The Evolution of Skin Colour

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1. What are the causes of skin cancer?
Exposure to abnormally high frequencies of light can potentially alter the DNA of a cell and turn it cancerous, the resulting cancerous cells are known as skin cancer. The most common cause is ultraviolet light emitted by the sun although it has been known to be caused by tanning booths, unusually high levels of x-rays, exposure to some chemicals and in rare cases the abnormal genes that cause skin cancer can be inherited by children from their parents.

2. Why are Caucasians more at risk of skin cancer than other populations? Melanin is the pigment in the skin that absorbs ultraviolet light and protects the skin from sun damage, so the less pigment in a person’s skin the less protection they have from ultraviolet light. So light skinned people (Caucasians), particularly those with light coloured eyes and light coloured hair are more susceptible to sun damage and skin cancer than people with more pigment in their skin. People, who have no melanin at all, like people with albinism or vitiligo, are much more susceptible to skin cancer.

3. At what age does skin cancer typically occur? Is the incidence of skin cancer greater in youth or old age? A person's lifetime sun exposure is a known factor in skin cancer because of this the risk of developing skin cancer increases with age, most diagnoses are made in people over 50 years of age. However people who are exposed to large quantities of ultraviolet radiation can develop skin cancer as early as 20 to 30 years of age. The average age of skin cancer diagnosis occurs at 53 years old.

4. Does the amount of UV light reaching the Earth vary in a predictable manner? If so, describe the pattern you observe. The earth’s axis is slightly tilted relative to its orbit, this means that during stages of the earth’s orbit one side of the earth is closer to the sun than the other and becomes exposed to higher levels of ultraviolet light. Half a year later when the earth is on the opposite side of the sun the other half of the earth is exposed to higher levels of ultraviolet light. This cyclic process of varying ultraviolet light exposure causes the areas exposed to most ultraviolet light to increase and decrease in latitude and the year progresses. The earth’s axis is only slightly tilted, because of this the ultraviolet exposure band only varies in latitude a small amount and never reaches the northern and southern most parts of the earth. Over a year the mean of daily ultraviolet radiation that hits the earth’s surface generally increases the further away a place is from the north and south poles. The earth’s atmosphere plays a large role in absorbing the ultraviolet light from the sun, the atmosphere is thinner in certain places and causes a greater amount of ultraviolet light to hit the earth’s surfaces beneath it, one such place is over Australia where it receives much larger ultraviolet radiation than it should.

5. What latitude receives the greatest amount of UV light? The least? On average the equator receives the greatest amount of UV light because it spends the greatest amount of time nearest to the sun and varies from this by a small amount, the UV light also has to pass a smaller distance through the atmosphere as it passes directly above it and the ozone is naturally thinner above the tropics so there is less ozone to absorb the UV light. The equator is at 0° latitude. The north and south poles receives the least UV light as the sun is usually furthest away from these points, ozone holes have formed above the north and south poles but because they spend much of each year without sun and the parts of the year they spend with constant exposure to sunlight the sun is so low in the sky the UV light does not go through the ozone hole but has to pass a considerable distance through the normal atmosphere and becomes largely absorbed by the ozone. The north and south poles are at 90° latitude.

6. Based on these data, where might you...
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