Why Do Humans Have Different Skin Colors?

Why Do Humans Have Different Skin Colors?
By Arielle Moss
Among living primates, humans are the only ones who have a mostly
hairless body with a range of skin colors. Originally, scientists
thought that the darker skin tones, often found in Africa and along the
equator, evolved to protect humans from skin cancer. Recently, however,
scientists who study chemical processes in the body and those looking at
the prevalence of different diseases in large populations have gathered
evidence that suggests a different reason. Their evidence suggests
that human skin color differences are the result of nature selecting the
right amount of skin pigmentation to allow skin cells to receive the
right amount of ultraviolet (UV) radiation from the sun in order to
produce and preserve nutrients that are needed to allow humans to
reproduce successfully.

Dark skin is thought to have originated with the loss of hair in
humans. Chimpanzees have light skin covered in hair. The pink faces,
hands, and feet of their young freckle or darken with sun exposure.
Early bipedal primates (hominids) were small with skeletons built more
for climbing in trees than running. Hominids of the genus Homo were the
first hominids with skeletons built for walking or running great
distances.

Because these humans were more active, they had the problem of
staying cool and protecting themselves from overheating. These problems
were solved by an increase in number of sweat glands and the loss of
most insulating body hair. With the loss of hair, the human body had to
protect itself from the sun's UV radiation in another way.

The skin found on the hairless parts of the bodies of
chimpanzees contains cells called melanocytes. Melanocytes make the
dark-brown pigment melanin when they are exposed to UV radiation. After
humans became hairless, the ability to efficiently produce melanin
became more important. Melanin is the body's natural sunscreen.
Melanin is a large, organic molecule that absorbs UV rays and
neutralizes harmful molecules called free radicals that form in the skin
after it has been damaged by UV radiation.

In 1978 Richard Branda and John Eaton found that light-skinned
people who had been exposed to strong sunlight had low levels of the
essential B vitamin folate in their blood. What makes this finding
important is its correlation with reproduction.

Folate deficiency in
pregnant women has been shown to cause neural tube defects in babies.
In a 1996 study, pediatrician Pablo Lapunzine also concluded that dark
skin evolved to protect the body's folate stores from being destroyed.
He had observed birth defects in infants whose mothers had used tanning
beds in the early weeks of their pregnancies.
Folate is not only important in preventing birth defects, but it
is also essential for the replication of deoxyribose nucleic acid, or
DNA, in dividing cells. Therefore folate is necessary for the
production of sperm.

While UV rays can destroy folate, inhibit sperm production, and
cause cancer by damaging DNA, they also stimulate the production of
vitamin D in skin cells called keratinocytes. UV rays that reach the
keratinocytes stimulate those cells to convert cholesterol into
pre-vitamin D, which is later converted to vitamin D by the kidneys.

The oldest human fossils have been found in Africa, but about
120,000 years ago, modern humans, driven by the nature of curiosity,
ventured out of the tropics into new territory. Their darkly pigmented
skin, suitable for the conditions of UV radiation at the equator, would
have proved to be detrimental as they moved north into areas that
receive less concentrated UV radiation.

Those with large amounts of melanin in their skin would not have
absorbed adequate UV radiation to produce required amounts of vitamin D.
In children, vitamin D deficiency causes rickets, which results in
skeletal deformities. In adults, vitamin D deficiency can lead to
osteomalacia, which results in muscular weakness in addition to weak
bones. As people migrated into more northern areas, those with less
melanin in their skin would have had a survival and reproductive
advantage, ultimately producing entire populations with lighter skin.
For more information contact the ASU Department of Biological
Science at biology@astate.edu.

Photo Caption: ASU Biology of the Cell students show off their different
skin tones inherited from their ancestors who were adapted to different
amounts of UV radiation at different earth latitudes. Students (left to
right) are Bethany Boyle, Angela Tims, Divya Patel, and Rasheda Wright.
Photographer: Joy Trauth