[The evolution of the human species: a long journey for the respiratory system]

Evolution has involved important changes in hominids, particularly in relation to the process of encephalization and the transition to bipedalism. Some of these changes involved structures related to the respiratory system and altered its functional behavior. Changes affecting the relationship between the skull and the spinal column, together with an improved laryngeal structure (allowing vocalization), resulted in a soft and elongated oropharynx, with part of the tongue integrated into its anterior wall, and thus in an increased tendency towards upper airway collapse during sleep. Vertebral bodies moved inwards into the thorax, which became slightly shorter and went from a bell-shaped appearance to that of a flatter barrel-shaped one. This resulted in respiratory muscle mechanics that were more efficient for upright posture. The pulmonary ventilation and perfusion gradients moved from a dorsoventral to a craniocaudal axis, while the structural organization of the respiratory muscles underwent only minor changes.