I have blue eyes, not terribly unusual, but apparently of an interesting origin. According to this article all people with blue eyes have a single ancestor. A mutation arose in that ancestor some time in the past 6,000 to 10,000 years. The mutation occurs in a gene adjacent to the OCA2 gene which regulates the production of melanin. Apparently in the iris the mutation “slows down” the OCA2 gene leading to relatively less melanin and thuse the blue shade of the iris.
The study tracked down this mutation across a wide population and through analysis of the long haplotypes concluded when the mutation occurred (obviously only approximately). The study only covered people from Jordan to Denmark but for me it raises the question of how these mutation spread, i.e. how related are the people from these regions over only such a short period. The study humorously concludes that the fixing of the mutation in 20-30% of the population must be due to mates finding blue eyes more attractive (although, WHY, would be an interesting question for evolutionary psychology). But all this does indicate a lot more mixing up of the populations of Europe and the near Middle East than I’d previously thought was the general prediction.
Now ‘ancestor’ is one of those misleading thing. It doesn’t mean all the people with blue eyes are my cousin. AFAIK the mutation is recessive, so my dad, who had blue eyes had two copies, and my mother, who didn’t, had one copy, and thus I managed to get the mutation from both parents. But let’s think about ancestors. For convenience let’s assume three generations per hundred years and assume the most recent date for emergence of this mutation, i.e. 6000 years, or 60 centuries, which means 240 generations ago. That nominally means I have 2^240 ancestors, one of which includes that original individual with the first instance of the mutation. I’ve seen estimates that the total number of humans that have ever existed is around 100 billion (let’s use 64 billion for convenience) which is 2^36 people. That means vast numbers of my possible 2^240 “unique” ancestors are in fact the same person but one thing is certain, all my actual ancestors had at least one copy of the mutation so that eventually I’d end up with two copies.