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How did jaws evolve from gill arches in vertebrates?

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Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

The evolution of jaws in vertebrates represents a significant transition from the ancestral condition seen in jawless fishes (Agnatha). In early jawless vertebrates, the gill apparatus functioned primarily as a structure for filter feeding and respiration, supported by a series of curved bars known as branchial arches located between the pharyngeal pouches.123

The transition to jawed vertebrates (Gnathostomata) involved the modification of these existing skeletal elements. Specifically, the cartilages of the first two anterior arches were repurposed: the anterior arch evolved into the upper jaw (palatoquadrate) and the mandible, while the second, or hyoid, arch underwent its own structural modifications.4

This evolutionary shift allowed for more efficient feeding mechanisms compared to the simple sucking or nibbling actions of early agnathans. While the exact fossil ancestors of these jawed forms remain elusive, the structural transformation of these visceral arches remains a fundamental principle in vertebrate anatomy, with remnants of these arches persisting in higher vertebrates as components of the hyoid apparatus, the larynx, and even the auditory ossicles in mammals.123