I Accept the Theory of Evolution

When you inhale through your nostrils, the flow of air branches into four pairs of large chambers tucked in the bones of your face… the nasal sinuses, which are cavities inside our heads. The air comes into contact with mucous membranes… wet and sticky tissue that catches dust and other particles, including bacteria and viruses, so that they don’t reach your lungs.

The mucous membranes produce a slow and steady flow of sticky mucus. This mucus is swept away by tiny, pulsating, hairlike structures called cilia. The mucus drains into several spots and is ultimately swallowed and sent to the stomach (where the acid destroys it).

Sometimes the system gets gummed up, and that can lead to a sinus infection. Bacteria can establish an infection that may spread throughout the sinuses and beyond. Mucus, normally thin and mostly clear, becomes thick, viscous, and dark green when you have an infection.

Have you ever noticed that dogs, cats, and other animals don’t seem to have head colds nearly as often as humans do? Most humans suffer between two and five head colds (also called upper respiratory infections) per year, and these are often accompanied by full-blown sinus infections. For dogs it’s different. Dogs can get sinus infections, which show up as a runny nose, but it’s rare for them. Most dogs will go their whole lives with no major episodes of infection in their nasal sinuses.
 
In fact, sinus infections are possible but rare in all non-human animals, although they are a little more common in other primates. Why is it so bad for us?

The reason is that our mucous drainage system is a mess. Specifically, the most important drainage-collection pipes are installed near the [i]top[/i] of the largest pair of cavities, the maxillary sinuses, located underneath the upper cheeks. Having the drainage-collection point high within these sinuses is a problem because of gravity. While the sinuses behind the forehead and around the eyes can drain downward, the largest and lowest two cavities must drain upward.

Because the mucous collection duct is located at the top of the chamber, gravity cannot help with drainage. This is the reason why sinus infections are so common in humans but unheard of in other animals.

This is why some people with colds and sinus infections can briefly find relief by lying down and tilting their head back. However, the relief is only temporary. Once a bacterial infection takes hold, drainage alone can no longer combat it, and the bacteria must be defeated by the immune system.

What kind of plumber would put a drainpipe anywhere but at the bottom of a basin?
Why is the drainage system at the top of the maxillary sinuses instead of below?

The evolutionary history of the human face holds the answer. As primates evolved from earlier mammals, the nasal features underwent a radical change in structure and function. In many mammals, smell is the single most important sense, and the structure of the entire snout optimises the sense of smell. This is why most mammals have elongated snouts... to accommodate huge air-filled cavities full of odour receptors. As our primate ancestors evolved, however, there was less reliance on smell and more reliance on vision, touch, and cognitive abilities. Accordingly, the snout regressed, and the nasal cavities were squashed into a more compact face.

Nowhere are there more differences between humans and nonhuman primates than in the facial bones and skull. Humans have much smaller brows, smaller dental ridges, and flatter, more compact faces. In addition, our sinus cavities are smaller and disconnected from one another, and the drainage ducts are much skinnier… a side effect of making room for our big brains.

This rearrangement produced a suboptimal result that has left us more susceptible to painful sinus infections than any other animal.