The vast majority of people have brown eyes. But have you ever wondered why some (maybe even yourself) have blue eyes?
The story is fascinating.
There is strong evidence to suggest that everyone with blue eyes – be it Frank Sinatra, Daniel Craig, or any one of the millions of blue-eyed people – can trace their ancestery back to a single person.
Scientists at the University of Copenhagen have been studying the genetics of eye colour. Their findings suggest that sometime between 6-10,000 years ago, something unusual happened – someone was born with blue eyes. And from this one person, blue eyes have been passed down over thousands of years.
Originally, everyone had brown eyes. This is because of a gene known as OCA2, which produces melanin in a person’s iris (the coloured area of the eye). Melanin is a dark brown pigment, which also determines the colour of a person’s hair and skin. It’s because of melanin that people have brown eyes.
But there’s another substance in the eye called collogen, which is blue. When an eye has a very low level of melanin, it will be blue.
But then thousands of years ago, a genetic mutation took place. Effectively, this mutation switched off the OCA2 gene, preventing it from producing enough melanin for brown eyes. Though genetic mutation is a fundamental part of the theory of evolution and the survival of the fittest – it doesn’t always provide an advantage or disadvantage. The presence of blue eyes rather than brown would not have left the ancestor at a disadvantage, so they were able to pass the switched-off OCA2 gene on to their descendants.
One question which of course arises from this study is “how can we know that this is all because of one person?”.
In people with brown eyes, there is a large amount of variation in the DNA sequence which is responsible for eye colour. However, blue-eyed people have very little variation in the same DNA sequence. If blue eyes had occurred in a number of locations, and not originated from one person, there would be significantly more variations in DNA. Instead, the mutations in the section of DNA which controls eye colour are identical in almost every blue-eyed person.
Despite this discovery, there will always be an air of mystery to those with blue eyes. We still don’t know – and probably never will – why blue eyes have become so prominent in some countries. In some Scandinavian countries, the proportion of blue-eyed people is as high as 95%. Many people can take a guess at the reason, but but it’s unlikely we’ll ever know for sure. One secret may have been revealed, but many still remain.
Jon lives in the UK and likes to write about science. He works for Lenstore.co.uk, an online retailer of prescription and coloured contact lenses from leading brands such as Acuvue and Freshlook.