Human Adaptation II: Skin Pigmentation
UV Radiation and Skin Pigmentation
- Skin pigmentation is one of the most variable phenotypes in humans.
- The variation of skin pigmentation is highly correlated with latitude, as you can see in the
- This was in the early 1900s, and is based on his study of pigmentation
- Pigmentation is correlated with latitude: people living in the equator tend to have darker
- And people living in higher latitudes, tend to have less melanin
Skin Pigmentation and Melanin
- Skin color is mainly due to the pigment melanin, which is produced by specialized
epidermal cells known as Melanocytes.
o Present in the skin, iris and hair
o In the skin, these cells (melanocyte) is where melanin is going to be synthesize
o Kerathinoxtyes is where the melanin passes through to go to other cells
A Review of Melanin Synthesis
- Within the melanocytes, melanin is synthesized in organelles known as melanosomes.
- Melanosomes are then transferred, through dendritic structures, from the melanocytes
to surrounding cells in the epidermis, known as keratinocytes.
- Tyrosinase is the key enzyme involved in melanin synthesis.
o This is the key enzymes (and there are more enzymes)
o This is the major enzyme that is involvedd in the synthesis of melanin in the
melanosomes - This is where melanin will be synthesis
- Melanin will protect the cells, as it is found in the nucleus in the cells
- Protects from ultra violet radiation
Skin Pigmentation Variation
- The number of melanocytes is similar in all human populations. Differences in skin
pigmentation are mainly due to:
- Type of melanin synthesized by the melanocytes
o Eumelanin: brown/black.
o Pheomelanin: red/yellow.
Red head synthesized pheomelanin in hair cells
- Number, size and distribution of melanosomes.
o In persons with light skin, melanosomes are small and packaged in groups.
o In dark-skinned persons, there are more melanosomes, which are larger and
packaged as a single entity.
The Evolution of Skin Pigmentation
- How can we explain the distribution of skin pigmentation in human populations, and
more particularly, the strong correlation of skin pigmentation and latitude?
- In this case, the selective factor seems to be UV-radiation:
o Populations living near the equator receive high doses of UV radiation and they
tend to have dark skin.
o Populations living far from the equator receive much lower doses of UV
radiation, and they show light skin color.
- Let’s review in more detail the evolutionary factors that could be involved.
Dark Skin Color and UV Radiation
- Many hypothesis have been put forward to explain the evolution of dark skin color in
equatorial and tropical environments with high UV radiation.
o Dark skin protects against skin cancer.
This is something evolutionary biologists are spectacle about because
people believe skin cancer happen relatively late in their life time where
passing on genes has already occurred.
o Dark skin protects against sunburn (severe sunburn can lead to infection and
interfere with an efficient sweating mechanism).
Severe sunburn can be bad in some situations
o Dark skin protects against folate destruction by UV radiation.
Folate is required for adequate fetus development, and also important in
reproduction (lack of folate can result in fetal defects and also male
Folate is an important component related to one of the vitamins has
important role in the synthesis of DNA. Can be destroyed by high UV
radiation. The major reason for this strong relationship is for the need to
have the protection because the protection of folate destruction
The Evolution of Light Skin Color
- When modern humans migrated out of Africa, they began to occupy regions with lower
- The protection provided by high melanin levels was no longer necessary.
o Less risk to skin cancer, sunburn and folate destruction.
- Additionally, a new selective factor also played an important role, vitamin D synthesis. o Melanin will interfere with the production of vitamin D
Light Skin Color and Vitamin D Synthesis
- Vitamin D is necessary for a proper absorption of calcium, which is important for bone
and teeth growth.
- In our body, vitamin D is synthesized through the action of UV radiation.
o Vitamin D is synthesized in the skin
- In environments with low UV radiation, dark skin would be harmful, because it would
interfere with proper production of vitamin D.
- Thus, there was likely a selective pressure for lighter skin pigmentation in regions far
from the equator, with low UV radiation.
- Current evidence indicates that the evolution of skin color has been driven by a balance
between protection against the harmful effects of UV in equatorial and tropical regions,
and the need to facilitate vitamin D synthesis in regions far from the equator.
- However, very little is known about the genetic basis of pigmentation variation and the
evolutionary history of the genes responsible for pigmentation.
o How many genes are involved?
o Which variants in each gene?
o Are the mutations different in different geographical regions?
Some Important Genes of the Pigmentary System
- Some of the genes are expressed in the melanamosomes