is extremely sensitive to carbon dioxide. Your brain generates a rhythmic signal to trigger breathing, but its rate is regulated by the amount of carbon dioxide in the blood. Scientists searching for the evolutionary roots of this finely tuned regulatory mechanism have sought answers in the sea from which they believe air-breathing life emerged. Neuroscientist Michael Harris of the University of Alaska Fairbanks and colleagues believe that have discovered its evolutionary prototype in lamprey larvae.
“To breathe air with a lung you need more than a lung, you need neural circuitry that is sensitive to carbon dioxide,” says Harris. “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. I'm interested in where that carbon-dioxide-sensitive neural circuit, called a rhythm generator, came from.”
So, in the evolutionary view, which came first, the lung or the neural stimulation to breathe? Harris believes the carbon dioxide sensitive rhythmic breathing control evolved before lungs. However, he believes the neural control mechanism served a different purpose. Harris explains, “We try to find living examples of primitive non-air-breathing ancestors, like lamprey, and then look for evidence of a rhythm generator that did something other than air breathing.”
The lamprey is a cartilaginous vertebrate believed by evolutionists to be primitive. “For biologists, lampreys represent an opportunity to envision the evolutionary past, because of their status as ‘living fossils’ that haven't changed in millions of years. Lampreys are thought to be an early offshoot on the evolutionary tree, before sharks and fish. Their lack of jaws distinguishes them from sharks or other types of fish.”’1
Lamprey larvae live in tubes buried in mud and pump water through their bodies to obtain food and oxygen. When the tube gets clogged with debris, a cough-like reflex blows the debris out. “We thought the lamprey ‘cough’ closely resembled air breathing in amphibians,” Harris explains. “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.”
Harris and his colleagues conclude, “The evolution of lung breathing may be a repurposing of carbon dioxide sensitive cough that already existed in lungless vertebrates, like the lamprey.”
The discovery of carbon dioxide sensitive neural circuitry in lampreys does not demonstrate common evolutionary ancestry. Clearing the lamprey’s larval tube or reminding mammals to take a breath are both actions designed to replenish oxygen. In a world full of animals designed to utilize oxygen and release carbon dioxide as a cellular waste product, such a trigger to stimulate oxygen-replenishing behavior is a sensible design. Unlike oxygen, which is often carried on molecules like hemoglobin, carbon dioxide readily dissolves in body fluids to form chemicals that can be detected and responded to instantaneously. The ubiquitous presence of this sophisticated neural circuitry is consistent with a world in which a common Designer, God, created all kinds of living things equipped to live in the world He made.
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