The blue eye mutation dates back to 6-10 thousand years ago when the main gene associated with the eye color, OCA2, underwent a mutation in a single human. Ever since that single event, no mutations have arisen in this gene and thus is considered most likely to be the cause of all the blue-eyed humans alive presently.
OCA2 gene, also known as the P gene, synthesizes a protein called P-protein in humans. It is located on the chromosome number 15. It, like other genes, has a set of instructions to code for a particular protein. Genes synthesize proteins which in turn play role in the expression of a character. Similarly, OCA2 gene (as mentioned before) synthesizes P- protein which is located in melanocytes. These cells produce melanin for the body use and function. Melanin protein is known to be responsible for the color of eye, skin and hair. This gene has also been found to be responsible for variation in the color of eye, for example: Production of large amounts of melanin cause brown eyes while little to zero melanin results in blue eyes.
Now the mutation! That occurred some thousand years ago leading to the emergence of people with blue eyes. According to a scientist, Prof. Eiberg and his studies, originally Homo sapiens had brown eyes. But due to a genetic mutation, the factors leading to brown eyes were typically "turned off" in the OCA2 gene. However, this mutation was specific and limited to the iris only. It didn't change the color of skin or hair. In brief, the mutation caused the gene OCA2 to limit the production of melanin in iris only- not effecting other body parts.
On the basis of recent studies, it's been clearly pointed out that the locus of OCA2 gene plays a key role in expression of eye color. Scientists mapped the genome of a Danish family, with OCA2 locus and a 166 Kbp region within the HERC2 gene. By association analyses, they tracked down the two SNPs responsible for the expression of blue and brown eyes, namely rs12913832 and rs1129038. In comparison to the blue eye allele of rs12913832, the brown eye allele of rs1129038 was found to be highly conserved throughout a number a number of species. Moreover, there is a conserved region too around the rs12913832 which regulates the expression of OCA2. Also, the C allele present at rs12913832 limits the expression of OCA2 in iris. Successive experiments led to the conclusion that due to a mutation in HERC2 gene, the expression of OCA2 gene changes causing it to limit it's expression in melanin production in iris.
Alteration in OCA2 gene are known to have severe effects on pigmentation of eye, hair and skin. Deletion of OCA2 gene in Prader-Willi and Angelman syndromes lead to hypo-pigmentation of skin, hair and eyes, while duplication or presence of extra copies of OCA2 can lead to hyper-pigmentation in skin, as observed. Mutation in the locus of OCA2 gene can cause oculocutaneous albinism, a medical condition in which eyes and skin are severely affected by albinism.
There is a huge variation in eye color that is spread all over the population. From brown to green eye color, this variation can be understood and explained on the basis of different amounts of melanin present in the respective individuals. However the individuals having blue eyes have melanin in small amounts and also, the degree of variation is found to be very low. From this thorough explanation, it was concluded that since the variation is significantly low in blue eyed individuals, they have inherited from the same ancestor. Also, the mutation was acquired during a same period of time from a same ancestor.
Scientists further point out that though this discovery is a significant one in the area of genetics and genomics but the mutation has neither a good nor a harmful effect. It's just another mutation shuffling the whole gene pool of humans. Similar mutations have also been discovered in past too, like the appearance of beauty marks etc. Such mutation aren't positive or negative by nature but do contribute to the whole complex genomic understanding that exists already. Discoveries like this are surely providing a whole new insight in the "yet-un-examined" genomic regions and leading scientists to explore and examine the genome more accurately.
All the blue-eyed individuals, thus, have been originated from a single parent ancestor and it would not be exaggerating the statement but pointing out the fact that this recent discovery has only contributed to the genetic cocktail of traits and would continue to do so.
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