Fig 3. Evolution of Air-Breathing Central Rhythm Generator.

Schematic illustration of the speculative evolution of ventilatory rhythm control in vertebrates: basic elements critical for ventilatory control and producing phasic pharyngeal basket ventilation and periodic “cough” were present in a basal, common vertebrate ancestor. These elements transition during vertebrate evolution through a process of exaptation or functional retasking. Mechanisms generating rhythmic pharyngeal ventilation are exapted to produce gill ventilation, buccal cavity ventilation, gular fluttering and suckling. Distinct mechanisms producing “cough” are sensitive to CO₂ and are exapted to produce air breathing. In this scheme, as described in the text, pharyngeal pumping in the basal vertebrate resulted from the pool of pump muscle motor neurons (MNP) being activated by relatively continuous activity from a central rhythm generator (CRG, ~) generating a pharyngeal ventilatory rhythm and, periodically by a distinct CO₂/pH-modulated CRG_cough producing water flows to clear debris from filter feeding areas and habitat. A first exaptation occurred with the evolution of unimodal gill ventilation when the pharyngeal pump was retasked to generate rhythmic gill ventilation (CRG_P becomes the CRG_gill) and the CRG_cough was retained. A second exaptation facilitated bimodal gill and air breathing; a retasked CO₂/pH modulated CRG_cough functions as the CRG for air-breathing (CRG_AB), while the CRG_gill is retained for gill ventilation. Unimodal air breathing continued to be generated by the CO₂/pH modulated CRG_AB in reptiles, birds and mammals, while the CRG_gill is retasked to produce periodic rhythmic gular fluttering and suckling.