Parathyroid Hormone-related Protein (PTHrP) and its receptor are ubiquitously expressed in vertebrate phylogeny from the swim bladder of the fish through the lungs of amphibians, reptiles, mammals and birds. Structurally, PTHrP expression is necessary for lung development since the mouse knock out does not form alveoli (Rubin et al 2004), the key to the phylogenetic increase in lung surface area throughout phylogeny and ontogeny. Functionally, PTHrP/PTHrP receptor signaling is a stretch-regulated mechanism that up-regulates surfactant production to accommodate increased alveolar surface area, providing a rationale for the positive selection pressure on PTHrP signaling in ontogeny and phylogeny (Torday and Rehan, 2004). We hypothesize that the amplification of the molecular PTHrP signaling pathway for surfactant synthesis underlies the increased surface area for gas exchange through vertebrate phylogeny. To test this hypothesis we have examined the PTHrP signaling pathway from endoderm to mesoderm in the fish swim bladder (the structural origin of the lung), as well as the lungs of frogs, alligators, rats and chicks. PTHrP, its receptor and pulmonary surfactant are expressed in all of these tissues; PTHrP/PTHrP receptor signaling is up-regulated in the frog, chick and rat just before the onset of surfactant production. All of these tissues were cultured as explants and incubated with 5 × 10-7M PTHrP (1-34) for 24h. The tissues were incubated with 3H-choline chloride for the last 4h and subsequently analyzed for 3H-saturated phosphatidylcholine (SatPC) content (surfactant phospholipid). The alligator lung showed a 25% increase in de novo SatPC synthesis, and the rat lung showed a 200% increase, consistent with the hypothesized increase in PTHrP signaling through phylogeny. These experiments are an empiric test of the molecular basis for the ontogeny, phylogeny and ultimate evolution of the vertebrate lung.