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Hello, this question is based on vertebrate evolution.
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Evolution is studied based on fossil evidence with relative or radioactive dating.
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Morphological or physiological comparisons and also current advances in developmental genetics are shedding light on evolution.
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To answer the first question, scientists have established based on fossil evidence that jaws originated from the pharyngeal arches that supported gills in the now extinct chondrycteus the placoderms that existed in the silurian period.
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It was first believed that these fish had nine pharyngeal arches to support gills.
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As you can see in this image, out of these the first two were lost due to evolution and the third became modified to form a breathing jaw.
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This third arch is called as the mandibular arch.
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The jaw formed is called as a breathing jaw because it helped to make the breathing of these fish more efficient.
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As evolution proceeded, the fourth arch called as the hyoid arch became modified to support the breathing jaw and suspend it from the skull or brain box.
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This jaw formed by both the arches was stronger and more efficient and it is called as a feeding job.
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This theory came to be known as the neoclassical theory as other theories were being proposed.
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The disadvantage of this theory is that it assumes that the pharynx or the pharyngeal arches of placoderms were metamerically segmented.
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Metamirically means having similar repetitive units but this is not so fossil and anatomical and developmental evidence do not support this...