• Skin colour: One of the human race's most defining signatures of identity (Illustration by Matt Roden for SBS) (SBS)
We’ve treated skin colour as one of our most defining signifiers of identity for centuries even though, at a genetic level, we’re actually all very similar.
By
Illustrations: Matt Roden

27 May 2016 - 9:58 AM  UPDATED 27 May 2016 - 10:10 AM

So if we are genetically so similar, then why do we have different skin colour?

 

Below are two of the most popular scientific explanations to date:

 

THEORY ONE: If we live close to the equator, we’ll have darker skin

 

UV exposure has been advanced as the adaptation that explains current skin colour distribution. Skin colour distribution is non-random, meaning that the darkest skinned people tend to live in the tropics closer to the equator, while lighter skinned people reside in higher latitudes closer to the north and south poles.

 

So how has this hypothesis been backed up?

 

The skin colour of early forms of man (not humans) would have been white but covered in hair, much like the great apes.

 

Where skin is habitually exposed then primates do have darker skin, while less exposed areas, such as the palms of the hand, remain lighter.  

 

As man evolved into a large brained hunter-gatherer who walked on two feet, there would have been selective pressure for an efficient body cooling system.

 

The response is seen in the evolution of specialised sweat glands and the loss of hair from most of the skin enabling heat to be lost by sweating and evaporation. However, exposed skin is at risk of UV radiation damage.

 

Melanin, which is the pigment in the skin that gives it a darker colour, protects us against UV damage by:

 

a) Preventing UV entering beyond the skin’s surface by absorbing it; and

 

b) It also absorbs the chemical products of UV damage.

 

If this did not occur, these products could be toxic and carcinogenic elsewhere in body.

 

Melanin is a substance that gives us our skin, hair and even eye colour. Variation in the level of skin colour results from differences in individual levels of melanin in our skin cells.

 

Importantly these differences are found at birth, but some change in skin colour can occur after exposure to UV from the sun - tanning.

So under this hypothesis:

  • The immediate ancestors of all humans would have been dark skinned and so we have to explain why some human groups have lost dark pigmentation.
  • UVR effects are clearly damaging to skin. 
  • Sunburn is very painful and can be lethal - therefore light skin is a disadvantage.
  • UVR causes nutrient degradation in the skin cells. Micronutrients, esp. flavins and folate, are very sensitive to UV and lots of melanin would protect against this damage (so darker skin would protect against this damage)

 

But there are some ideas stacked against this hypothesis.

Catch up on DNA Nation: Episode 1
After having their DNA tested, Ian Thorpe, Ernie Dingo and Julia Zemiro set off on an epic journey of genetic time travel to find out where they, and the rest of humankind, come from.

 

The UVR hypothesis of explaining skin colour differences is as follows:

 

As darkly pigmented humans moved from the tropics (Africa) to higher latitudes, there would have been a need for them to adequately synthesise vitamin D in these low UV regions.

 

Under Darwin’s theory of Natural Selection, over time, those with lighter skins would be strongly selected as they have a reproductive advantage and their genes would predominate in those populations and the genes for dark skin would reduce and then disappear.

 

Why would they have a reproductive advantage?

 

Having enough Vitamin D is crucial to a healthy life. Vitamin D is essential for bone growth for when babies are still in the womb, as well as for young children.

 

UV is vital for our bodies to synthesise vitamin D, and in most people, the majority of vitamin D comes from the action of UVR on the steroid alcohols, or sterols, that occur naturally in the skin.

 

Insufficient vitamin D results in bone malformations such as rickets in children and osteomalacia in adults. Pelvic deformities as a result of vitamin D deficiency in women are a major risk in childbirth. Also, it seems vitamin D may play other roles such as in resistance to disease and infection.

 

This apparent link between skin colour distribution and vitamin D absorption due to UV strength is used to support this hypothesis. However, it is important to understand that there have been cultural responses to any possible deficiency. For example, vitamin D can be obtained from some foods – fish oils, liver and egg yolk, so the hypothesis isn’t bulletproof.

 

The genetic evidence confirms there is evidence for natural selection acting on skin colour types. Perhaps, the strongest evidence comes from Europeans in whom a single genetic variant accounts for a considerable component of variation in their skin colour as it is associated with paler skin and in some European groups the variant is fixed (i.e. 100%). The genetic evidence also suggests that depigmentation in Europeans is quite recent. All Europeans are actually descended from darker-skinned people with the occurrence of lighter-toned skin only being dated back as far as the Bronze Age (around 3000 BC). This fits in with the story of  the Yamnaya pastoralists, featured on the TV documentary DNA Nation, moving into Europe from the Near East during the Bronze Age.  

 

However, this  genetic variant does not appear to play the same role in Asian or other lighter pigmented groups, so other genes are clearly involved in explaining the lighter pigmented skin in world populations.

 

So, basically, different groups of lighter-skinned people have different genetic reasons for the colour of their skin. This means that UV exposure may not be the only explanation.

 

Clearly, from the story about the identical twins being of different skin colour there are other factors, genetic and non-genetic ones that can influence an individual’s skin colour at birth.

 

THEORY TWO: FARMING

 

Given the evidence that lighter skin may have only evolved as few as 1,000 years ago during the Bronze Age, there is a hypothesis that it’s due to the rise of farming and agriculture.

 

The idea is that the change in our diets was based on no vitamin D in our food: we were eating more cereals and grains – possibly less fish oils, liver and egg yolk – and so our skin became lighter to compensate and synthesise more Vitamin D from the sun's UV rays.

 

Under this hypothesis, our diet - plus the route some humans took across Europe, and also UV, are all interrelated in the processes that resulted in some of us having lighter skin.

 

However, this does not explain what happened to the other groups of our ancestors who travelled across other parts of the world.

 

We must not let what we see possibly occurring in Europe determine what happened elsewhere.

 

The genetic data do suggest that skin depigmentation in Europeans occurred relatively quickly but we have no equivalent data for other human groups yet. There is still more research to be done.

 

Dr John Mitchell is a human geneticist who is also the series consultant for DNA Nation and will act as a guide to intrepid adventurers Ernie Dingo, Julia Zemiro and Ian Thorpe throughout the show.

DNA Nation airs Sundays, 8.30pm on SBS.

Watch a sneak peek of the upcoming leg of Ernie's journey for DNA Nation below:

 

Read more from DNA Nation:
Catch up on DNA Nation: Episode 1
After having their DNA tested, Ian Thorpe, Ernie Dingo and Julia Zemiro set off on an epic journey of genetic time travel to find out where they, and the rest of humankind, come from.
Science confirms how we all really are descended from ‘Eve’
It turns out that DNA science supports the idea that we all really do share the same ancient mother. Here’s why everyone on earth is actually linked back to this one woman in Africa.
DNA Nation: Meet the famous Australians journeying to uncover their true identities
Ian Thorpe, Julia Zemiro and Ernie Dingo are three high-profile Australians from very different backgrounds who set off on this intrepid journey into their genetic history with the help of high profile human geneticist and series consultant Dr John Mitchell.