Gills evolved before lungs. This is supported by extensive fossil evidence and the evolutionary history of aquatic and terrestrial vertebrates.
Evolutionary Timeline
- Early Aquatic Life: The earliest fish possessed gills, structures adapted for extracting oxygen dissolved in water. These evolved hundreds of millions of years before lungs appeared. The evolution of gills predates the evolution of lungs by several hundred million years. [Reference: In fish, gills predate lungs by several hundred million years.]
- Lung Development: Lungs evolved later, initially as a supplementary respiratory organ in some fish, likely used for supplementing oxygen intake in oxygen-poor waters. This swim bladder or air bladder then gradually developed into the lungs we see in terrestrial vertebrates. [Reference: The lungs of higher animals evolved from the swim bladder or air bladder in fish.]
- Transition to Land: As some fish ventured onto land, lungs became more crucial for respiration in the air, while gills became less important. Many amphibians, for instance, begin life with gills and later develop lungs as they mature. [Reference: Amphibians, such as frogs, and salamanders start out with gills, and grow lungs as they develop into their adult form.]
The Evolutionary Advantage of Gills and Lungs
- Gills: Efficiently extract dissolved oxygen from water, crucial for survival in an aquatic environment.
- Lungs: Adapted to extract oxygen from the air, a necessity for life on land. The development of lungs was a critical step in the vertebrate transition from aquatic to terrestrial environments.
Some fish have both gills and lungs, highlighting the evolutionary transition. The presence of both demonstrates an intermediate stage in the evolutionary journey from aquatic to terrestrial respiration.
