Blue eyes first appeared somewhere in the European tundra about 10,000 years ago as a simple mutation in a gene. Assuming that only those mutations that are advantageous—or at least not disadvantageous—survive, the question arises: what evolutionary advantages must blue eyes have had to survive to this day? Especially as more cons than pros have been identified so far.
Various hypotheses have been put forward to answer this question. Some have suggested that their success was a consequence of greater sex appeal, or that blue eyes were simply a side effect of having light skin and hair, which would be an advantage in these latitudes when it comes to capturing the UV solar radiation needed to produce vitamin D.
Now, according to a new study by researchers at Liverpool John Moores University, the key may lie in the combination of an a priori evolutionary disadvantage and its prevalence in Nordic countries.
The mission of melanin
Eye colour is determined by the amount of melanin present in the iris. Melanin is a photoprotective pigment that absorbs solar radiation (including potentially harmful UV radiation). So the blue colour of the eyes is actually the result of a lack of pigmentation, or to be more precise, a very small amount of melanin in the iris, where it is widely dispersed. There is not enough to darken the eye, but enough to scatter the incident radiation and make it appear blue, in the same way that the molecules in the atmosphere scatter the incoming sunlight and colour the sky blue. It is therefore a structural colour, not a chemical or pigmentary one.
But in reality, the main purpose of melanin in the iris is different. In dark eyes, the abundance of this pigment prevents light from reaching the retina beyond that which enters through the pupil. In blue eyes, however, the lack of melanin allows some of the incident light to pass through the successive layers of tissue until it reaches the back of the eye, the projection screen, in the form of stray light. This is unwanted extra illumination that not only damages the eye’s tissues, but also degrades the quality of the image by creating glare and, most importantly, masking the contrast. One way of seeing this is to notice how difficult it is to make out the picture on a television when the light is falling more or less directly on the screen (which is why it’s not a good idea to have your TV facing a window).
This is how it works in sufficiently bright conditions. But what if this apparent disadvantage turns out to be an advantage in dim light? This is the conclusion of the new study, which postulates that blue eyes improve visual acuity in low-light conditions precisely because of the “stray” light that passes through them—conditions that prevail for much of the year in the northernmost countries.
Two ideas for dim light experiments
However, given that the study was carried out on a small group (40 subjects), experts point out that the conclusions reached, although plausible and meaningful, will need to be confirmed with new studies…
… or with experiments such as the one described below, which consists of putting yourself in a darkened room, with the door ajar and the hallway light on. The idea is to find out from what point (using the angle of the open door as a measure) you can read a newspaper article or the pages of a book held at the usual distance or on a table while sitting down. And then compare your results with those of friends and/or family members to see if people with blue eyes have better visual acuity.
Perhaps even more interesting is another aspect of this research: several studies have linked blue eyes with a reduced ability to respond to external stimuli (visual, sound, etc.). The explanation is that the colour of the iris is thought to be an indicator of melanin synthesised in other parts of the body, in this case neuromelanin, a molecule involved in the speed at which nerve impulses are transmitted. But this would seem to be an evolutionary disadvantage in the struggle for survival…
… which brings us back to the more boreal regions and their dim light for much of the year. And whether, under these conditions, higher visual acuity makes up for, or even outweighs, the slower speed of nerve signal transmission. In other words, whether the ability to detect a visual stimulus earlier compensates for a slower reaction speed. To test this, you can do a second experiment in the same darkened room, but with another person standing in front of you, both of you with your arms outstretched, one of you holding a coin or something similar, ready to release it at any moment.
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