The Science of Blue Eyes
How do you tell if you’re related to the famous blue-eyed Julius Caesar? Check the mirror.
For thousands of years, people have been fascinated by eyes. A famous maxim calls the eyes the windows to the soul, and it is no question why: eyes connect invisible feelings to tangible expressions. But throughout literature and culture, blue eyes in particular are considered special (Gibbons, 2014). Recent research has cast a new light on the origins of blue eyes, suggesting that nearly all blue-eyed people share a common ancestor who lived in the Stone Age (Weise, 2008).
Hans Eiberg began blue-eye research in 1973 at Copenhagen University. The geneticist studied a Danish family with 17 children who carried both the blue and brown eye phenotypes (traits). Eventually, he was able to pinpoint the region near the OCA2 gene as a chief suspect for the difference. Eiberg expanded his studies to include blue-eyed individuals from Turkey, Scandinavia, Jordan, and India. He sequenced these individuals’ genomes and compared the OCA2 regions—and shockingly, found a genetic marker that was identical in 97% of the individuals tested. Individuals from Denmark shared the same haplotype of individuals far away as Jordan (Eiberg et al., 2008). Haplotypes, specific sequences of a gene, are sometimes used to sort lineages. Since genes are shuffled during reproduction to form new haplotypes, this finding suggested that blue eyes first appeared in relatively recent history, possibly 6,000 to 10,000 years ago.
The sudden appearance of a new phenotype signals that it was brought about by a random DNA mutation, probably due to an error made during DNA replication. Mutations are usually detected and repaired by mechanisms within the pre-mitotic cell, but sometimes mistakes go by unnoticed. When this happens in a gamete, a sex cell, it can result in an organism with a mutated phenotype. In this case, blue eyes were the mutation, an unsettling aberration in a world with only brown eyes. While it is not possible to tell how humans first reacted to this development, blue-eyed individuals must have survived to pass on the mutation to their posterity—us. But who was the first human being to have blue eyes?
The best candidate, found in Spain in 2006, lived around 7,000 years ago. By sequencing DNA fragments extracted from the Mesolithic man’s tooth, scientists discovered that this middle-aged man displayed a medley of unexpected characteristics—including blue eyes and dark skin. The man possessed a mutation in the HERC2 gene almost identical to that of modern humans with blue eyes (Olalde et al., 2014).
Eiberg postulated that the HERC2 gene regulates the activity of the nearby OCA2 promoter. (Eiberg et al., 2008). The point mutation is just enough to ‘switch off’ the transcription of the OCA2 gene, dampening its effects. OCA2 is involved in producing melanin, a pigment that absorbs light, determining the colors reflected from the living cell. In normal concentrations, melanin forms a thick deposit in the stroma, the outer layer of the eye. Since there is less melanin in blue eyes, they reflect more light, creating a blue appearance (Gunston, 1987).
So why do we still have blue eyes today?
Any given mutation is able to continue based on two factors: chance and advantageousness. Scientists speculate that both may have been on the side of blue eyes. A founder effect could have occurred when blue-eyed individuals migrated from the Black Sea to Northern Europe, thus increasing the frequency of blue eyes in the new population. Another idea is that blue eyes improved mating prospects. Dr. Eiberg reflected, “Blue eyes are seen as powerful. I have recently been in China and in the temples there all the gods are painted with blue eyes, but everyone else has dark eyes. There is something attractive about blue eyes, maybe because they used to be so rare” (Gibbons, 2014).
Indeed, over time blue eyes became the symbol of cultural supremacy. As fair-skinned Europeans rose from the Middle Ages in the Renaissance and Age of Exploration, this trait would mark the western presence. Everywhere Europeans journeyed, blue eyes were introduced into the population. Even in India, the vestiges of British colonialism are still seen in blue-eyed, dark-skinned individuals. In the twentieth century blue eyes were a defining characteristic of Hitler’s ‘Aryan’ race, a key part of his quest to ‘purify’ Germany. After the war ended, blue eyes became a commercial standard of beauty, popularized by the hit toy Barbie (Rickett, 2014).
The fascination with baby blues hasn’t abated. A common misconception that blue eyes have weaker vision was countered by the World War II observation that a “remarkably” large proportion of Britain’s best fighter pilots were blue-eyed. Several ophthalmologists have similarly noted that blue is the most common eye color among navy officers (Gunston, 1987). But some researchers speculate that, as a part of the genome, eye color is intrinsically related to brain function and personality. Traditionally, blue eyes are often viewed as cold, sharp, or pure. According to Drs. John Glover and A.L. Gary, people with blue eyes are better listeners and are more considerate and patient (Gunston, 1987).
Without a doubt, blue eyes have shaped culture for thousands of years. And they will continue to. Myths abound that blue eyes will disappear due to their recessive nature—but the opposite is true. The mutation, whether masked or unmasked, will continue to be passed down to our descendants. From 10,000 years ago to today, blue eyes have captivated people across the world. And not much will change. Blue eyes are here to stay.
This article originally appeared in the 2017 issue of Hybrid Vigor magazine.
Eiberg, H., Troelsen, J., Nielsen, M., Mikkelsen, A., Mengel-From, J., Kjaer, K. W., & Hansen, L. (2008). Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression. Human Genetics, 123(2), 177-187. doi:10.1007/s00439-007-0460-x
Gibbons, K. (2014, August 30). Blue eyes are peeping across Britain; The top colour used to be brown but scientists have found that a new look is developing, reports Katie Gibbons. The Times, p. 3.
Gunston, D. (1987, August 4). Blue Eyes: Facts and myths. The Advertiser.
Olalde, I., Allentoft, M. E., Sánchez-Quinto, F., Santpere, G., Chiang, C. W., Degiorgio, M., Prado-Martinez, J., Rodriguez, J., Rasmussen, S., Quilez, J., Ramirez, O., Marigorta, U., Fernandez-Callejo, M., Prada, M., Encinas, J., Nielson, R., Netea, M., Novembre, J., Sturm, R., Sabeti, P., Marquès-Bonet, T, Navarro, A., Willerslev, E., Lalueza-Fox, C. (2014). Derived immune and ancestral pigmentation alleles in a 7,000-year-old Mesolithic European. Nature, 507(7491), 225-228. doi:10.1038/nature12960
Rickett, O. (2014, January 27). Why are blue eyes so fascinating? The Guardian.
Weise, E. (2008, February 6). More than meets the blue eye: You may all be related; Trait linked to single gene mutation. USA Today, p. 7D.