How we got our skin and what it does for us
Skin is the body’s largest organ; the average adult’s skin weighs 13kgs. It protects the body from injury and from harmful microbes and chemicals. Skin also plays an important role in producing hormones, making vitamin D3 and regulating both the temperature and the amount of water in the body. The colour of skin is the result of many substances but principally melanin, the more melanin, the darker the skin. There are two main layers of skin, the inner layer, called the dermis and the outer layer, the epidermis. The epidermis has four or five layers, depending on the region, and located in the bottom or basal layer are the melanin producing cells called melanocytes. From these cells melanin is transferred to keratinocytes, cells which make up 90% of the epidermis. Exposure to ultra violet radiation (UVR) increases the production of melanin; this link between UVR and human skin colour goes deep into our evolutionary history. Skin is an important source of vitamin D3 and is synthesized by the action of UVR on the skin. Vitamin D3 promotes the absorption of essential calcium and phosphorous in the body, but the amount of UVR decreases with distance from the equator. Light skin absorbs more UVR than dark skin so in the higher latitudes humans evolved lighter skin to capture sufficient UVR to produce vitamin D. But too much UVR can be dangerous, damaging the skin and causing cancer. In regions of high year-round UVR, the equatorial regions, humans evolved dark skins – full of protective melanin. Skin colour at a given latitude is a compromise between having enough melanin to protect the skin from the harmful effects of UVR and the requirement for sufficient exposure to UVR to allow vitamin D3 synthesis to take place. Anatomically modern humans evolved from a branch of hominids in East Africa about 200,000 years ago. They had black hair and dark skin; some migrated north out of equatorial East Africa and as populations moved north over millennia, they experienced natural selection in favour of lighter skins. There is scientific debate about why our ape-like ancestors lost their body hair. The generally accepted theory is that climatic changes caused the forested regions, where these hominids lived, to become treeless, sun drenched hot savannah. The hominids overheated in their hairy bodies, so natural selection favoured loss of body hair, more sweat glands and bipedalism as a means to keep cool. A bipedal animal is subjected to only 40% of the direct sunlight exposure of a quadruped of the same volume. Managing to keep cool efficiently enabled ancestral humans to walk and run long distances stalking prey in the open savannah. But UVR exposure has its dangers and to protect themselves from the tropical sun, these hominids produced more melanin and became dark skinned. Evolution of the optimal skin colour in a given region takes about 20,000 years. Today we have a rainbow nation of skin colours and the genes favouring fairer or darker skin have become mixed across many peoples; the optimum melanin protection has been lost in this racial diversity. This means that people with darker skin living in higher latitudes should be aware of their decreased ability to make vitamin D and people with lighter skin living in lower latitudes should be aware of their increased danger to UVR exposure. New Zealand has 40% higher UV levels during the summer than at corresponding latitudes in the northern hemisphere. Dr Richard Mackenzie of the National Institute for Water and Atmospheric Research (NIWA) gives three reasons for this. Firstly, Earth’s orbit is closest to the Sun in December/January, the southern summer. Secondly, during the summer months there is less ozone in the southern hemisphere than in the northern hemisphere. Ozone absorbs the shorter UV wavelengths; it is generated in the equator and the efficiency of transport of ozone is different in the southern hemisphere from the northern hemisphere. Thirdly, the cleaner southern hemisphere air means the UV intensity is greater here than similar latitudes in the northern hemisphere. All New Zealanders, fair and dark skinned should be aware of the potential dangers of too much summer UV, but noting also the dangers of too little winter UV.