Final answer:
The modification of pharyngeal slits for respiration first occurred in early chordates. These structures are present in various forms across the deuterostomes and played a key role in the transition from aquatic to terrestrial life, especially in vertebrates where they were further adapted into gill supports and later, into parts of jaw and ear structures.
"The correct option is approximately option A"
Step-by-step explanation:
Modification of the pharyngeal slits for first use in respiration occurred in early chordates. These slits are crucial morphological features and are significant for understanding the phylogenetic relationships among deuterostomes. The early chordates, which later diverged into vertebrates, are understood to have modified these slits primarily for respiration. However, pharyngeal slits function differently in various animals. In aquatic invertebrate chordates, such as tunicates and lancelets, they are involved in feeding by filter-feeding mechanisms. These animals retain the pharyngeal slits into adulthood, but in the vertebrates like fishes, the slits are further modified to form gill supports, and later in jawed fishes, into jaw supports.
Over the course of evolution, tetrapods have highly modified the pharyngeal slits into other structures such as the ear, tonsils, and thymus glands due to the move from water to land, making the enhancement of gill structures unnecessary. The superphylum Deuterostomia encompasses both Echinodermata and Chordata, which includes humans, echinoderms like starfish, and hemichordates. All of these undergo deuterostome development, which is distinct from protostome development found in arthropods and other non-deuterostome invertebrates. This evolutionary path highlights the close relationship between chordates and echinoderms.
Thus, in response to the question, the early chordates are the ones that first modified their pharyngeal slits for respiration, marking a significant transition in the evolution of these structures.