“My, what beautiful blue eyes you have!”
It looks like a single common ancestor is to thank for blue eyes. Researchers have roughly determined the origin of blue eyes to have arisen from a single common ancestor nearly 10,000 years ago. A mutation caused a change in the way the OCA2 gene is expressed phenotypically. For whatever evolutionary advantages, this mutated gene spread rapidly throughout norther European populations.
How Does the Blue-Eyed Trait Operate?
The way the blue-eyed trait works is by acting like a switch that turns off the ability to produce the full range of colors necessary to give the iris a brown hue. The blue-eyed trait does not turn off the exclusive ability to produce color, otherwise this would result in characteristics associated with albinism. Instead, the blue-eyed trait limits the production of melanin needed for brown eyes and produces Eumelanin. Blue eyes are in fact brown eyes with less melanin.
Genetically Speaking, What Does it Mean?
This single blue-eyed mutation spawned from a common ancestor and has been the foundation for all individuals with blue eyes. Eye color is a polygenetic phenotypic character meaning that multiple genes are responsible for a single trait. The snippet of DNA responsible for eye-color is closely linked to pigment regulation.
People living in northern Europe may have triggered this mutation by favoring light skin – a phenotypic condition regarding a lower production of melanin. Since the blue-eyed trait is the result of less pigmentation, it may have arisen in response to the higher survival rates of fair-skinned people in areas lacking sunlight. Since Vitamin D is crucial to human life, pale skin absorbs sunlight more readily because the receptors are not blocked by thick levels of melanin.
In the context of evolution, the blue-eyed trait may have been attached to segments of DNA controlling skin-pigmentation. As human populations settled in northern Europe, they needed to adapt to a cloudier climate, and as lighter skinned people prevailed in these regions, the blue-eyed mutation may have tagged along.