Bolder-dash's very own little thread

One school of thought holds that the evolution of "air breathing" in early tetrapods led to the tear duct.Breathing air in early tetrapods involved the choana, a structure connecting the external nostril to the inside of the mouth. The choana also had an opening that lead to the eye orbits via a small duct.The choana is found only in osteolepiform lobe-fins and tetrapods. (The choana is also homologous with the primitive paired external nostrils that are still found in lampreys, sharks, Latimeria and ray-fins. These external nostrils are used for smelling in water and not for respiration.)The duct leading to the orbits is comparable to the tear duct found in modern tetrapods. Similar secretions may have enabled certain lobe-fin fishes to operate near or at the water’s surface. Edited by molbiogirl, : No reason given.

Ancestral glands are usually assumed to have produced a copious watery secretion which moistened the cornea and provided protection against potential microbial pathogens.These glands were evolutionarily derived from sebaceous cells, just as crash said.The duct itself was formed by the bones of the skull.

It's not "my" theory. Google "lacrimal duct evolution sebaceous gland" and see for yourself.You asked about the evolution of tear ducts, I answered.Why is it difficult to believe that a duct lined with cells that secrete watery substances evolved into a duct lined with cells that secrete tears?