Hint: Gills are known to be respiratory organs that are present in mostly aquatic organisms for the intake of oxygen from water and also excretion of carbon dioxide. They provide a large surface area which makes the exchange of gases easier.
Complete answer:
The respiratory systems of humans are different from those of aquatic organisms for the sole reason that humans live on land. Instead of gills humans have lungs which is the main component of the respirator system. the air from outside enters through the nostril and passes onto the larynx, pharynx and finally to the trachea. The trachea branches into two bronchi at the middle of the chest and these bronchi enter into the lungs and branch into smaller bronchioles.
The bronchioles lead to the alveoli which are small sac-like structures that are rich in blood. It is at the alveoli that the exchange of gases takes place. The oxygen from the atmosphere is dissolved into the blood and carried to different parts of the body and the carbon dioxide is exchanged and sent back through the same route and finally expelled outside through the nostrils. So humans cannot grow gills because they already have a respiratory system that is much more developed than aquatic animals and also being land inhabitants they do not require gills.
Thus humans cannot grow gills.
Note: Though humans do not require gills but recent research is being made to develop artificial gills for humans as it will help them remain under water for much longer times.
