Evo-Ed: The Sun and the Evolution of Human Skin Color

Have you ever had a sunburn? Many of us have, and some of us probably haven't. A sunburn happens as our bodies respond to cell damage caused by sun exposure. A sunburn is actually an immune response. As your skin cells get more and more damaged from UV radiation from the sun, a signal gets sent to your immune system to send specialized response cells through the lymphatic and circulatory systems to the skin to start breaking down damaged cells and to stimulate healing. As this happens, your skin gets red and inflamed. And this inflammatory response explains why there's often a little bit of a delay between your sun exposure and when the sunburn appears. It's the immune response that needs a little bit of time to get kickstarted as your body responds to cell damage. Welcome to the EvoEd series on skin. Our skin is pretty cool. It does a number of things for us. For example, it acts as a barrier. It helps to keep our insides inside of us. It's also a sensory organ. It's the home of something called the somatosensory system. This system is composed of specialized nerve cells that are particularly active in places where you feel things the most, like your fingertips or your lips. The somatosensory system also helps you to detect temperature differences in the air around you. Your skin also hosts an incredible array of bacteria, something like a thousand different species. I know, that sounds kind of gross, but it's not. The bacteria living on your skin typically have no negative effect on you whatsoever. They don't make you sick, for example. But they can play a role in protecting you from harmful bacteria. See, if your skin already has a thriving and full community of bacteria, then it makes it a lot more difficult for newcomers to compete for space. One last thing. Your skin is also the primary part of you that interacts with the light that shines down from the sun. There's a type of cholesterol in our skin cells called 7-dehydrocholesterol. This is just regular cholesterol that's missing a hydrogen on the seventh carbon in the molecule. When 7-dehydrocholesterol intercepts photons of light from the sun, it gets converted into a molecule called pre-vitamin D. Your liver and kidneys then take pre-vitamin D and convert it into ordinary vitamin D. But the whole process starts in your skin cells. You may have heard of something called seasonal depression, or seasonal affective disorder. This is a type of depression that's triggered by changes in seasons, particularly for people who live at extreme latitudes. The theory is that vitamin D plays a role in preventing depression. So in dark months, since you're intercepting less solar radiation, you produce less vitamin D, and therefore may be prone to experiencing seasonal depression. And then there's the matter of sun and skin cancer. Solar UV radiation can cause mutations in your skin cell DNA. Now the vast majority of the time, these mutations are either repaired, or are inconsequential, or are so catastrophic that the cell itself will die or be destroyed. But every once in a while, these UV-induced mutations in the DNA of your skin cells can lead to uncontrolled cell growth and skin cancer. One way we can prevent this is to use sunscreen. Sunscreen tends to scatter the sun rays away from our skin so that the UV radiation doesn't damage our cells. Natural skin pigmentation can also protect your cells from solar UV radiation. Specialized cells called melanocytes produce dark pigment known as eumelanin. This eumelanin is exported to surrounding cells in special packets called melanosomes. The eumelanin intercepts the UV solar radiation before it can damage DNA, thereby protecting you from cancer. And that's not all. Solar UV radiation can also break down a molecule called folate, also called vitamin B9. Folate helps to repair damaged cells, which means it too can play a role in helping to limit something like skin cancer. And folate also plays a role in fertility and in fetal growth and development. So too much sun can really be a bad thing, both because it can directly contribute to DNA mutations that lead to cancer, and because it breaks down folate. In this video series, we're going to explore the interaction between the sun and your skin. We'll look at skin pigment as a feature that can protect your skin cells from sun-related damage. Connected to that, we'll also examine why skin cancers form, and figure out what geography has to do with any of it. Thanks for tuning in. I'll see you next time.