Chapter 17: Cell–Cell Signaling, the Energy Flow from the Big Bang to Civilization Check Access

Life has accommodated metabolism in order to cope with the rising and falling levels of oxygen in the atmosphere over the last 500 million years through the thinning of the wall of the gas exchanger, progressively ratcheting up the surface-area-to-blood-volume ratio from fish to amphibians, reptiles, mammals and birds. To work physiologically, the alveoli had to produce lung surfactant, a soapy material that lines the surfaces of the alveoli, reducing the surface tension that would otherwise have caused alveolar collapse, or atelectasis. By following the changes in the composition of lung surfactant backwards from the mammalian lung to the unicell, the evolution of the lung can be documented, beginning (and ending) with elemental lipids and calcium.