Some muscles get all the glory. Bodybuilders show off their swollen triceps; sprinters flash their sharp-edged calves. But deep inside all of us, a sheet of muscle does heroic work in obscurity.
In order to breathe in, we must flatten the dome-shaped diaphragm; to breath out, we let it relax again. The diaphragm delivers oxygen to us a dozen times or more each minute, a half-billion times during an 80-year life.
“We are completely dependent on the diaphragm,” said Gabrielle Kardon, a biologist at the University of Utah. “But we take it for granted every moment we’re breathing.”
To Dr. Kardon, the diaphragm is not just underappreciated but puzzling. All mammals, from platypuses to elephants, have a diaphragm. But no other animal has one. “We have a very different solution for breathing than reptiles and birds,” said Dr. Kardon.
Before the evolution of a diaphragm, our reptilelike ancestors probably breathed the way many reptiles do today. They used a jacket of muscles to squeeze the rib cage.
Once the diaphragm evolved, breathing changed drastically. Mammals gained a more powerful, efficient means to draw in a steady supply of oxygen. The evolution of a diaphragm may thus have made it possible for mammals to then evolve a warm-blooded metabolism. Without a diaphragm, humans might not have been able to evolve giant — but oxygen-hungry — brains.