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Introduction to Biological Anthropology
Week 11
Solar Radiation
There are many aspects that will affect the intensity of Solar radiation on the human body
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Intensity on the earth’s surface
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Daily

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Seasonally

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Latitude and longitude

The effects of solar radiation will be most intense at noon, in the summer, at the equator and at
higher altitude
...
Reflective surfaces such
as water and snow will also increase the intensity of solar radiation
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One
general observation that we know to be true is the level of pigmentation of populations prior to
European contact
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What we tend to see is that the darkest skin colours tend to be
concentrated around the equator
...

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Haemoglobin

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Carotene

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Melanin

Melanin
This is a substance that is produced by melanocytes – it is the outer layer of skin that protects
against harmful UV rays, which can cause genetic mutations in skin cells - such as cancer
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It occurs in everyone except those who have albinism
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There is a suggestion that UV radiation is a powerful selection pressure acting on skin colour
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Cancers usually occur after they reach the
reproductive age
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Folate hypothesis to explain skin colour cline

Folate is a B vitamin that tends to come from green leafy vegetables and fruits
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One of the things about folate
serum levels, is that they are depleated very significantly by UV exposure
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There is suggestion that in areas of high UV
exposure, it is beneficial to be dark skinned to protect against folate insufficiency and foetal
abnormalities
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It enables the body to absorb
calcium from dietary sources such as fish oils, egg yolk, butter, cream and liver
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Inadequate vitamin D can cause rickets in childhood
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Women with inadequate Vitamin D status produce vitamin D
deficient new-borns
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Poor vitamin D status can lead to increased morbidity and mortality, as well as reduced completed
fertility
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In low UV radiation
conditions, selection favours genotypes with less melanin that can maintain vitamin D and calcium
levels more easily
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In areas closer to the equator, people need to balance their folate levels in
particular, which leads to a higher melanin content of the skin, and darker skin colour
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After the age of about 5 years old, the majority
of people cannot digest milk
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1
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Lactase-persistent phenotypes absorb calcium better than late-onset lactase deficiency
phenotypes

Natural Selection would favour the lactase persistence gene under UV conditions
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Summary
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Skin colour variations – melanisation – biological adaptations to environmental stressors

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Melanised skins protect against harmful effects of UV radiation and resultant cancers - X

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Folate hypothesis – melanin protects against folate depletion

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Vitamin D hypothesis – lighter skins in northern regions reflect selection for reduced UV
radiation conditions

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Lactase persistence and calcium metabolism

Living at altitude
Population responses to living at altitude
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<1% of the world’s population or approximately 25 million people

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High altitude 2,500m or approximately 8200 ft
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This can be
biological (genetic usually) or cultural

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Acclimatisation – changes that occur during an individual’s lifetime to reduce the strain
caused by stressful environmental conditions
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MAY help
the individual survive, but at a cost

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Developmental Acclimatisation – special form of acclimatisation referring specifically to
changes that occur during an individual’s development

Stresses of living at high altitudes
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Low oxygen levels

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High levels of solar radiation

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Cold

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High winds

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Lack of moisture

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Rough terrain

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Relatively limited plant and animal life

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High altitude hypoxia

Hypoxia
Responses to hypoxia:
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Increased breathing rate

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Reduces as become acclimatised

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Increased oxygen uptake by red blood cells

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Compensates for overall reduced oxygen uptake

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Number of capillaries in body increases

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Increase in number of red blood cells – accommodation rather than acclimatisation

Although the oxygen levels in the air does not decrease as you climb, your ability to access it does

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