Which came first, the lung or the breath?

Area of Science:

• Comparative anatomy
• Evolutionary biology
• Vertebrate paleontology

Background:

• Lungs are characteristic air-filled organs (AO) in lungfish and tetrapods, while swimbladders are ancestral in other bony fish.
• The homology and evolutionary origin of these air-breathing organs remain subjects of debate.
• Existing air ventilatory mechanisms in actinopterygians differ from lungfish and tetrapods, suggesting varied evolutionary paths.

Purpose of the Study:

• To investigate the evolutionary origins and homology of air-filled organs (AO) in bony fish.
• To compare air ventilatory mechanisms across different vertebrate lineages.
• To explore the potential ancestral forms of vertebrate air-breathing organs.

Main Methods:

• Comparative analysis of lung and swimbladder morphology and vascularization.
• Review of existing literature on air-breathing mechanisms in various fish groups and tetrapods.
• Examination of neurophysiological data from gar brainstem preparations.

Main Results:

• Homologous pharyngeal AO likely arose in early bony fish ancestors, preceding specialized lungs and swimbladders.
• Primitive AO may have originated from posterior gill pouches or modified gills, supplied by the sixth branchial artery.
• Gar brainstem activity patterns suggest similarities to amphibian air-breathing mechanisms.

Conclusions:

• Vertebrate air-breathing organs likely originated from a common pharyngeal structure in early bony fish ancestors.
• Dorsal development led to respiratory swimbladders, while ventral development resulted in lungs.
• Behavioral mechanisms for surface breathing may have predated the evolution of specialized air-breathing organs.