The hemoglobin family of proteins, ubiquitous in all domains of life, evolved from an ancestral protein of primordial function to extant hemoglobins that perform a myriad of functions with diverged biochemical properties. Study of homologs in bacterial hyperthermophiles may shed light on both mechanisms of adaptation to extreme conditions and the nature of the ancestral protein. A hemoglobin was identified in Aquifex aeolicus, cloned, recombinantly expressed, purified, and characterized. This hemoglobin is monomeric, resistant to thermal and chemical denaturation, pentacoordinate in the ferrous deoxygenated state, and oxygen-avid. The oxygen equilibrium dissociation constant is approximately one nanomolar at room temperature, due in part to a hydrogen bond between the bound ligand and a tyrosine residue in the distal pocket. These biochemical properties of A. aeolicus “thermoglobin,” AaTgb, may have been shared by the ancestral hemoglobin, thus suggesting possible primordial functions and providing a starting point for consequent evolution of the hemoglobin family.