Final answer:
The hypothesis suggests that fish jaws evolved from the first pair of gill arches in jawless fishes, giving rise to gnathostomes. This development significantly expanded their dietary options and allowed them to become effective predators. Modern examples of jawless fishes are hagfishes and lampreys, while sharks and other fish species are members of gnathostomes.
Step-by-step explanation:
The question pertains to the evolutionary hypothesis that fish jaws developed from a modification of earlier structures. Indeed, the development of the jaw is one of the most significant events in the evolution of vertebrates, particularly for the subgroup known as gnathostomes, or "jaw-mouths." These jawed vertebrates, which include both cartilaginous and bony fishes, evolved from jawless fish.
The first fish jaws are believed to have derived from the first pair of gill arches that supported the gills in these pre-gnathostomic fishes. Jaws proved to be a major evolutionary advantage, allowing fish to grasp and tear food, and expanded their dietary options beyond what jawless fish could manage, facilitating a shift from filter-feeding to active predation.
Several other adaptations accompanied the evolution of jaws in fish, like the presence of two sets of paired fins—pectoral and pelvic—which significantly improved mobility and maneuverability in water. This suite of features enabled gnathostomes to outcompete and eventually replace most jawless fishes during the Devonian period.
Examples of jawless fishes include hagfishes and lampreys, which continued to rely on suction feeding or parasitic lifestyles. In contrast, the evolutionary lineage that led to modern sharks and other cartilaginous and bony fishes developed jaws that allowed these creatures to become efficient predators and exploit a variety of food sources.