Blue eyes have been around for at least 7,000 years but we still don't know exactly why they evolved.

Brad Pitt has them, Paul Newman had them — but when it comes to the human population as a whole, blue eyes are not that common.

Experts are not sure when blue eyes first evolved, but there are some interesting theories out there as to why they evolved.

In Africa dark eyes, skin and hair are the norm, but blue eyes are more common in southern Europe and even more common in northern Europe, where 70 per cent of people have blue eyes.

This gradient gave rise to the 'vitamin D hypothesis', which is the idea that light coloured skin, hair and eyes co-evolved as humans moved into latitudes where shorter days and summers meant they got less sunlight.

But, there's a problem with this idea, says molecular geneticist Associate Professor Rick Sturm of the University of Queensland.

There is no evidence that light-coloured irises let in more light or help you see better in low light than dark coloured irises.

More importantly, there is evidence that blue eyes evolved before light skin — at least 7000 years ago.

In 2014, Sturm and colleagues reported on ancient DNA from a 7000-year-old tooth belonging to a hunter gatherer dubbed La Brana 1, unearthed from the north-west of Spain.

His genes told them that while this man had dark skin and dark hair, he also had blue eyes.

"This individual had light blue eyes but dark skin and that was the great surprise because we always though these things were co-evolving and we expected light skin to evolve first," says Sturm.

Such archaeological evidence contradicts the idea that the need for sunlight to make vitamin D drove the evolution of blue eyes, along with light-coloured skin.

So scientists have come up with a range of other hypotheses to explain the evolution of blue eyes, including the idea that they were more sexually attractive than brown eyes — "The Paul Newman effect," quips Sturm.

There's also the idea that blue eyes were advantageous because they perceive stationary objects better than moving things. This could have been an advantage to hunter gatherer women who needed to identify and collect plant foods — indeed blue eyes may even have evolved in women first.

But Sturm has another idea. He says blue eyes have been linked to people coping better with seasonal affective disorder, a major depressive illness that occurs when there are long periods of low light.

Notably, he says, the eye has special neurones in the retina that can detect blue light and use this to help regulate circadian rhythms.

"Perhaps those with blue eyes may have been able to withstand the dark, depressing days of the Neolithic European winters better than those with brown eye colour?"

"They may have been actually active enough to go out hunting while all the rest were sitting in the cave depressed."

Contrary to what we might have once learned in school, it is possible for two blue-eyed parents to have a brown-eyed child, says Sturm.

This is because there are a lot of different factors that determine eye colour.

First there are genes that control the amount of melanin pigment in our irises — that's the part of our anatomy that acts like an aperture on a camera to control the amount of light that gets into the eye.

Around 74 per cent of our eye colour can be put down to a gene called OCA2 on chromosome 15. This gene also contributes to hair and skin colour, but to a much lesser extent.

In 2008, Sturm and colleagues mapped different genes associated with blue and brown eyes.

"What we're seeing with blue eye colour is a lack of melanin in the outer layer of the iris."

A variation in the HERC2 gene, which sits right next to the OCA2 gene controls whether melanin is produced in the outer layer of the iris.

"It's like turning a light switch on or off. If you turn the switch on the eyes will be brown. If you turn it off they become blue," he says.

Then there are genes that control the structure of the iris, such as it thickness and how much collagen it contains.

Eye colour is also influenced by the way light interacts with our irises.

Associate Professor Rick Sturm of the University of Queensland spoke with Anna Salleh