What is the Origin & Reason for Blue Eyes?

When did blue eyes first develop and for what reason? Now I have my own theory about why blue eyes develop, but I'll share that at the end. Firstly though, it does seem that all human beings initially had brown eyes. With brown eyes by far the most common eye colour still today, around 79% of the global population. They result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelength to be absorbed. Blue eyes are much less common however, with around 8-10% of the global population having blue eyes. They are most common in Europe, particularly Northern Europe, but they are found across the world, although many people that do have blue eyes around the world have ancestry that does connect to Europe. So this table shows the countries with the highest percentage of people that have blue eyes, and it does seem broadly accurate. At the top here we've got Estonia, and then we've got Finland, then we move down to Sweden, and Iceland at about 75%, and as we move down we can see Ireland in there, we can see Norway, we can see Scotland in there around 50%, and then we move down and you see United States at the bottom there, but obviously most of the countries here are from Europe, mostly from Northern Europe as well, which is quite interesting. Interestingly to note, there is actually no blue pigmentation either in the iris or in the vitreous body, as it is to do with how light is reflected. In people with blue eyes, longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same scattering that accounts for the blue appearance of the sky. Blue eyes however do have lower concentrations of melanin in the stroma of the iris compared to brown eyes. Now it does seem that blue eyes are more sensitive to light compared to darker eyes, but this perhaps means that people with blue eyes can actually see a This would make sense logically. I have blue eyes for instance, very light eyes, and I would definitely say my eyes are more sensitive compared to some of my friends that have darker eyes. It would therefore make sense that lighter eyes are essentially more efficient with light, and therefore are able to perform better in lower light conditions. This speaks to one potential theory about the evolutionary advantage that blue eyes may have given some of our ancestors, but more on that later. Firstly, let's look at the origin of blue eyes. The initial research came from a team from the University of Copenhagen in 2008 who located a single mutation that causes the phenomenon of blue eyes. The same DNA sequence of the OCA2 gene among blue-eyed people suggests that they may have come from a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in the Black Sea region sometime between 6,000 and 10,000 years ago during the Neolithic period. However, more recent ancient DNA research has identified human remains much older than the Neolithic period with the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele dates back to around 50,000 years ago, but is perhaps older than that and we just haven't found a sample yet. Interestingly to note as well, we know that Western hunter-gatherers largely seem to have blue eyes. This is different from Eastern hunter-gatherers who mostly had brown eyes. So now let's look at the reason for blue eyes. Well, there are numerous different theories out there that try and explain the evolution of blue eyes, and I have my own theory that I'm going to share just in a second. One theory is that blue eyes essentially allowed some of our ancestors to hunt better in lower light conditions, and potentially to defend themselves better in lower light conditions as well. This makes sense to some degree, but after the introduction of farming into Europe from Anatolia that reached the western islands of Britain and Ireland around 6,000 years ago, hunting essentially became less crucial with this change, and we would therefore not expect a dramatic decrease in blue eyes amongst the population that didn't appear to happen. Another theory is known as the sexual attractiveness theory, which argues that people may have considered blue eyes to be more attractive, and therefore people with blue eyes produced more offspring. This is difficult to prove however. A third theory is known as the vitamin D hypothesis, which is the idea that light-coloured skin, hair and eyes all co-evolved as humans moved into latitudes with shorter days, shorter summers and therefore less sunlight. A fourth theory is known as the seasonal depression coping theory. Some argue that people with blue eyes would have been able to withstand the dark days of the European winters better, and have more energy to hunt and do things, and this trait was selected for over time. Even today some research has shown that people with darker eyes tend to suffer more from seasonal depression during the winter months than people with lighter eyes. Now my theory incorporates a few of the elements from the different theories we've just heard there. I would definitely argue that blue eyes 100% evolved because of light, or to be more specific, because of the lack of light in certain parts of the world. I don't think it's coincidence that the countries that have the highest percentage of people that have blue eyes amongst the population also seem to have extremely low levels of light, particularly during the winter months. Now I live in Scotland and I can tell you during the winter months, the lack of light is brutal. On the day of the winter solstice last year, the shortest day of the year, which was the 22nd of December, the day length in Edinburgh was only 6.57 hours, just under 7 hours, with sunrise at 8.42 in the morning and sunset at 3.40 in the afternoon. And if the weather is bad during these short days, even the daylight period provides very little light. And Scotland isn't nearly as bad as parts of Scandinavia and places like Iceland, they get much less light during the winter. So as you can see, this table shows the cities in Europe by sunshine duration. And sorted for December, Norway's at the very top of this list, and it actually gets about zero hours of sunlight in December, which is pretty mad to think. And as we move down, Russia's very high on this list, Faroe Islands, other parts of Russia, Moscow and St. Petersburg, Estonia, Iceland, Latvia, Norway, and then into Lithuania, Estonia, the Baltic states, and then Belarus, Finland, Ukraine, Sweden. So if we return back to this map of light eye colour, there is a pretty strong correlation here. Now correlation doesn't equal causation, but it's interesting to know. And for me, the lack of light in the winter months was a strong selective pressure for blue eyes to become more common amongst the populations of Northern Europe. Let's not forget as well that your eyes are essentially part of your brain, they're just outside of your skull. I know it's a bit strange to think that way, but that's essentially the reality. Obviously your optic nerve goes all the way back into your brain, and your eyes are essentially a part of your brain. So this reality probably meant there was a very strong selective pressure on lighter eyes developing very quickly, in evolutionary terms at least, in Northern Europe, and perhaps quicker than lighter skin or lighter hair colour. But what's your theory on the reason for blue eyes? Please let me know in the comments below. What theory do you think is more plausible? Do you think it's also to do with light? Or do you think another theory, the sexual attractiveness theory, the vitamin D hypothesis, or perhaps the hunting theory, or a different theory that I've not covered in this video? Please let me know in the comments below. Speaking of blue eyes, the migration of the Bell Beaker people into Britain seemed to bring genes associated with a significant reduction in eye pigmentation, going on to largely replace a previous wave of migration into Britain by the first farmers who traced their ancestry back to the Anatolian region, and who were largely considered to have had brown eyes. This is one depiction of an early European farmer who brought farming into Europe from the Anatolian Near East region, and as you can see in this depiction at least, she has brown eyes, and in general early European farmers are considered to have darker eyes than the Western hunter-gatherers that were in Europe before them. But what was the genetic makeup of the Bell Beaker culture? To find out, please click here. Thanks for watching, please subscribe and hit the bell, and tell your friends and family about this channel. For ways to support, including through Patreon, all the links will be in the description below as well. Thanks again for watching, and I'll see you next time.