555-545 million years ago
Ancestral hemichordates evolved the precursor of gills although these pharyngeal slits were used primarily for feeding rather than gas exchange. The mucociliary mechanism which once functioned in food capture would be used by higher vertebrates to remove debris from the respiratory tract. Although ancestral hemichordates lacked a postanal tail, their digestive tract shared some features with that of vertebrates such as the presence of goblet cells and microvilli.
Ciliated pseudostratified epithelia (which lines the human trachea) exists
in the pharynx of hemichordates. (Benito, form Harrison 1997, p. 72.)
From the mouth, food passes through the pharynx to the intestine. The
ability to pass water through pharyngeal slits is involved in feeding
in primitive filter-feeding chordates and hemichordates: food from the
water becomes stuck in the mucus lining the pharynx and is carried to
the intestine by ciliary action. Pharyngeal slits became involved with
respiration secondarily and higher vertebrates use this mucociliary mechanism
as a way of directing microbes and dirt to the stomach and away from the
lungs. Most filter feeding animals make use of mucus to capture the food
and cilia to transport it. The development of a perforated, muscular pharynx
in chordates allowed the change from microphagy (feeding on small material
suspended in water) to macrophagy (feeding on larger material)(Barrington).