The common ancestors of birds and mammals were very early (stem) reptiles, which almost certainly possessed 3-chambered hearts.
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Mechanical digestion is the breaking down of food into smaller pieces physically. Mechanical digestion begins in the mouth as the food is chewed.
but in the case of chemical digestion, it breaks the food into simpler nutrients that can be used by the cells. Chemical digestion begins in the mouth when food mixes with saliva.
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