The cliche goes that the eyes are the window to the soul. Eye colour is often the first thing that anyone notices about someone they meet.
There is a spectrum of striking colours, with some, like blue, sought after by many due to its ‘celebrity’ appeal.
Then there are the rare colours, like green, which just two percent of the world’s population possesses.
Yet, there is an altogether rarer colour, one that doesn’t exactly exist today but which all ancient humans had — the colour black.
It’s quite difficult, perhaps even impossible to imagine a person with black eyes.
We’re most used to seeing black eyes in horror films, almost always given to those who have some sort of demon possessing them.
But wind back the clock at least 250,000 years ago, and black and extremely dark brown eyes were the dominant colour.
When humans arose in the Horn of Africa, OCA2, a gene known as P protein which transports a certain kind of protein which produces pigment called melanin, was expressed at high levels.
It led to higher production and volume of melanin — the amino acid that produces hair, eye and skin colour — which in itself led to a side-effect of nearly black eyes.
As an evolutionary process, darker skin was less likely to be sunburned or develop adverse skin diseases, which helped the survival of ancient humans in Central Africa’s sunny, equatorial climate.
When humans started to migrate out of Africa, somewhere between 50,000 and 70,000 years ago, the biological processes that drove up higher melanin production gradually began to fade.
When they pushed to the far north, into Europe, thousands of years of evolution found that having lighter skin was more advantageous in a land where sunlight was sparser than in Africa.
Lighter skin helped the absorption of more vitamin D from sunlight, in the process producing less melanin in the body which affected how eye colour mutated.
From this came into existence an array of different colours, particularly blue, which is believed to have first occurred along the Black Sea.
Despite evolution having taken its course, millions of people still have the OCA2 gene in their systems, around one person in every 36,000 white Europeans, and one person in every 3,900 to 10,000 Africans.
Scientists believe having blue eyes may help to regulate circadian rhythms — the body’s internal 24-hour clock — which makes them particularly helpful in higher latitudes where hours of sunlight differ drastically with the various seasons.
While it wouldn’t serve evolution in any way, as humans grow ever more diverse and settle in places far from their native homes, new eye colours could emerge.
Some people have actually experienced the process of their eyes and hair changing colour as they get older — and not just to grey.
Evidence suggests that a baby’s eyes changing colour depends a lot on the colour itself, with those born with blue eyes more likely to lose their colour to brown.
Change isn’t confined to infants, either. Injury can lead to a shock to the system which responds by changing the colour of the eyes.
Perhaps the most famous example of this is David Bowie, whose striking profile showed a dark left eye and a pale blue right eye, the result of a punch to the head that left his left pupil permanently dilated.
More commonly, infection is the root cause of the change, with things like heterochromia — present in actress Mila Kunis — destroying the pigmentation in one or both eyes.
