This allosteric molecule tells its evolutionary story by accepting and releasing oxygen in a complex dance with competitive and non-competitive inhibitors that is finely regulated by pH, pressure, and changes in concentration and conformation. In a clear example of homeostasis, the feedback mechanisms maintaining ideal concentrations of gases at the cellular level allow an understanding of cell respiration and the systems used to exchange gases with the environment. As a family of closely related genes, alpha, beta, gamma and delta forms of hemoglobin and myoglobin exemplify evolutionary fitness with the interplay of gene expression that changes throughout life due to environmental pressures. A sub-story of hemoglobin is the co-evolution of mutant forms of this molecule that have contributed to the survival of people in tropical regions of the world. This case study examines the structure and function of proteins using hemoglobin as a specific model for phenomena particular to this group of macromolecules. Molecular analysis and phylogenetic comparisons of gene sequences elucidate the history of this important molecule through taxa and through time.