External Defense

Every day I see dead people; and so do you! Well the people aren’t entirely dead, just the majority of what you see. All of the cells on the outside of you, except for your eyes, are already dead. Your hair, your skin, your fingernails: all dead. But don’t be sad about your dead cells, they died for a purpose, and you’re going to learn that purpose right now.

First and foremost, your skin serves as a means of protection. Like the walls to a city or the membrane around a single cell, the skin keeps the outside out, and the inside in. The skin is technically alive, though the living cells are buried underneath lots of layers of older, dead cells.

Your skin is constantly dividing and while the new cells push outward, the oldest cells lose their connection and flake off. This happens all the time, in fact your own skin and hair cells make up most of the dust in your bedroom! It’s why a shirt can smell like the owner, or how dogs can track people’s movements even without footprints. You are constantly growing and losing skin cells.

While the outer layer separates all the time, the inside layers mostly stay together. Some areas of your body, namely the palms of the hand and soles of the feet, have thicker layers of skin do endure more pressure or friction. If there is a break in the internal layers, to avoid further damage your body fills the breakage with water as a fluid cushion. If the layers break apart enough, the body will allocate more resources to hold on to more layers of skin. You know of that cushion as a blister and the thicker layers as a callus.

Blisters from burns
Blisters from burns

Not only does your skin provide a defense against invaders by just being there, it also secretes water with some salts or lipids. You know of the salty secretions as sweat and tears, and you probably call your fatty secretions mucus and earwax, depending on where it came from. The salty secretions remain on your skin even after the water evaporates, leaving the membranes of the dead cells slightly acidic. That acidity is very helpful for killing microorganisms that land on you throughout the day.

Your fatty secretions are found in your orifices and serve the function of catching debris that is headed into the body through the orifice. You know your orifices: nose, mouth, ears, eyes, etc. basically your holes. You wouldn’t want stuff to be able to wander inside you so the fatty secretions act as a sticky web to halt the invading particles in their tracks.

You’ve probably heard that UV rays can give you skin cancer. This is because light at ultraviolet frequencies is small enough to penetrate through cell membranes and manipulate the DNA itself. Over generations, a mutation occurred that produced a dark protein that could absorb many frequencies of light; we call that protein melanin. In the presence of melanin, photons had a lesser chance of damaging the DNA as they were mostly absorbed by the pigment. Those individuals who possessed more melanin were better able to survive with less damage from the sun.

Melanin in the skin

Melanin is an energetically expensive molecule to make. As some our ancestors migrated out of Africa and further north, the sun was less intense and darker individuals couldn’t gain necessary nutrients from the more scarce sunlight. Either by energy reallocation, random mutation, selective breeding, or a combination of all three, a lack of melanin became normal among the tribes of the north.

A little more time and those melanin-lacking humans would manage to travel far from their home coming across their own species, but with more melanin due to their environment. Unfortunately, the melanin-lacking humans did not understand their own anatomy at the time. Instead of recognizing their own roots, the “pale man” determined that the “dark man” must be an entirely different species. The rest is history.

Some cells produced a rigid molecule called keratin. Your outer skin contains a little bit of keratin for protection but it is your hair cells that contain the most and why they are so much more rigid. The higher concentration of keratin keeps the dead cells attached for longer, allowing your hair to grow into tall structures. On the tips of your fingers and toes, an even larger concentration of keratin allowed hairs to solidify forming hard and even sharp structures, you might know them as claws or nails. Every time you cut your fingernails, you’re giving yourself a haircut.

Hair was an advantageous adaptation because stalks could keep warmer air closer to the skin. Think about what happens to your arm on a cold day: the hairs stand up to create more volume for the warm air. If you’ve ever gotten a haircut during winter, you know that more hair means more warmth for your head. Hair surrounding the brain is an advantageous adaptation for a species whose survival depends on intelligence.

skinSome hair cells mutated in other ways and the advantageous ones are still seen today. Think about the shape of curly hair: very spring-like. Where do many of us have curly hair? Does that region likely need any resistance to friction? You have the tools to answer every question you can possibly think of.

Your skin is the largest organ of your body and it offers physical, chemical, and thermal protection to the Organism. It can heal itself when damaged. It can allocate resources to prevent further damage. It can stretch and it can wrinkle to fit your shape. It’s the majority of what people can see of you and it’s what you visually remember about everyone you meet. Remember to take care of your skin, but really it’s only one organ and most of it is already dead. Don’t get too caught up in judging yourself or others based on what the dead parts look like.

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