Human heart pattern began in lungfish

Brazilian scientists solve a mystery of physiology by tracing it back to its source. Andrew Jenner reports.

The South American lungfish's heart rate changes with its breathing -- an adaptation that is also found in humans.
The South American lungfish's heart rate changes with its breathing -- an adaptation that is also found in humans.

Did you know that your heart speeds up when you inhale and slows down again when you breathe out? If not, you’ve got a good excuse – it’s one of the many processes that your autonomous nervous system runs quietly in the background.

Scientists have long observed this phenomenon, known as respiratory sinus arrhythmia (RSA), in mammals, but they’ve struggled to explain exactly what it does.

Now, a research team in Brazil studying the South American lungfish (Lepidosiren paradoxa) has reframed that debate in several ways.

In a paper published in the journal Science Advances, the scientists document RSA in the species – the first time the mechanism has been found in a non-mammal. In addition, they have been able to clearly reveal its function.

“We verified that this mechanism in the South American lungfish is critical to improving gas exchange,” says co-author Cleo Leite, from Brazil’s Federal University of Sao Carlos. “Its respiration is more efficient because of this regulatory mechanism.”

Sometimes referred to as “living fossils,” lungfish are primitive members of the branch of the evolutionary tree in which lungs first appeared.

By carefully measuring the heartbeat and respiration of some test-subject fish in the laboratory, researchers found that they maximise oxygen uptake in their lungs by modulating their heart rate. This led Leite and his co-authors to suggest that RSA is vestigial trait in mammals that has no significant physiological function. They conclude that “highly evolved control systems can have primitive roots.”

“They show very clearly that RSA is present and strong in this species,” said Robin McAllen of the University of Melbourne, Australia, who was not involved in the study. While RSA doesn’t play as significant a role in optimising mammalian respiration, McAllen points out that it nevertheless does reduce the workload on an animal’s heart – a fact that could also explain why we’ve hung on to it ever since it debuted in the lungfish.

For his part, Leite hopes the study will help dismantle the idea that Homo sapiens represents the pinnacle of evolution, perched high above the lowly lungfish that’s been mucking around in swamps for the past few hundred million years.

“It isn’t correct, this kind of anthropocentric thinking that believes evolution has gone on all this time to culminate in human beings, and that human beings are are the most perfect animals of all,” he said. “Unfortunately, some scientists hold this view.”

Andrew Jenner is a science journalist based in Brazil.
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