Europeans have white skins and Africans black. The rest of the people around the world have intermediate skin colours between these two extremes. What determines skin colour and how the different shades have evolved in human beings or the homo sapiens?
Colour of the skin is caused by a pigment called melanin, which is an effective absorbent of light. It can dissipate 99.9% of absorbed UV radiation, which may otherwise damage the skin with propensity to cause cancer. Melanin is produced by melanocytes, the skin cells located in the epidermal layer of the skin also in the matrix of hair.
Melanocytes transfer melanin to skin epidermal cells through thin cellular projections.
There are two types of melanin, eumelanin and pheomelanin. Eumelanin is brown-black in colour and pheomelanin has red to pink hue. Presence of more eumelanin in the skin makes it darker and more pheomelanin make the skin lighter in colour.
The modern humans evolved from their ancestors, who walked on all fours and had hair covering their bodies.
When humans shed their body hair and started walking straight, their skins got exposed to the ultra violet radiation which comes with sunlight. Skin pigmentation evolved about 1.5 million years ago to protect it from deleterious effects of UV light along with sweat glands to keep the body cool while they ran to kill their prey or protect themselves from any attack (Gurche, Smithsonian magazine, 2010).Original skin colour of humans was probably black.
There are two theories to explain human origin and migration to different parts of the world based on archaeological and fossil records. Multi-regional hypothesis states that modern human beings evolved from their ancestors in several different locations.
The second, ‘Out of Africa theory’ states that modern humans developed in tropical Africa for several thousands of years before they spread throughout the rest of the world.
The reason for their migration is attributed to a mega drought, which occurred 135,000 to 90,000 years ago, when fresh water lakes, around which they had settled dried up (University of Arizona report, 2007).
A new tool, analysis of DNA of living human population, has been used by geneticists in1980s to trace back lineage of humans backward in time. This analysis showed that the modern humans (Homo sapiens) developed in Africa around 200,000 years ago.
Another genetic study with mitochondrial DNA, which is inherited only from mother also confirmed this hypothesis.
More sophisticated studies using the nuclear DNA from 935 people from 51 locations around the world support the African origin hypothesis (Guy Gugliotta, Smithsonian Magazine, July 2008).
Original skin colours of humans living in tropical Africa were probably black to protect them from injurious UV radiations. When they migrated northward, they received less UV radiation from sunlight leading to de-pigmentation of their skins.
Though injurious, UV radiation is also required to synthesize vitamin D in the skin, essential for human metabolism. Natural selection favoured two genetic solutions to this problem, i)Evolving pale skin that absorbs UV more efficiently and ii) Lactose tolerance to be able to digest the sugars and vitamin D naturally found in milk.
An international team of researchers examined genomes of 83 ancient individuals from archaeological sites of Europe and concluded that the Europeans today are a mix of at least three races of population who migrated to Europe in separate conglomerates over the past 8,000 years.
According to Ann Gibbons (2015) the modern Europeans do not look much like the early settlers. The early hunter gatherers of Spain, Hungary and Luxemburg had darker skins.
The modern Europeans have two fairness skin genes, SLAC24A5 and SLC45A2, which lead to depigmentation and pale skins.
When farmers from near east arrived in Europe 7,800 years ago, they transferred their fairness skin gene SLC24A5 to the Europeans through interbreeding. About 5800 years ago the other fairness gene SLC45A2 was also transferred and it spread throughout Europe.
About 4,500 years ago, massive migration took place from the steppes north of the Black sea. These people brought the Indo-European language to Europe. The early settlers could not digest milk, specifically the milk sugar, lactose, since they did not have the LCT gene, which enables humans to digest milk. They acquired this gene about 4,300 years ago and it spread quickly around Europe.
The people inhabiting the northern Europe, which receives less light, had fairer skins.
The DNA of seven people living in Motala archaeological site, 7,700 years ago in southern Sweden had the two gene variants SLC24A5 and SLC45A2, which cause white skins. They also had a third gene, HERC2/OCR2 which cause blue eyes and fair skin.
Exclusivity of Europeans not to intermingle with coloured races for generations has preserved their genetic integrity and skin colour.
(Dr Mitra is a former Dean and Professor, Orissa University of Agriculture and Technology. Email: email@example.com)