Ribs initially evolved to aid in movement, not breathing. A study suggests that early tetrapods (four-limbed vertebrates) used their ribs and vertebrae for side-to-side bending while walking. This locomotion function was crucial before their later adaptation for respiration. The upper (dorsal) ribs of early vertebrates are believed to be the ancestors of the ribs found in land-dwelling vertebrates. The development of a rib cage, including the attachment of ribs to a sternum (breastbone), first appeared in reptiles. In some lineages, like ray-finned fish, ribs are absent or significantly reduced, suggesting independent evolutionary paths for rib development across different vertebrate groups. The evolution of the rib cage also played a role in the development of the tall and narrow body shape seen in modern humans.
Key Stages in Rib Evolution:
- Early Tetrapods: Ribs were primarily used for locomotion, facilitating side-to-side bending during movement.
- Reptiles: The evolution of a rib cage, with ribs connecting to a sternum, marked a significant advancement.
- Mammals: Ribs continued to play a vital role in breathing and protecting internal organs.
- Humans: Rib cage structure contributed to the evolution of the human body shape.
Further Considerations:
- Independent Evolution: The absence of ribs in some early lobe-finned fish suggests that rib development might have evolved independently in different lineages.
- Adaptation for Breathing: While initially for movement, ribs were later co-opted for respiration, playing a critical role in breathing mechanics.
- Body Plan Diversity: Rib structure varies significantly across species, reflecting the diverse adaptations to different environments and lifestyles.
