About 375 million years ago, creatures that lived in the oceans began moving onto land. These animals are the ancestors of all vertebrate land dwellers, including humans. The eyes of these early land animals had evolved to see underwater — they were water-filled and needed to stay moist. If you were designing eyes from scratch for land animals, you wouldn’t want these features. The water inside the eyes bends light entering the eye as the fluid slows the transmission of light — having water-filled eyes solves the refraction problem for animals living underwater but creates problems for animals living on land. Likewise, the need to keep the cornea of the eye moist is complex and problematic. While our eyes have adapted to being on land through evolution, we still retain these remnants of our aquatic past.
The solution to keeping our eyes moist is having eyelids and using them to blink. This makes sense — fish and most aquatic animals don’t have eyelids and don’t blink — their eyes are always moist.
Lacrimal glands at the top of the eye produce tears and when we blink our eyelids spread the liquid over our corneas. If we don’t blink even for a minute our corneas begin to dry out and our vision blurs. We need a lot of fluid to keep our eyes moist so we blink a lot, about 15 times per minute or every four seconds. That translates into about 14,000 blinks per day. If you add up all those small moments that we are blinking it means that we spend about 10% of our waking moments with our eyes closed.
Our eyes produce about 10 ounces of fluid per day (nearly the amount in a canned beverage) or about 30 gallons per year. All those tears drain into our tear ducts at the bottom of our eyes and are carried to our nasal passage where they evaporate or get absorbed. or drain down to our throats where they’re swallowed.
In addition to our normal tears that keep our eyes moist (called basal tears), our eyes produce tears to flush out irritants (called reflex tears) or when we cry (called emotional or psychic tears). Reflex and emotional tears are produced in such quantity that it overwhelms our tear ducts and the tears roll down our faces. As an interesting side note, women have smaller drains for tears than men which is why their tears are more likely to run down their faces.
Humans are the only species to cry tears. Why is that the case? This IFOD has the (probable) answer: Why Do Humans Cry Tears?
Primary source for this IFOD: Evolution Gone Wrong: The Curious Reasons Why Our Bodies Work (Or Don’t)