We focus quite a bit on eye color. People find certain eye colors more attractive than others. Whether or not you can remember someone’s eye color is used as a gauge of how well you know them (much to the chagrin of men). In short, we have come to consider variation in eye color as an important part of who we are. Thus, it’s hard to imagine a time when everyone had the same color eyes: brown.
That’s the image a group of researchers from the University of Copenhagen are asking us to conjure up. They have recently reported finding a genetic mutation that occurred 6,000-10,000 years ago, which resulted in the first pair of blue eyes. The mutation, they assert, affected a gene called OCA2. This gene encodes for a protein involved in the production of melanin, a polymer responsible for the pigmentation of our skin, hair, and eyes. The mutation causes less melanin to be produced in the iris. This, according to the research group, is what caused eyes to become diluted from a universal brown, to blue.
How the heck could they know this, you might ask? The answer is by studying mitochondrial DNA (mtDNA). Every cell in our body contains DNA within its nucleus. This nuclear DNA is the type most of us are familiar with, and the type used for paternity testing, forensics, disease screening, etc. But mtDNA is a less publicized form of DNA that resides in the mitochondria, organelles outside the nucleus that provide energy for the cell (remember the “powerhouse of the cell” from grade school?). When two people procreate, their nuclear DNA is mixed, resulting in offspring that possess a combination of their genetic codes. The mtDNA from the father, however, never makes it into the egg with the sperm. It is left behind, and only the mother’s mtDNA is passed on. Thus, one’s mtDNA represents their pure line of maternal ancestry.
Geneticists can use mtDNA to obtain information about that ancestry back through many generations. To do this, they examine the mtDNA and compare it with samples from other individuals. They can then create a “family tree” of mtDNA. This was how scientists inferred the most recent common ancestor of humans was a woman from Africa (often referred to as “Mitochondrial Eve”).
The team from the University of Copenhagen studied the mtDNA of a group of 800 people from all over the world to estimate the time of the eye color mutation. A member of the team, Hans Eiberg, described the mutation as neither positive nor negative. This means it didn’t create a selective advantage or deficiency, but was simply the result of genetic shuffling. To me this can’t be said with conviction, however, as blue eyes may have possessed an advantage in sexual selection (meaning they were found attractive, leading to the trait being passed down more often through procreation). Or maybe I’m just inclined to think that way because I have blue eyes…