The colour of a person’s skin is no more than a sign of the natural evolutionary response of his or her ancestors to the amount of sun they experienced thousands of years ago. Ascribing labels or “racial” characteristics to that skin pigmentation is folly. Rather we should use the knowledge about the evolution of skin colour, seen in the marvellous sepia range of human pigmentation, as a way to educate people about the diversity of humankind.

This is the call to arms of Professor Nina Jablonski, anthropologist and palaeontologist from Penn State University, speaking in Stellenbosch on Tuesday this week as part of the African Genome Education Institute’s (AGEI) 2010 lecture series celebrating the 200 years since the birth of Charles Darwin.

Pigmentation is one of the foremost protectors of the organ, our skin, which is at the very interface between our physical environment and our inner selves.

“Skin pigmentation is not connected to any other qualities of our body. Once we strip off this protective layer of skin we are remarkably similar. Throughout the world, beneath our skin we’re so very much the same in our intellectual constitutions, in our physical bodies, in our susceptibility to disease, in our artistic predilections.”

Yet human beings are so conscious of colour because we are a highly visually-oriented primate, Jablonski says, so we measure people according to appearance, age, background and skin colour. But modern science shows that we have more in common than apparent superficial differences.

As far back as the Greeks, people have theorised about the relationship between skin pigmentation and the intensity of the sun. The first comprehensive map of skin pigmentation was drawn up in the middle of the 20th Century which showed that more darkly pigmented people are found closer to the equator and more lightly pigmented people are found closer to the poles, with a gradient between the two.

“So where does this beautiful rainbow of skin colour come from that we see around us in Africa and specifically in South Africa?” Jablonski asks rhetorically.

Fossil records show that modern humans evolved in equatorial east Africa, before moving out of the homeland and spreading into the rest of the world. But more recent science around the working of the skin, and its response to exposure to ultra-violet (UV) radiation shows how evolutionary processes drove a change in pigmentation in peoples’ skin as they moved away from places with a high UV “load”.

Human’s earliest ancestors were probably pale skinned and covered with dark hair, much like our nearest living primate cousin, the chimpanzee. But between about 2 million and 1.2 million years ago, our ancestors shed their body hair and develop the ability to sweat prodigiously. This is in response to a cascade of genetic changes, including the swelling of the human brain which is metabolically hungry and generates vast amounts of heat. The skin becomes an important cooling device for this expensive organ, explains Jablonski.

“When sweat glands produce sweat on the surface of our bodies, that evaporates, cooling the surface of our skin and the blood flowing through it. That blood flows back to the heart, in its somewhat cool state it is oxygenated and circulated back into the brain which receives it gratefully.”

This important change left the now sweating, but nearly hairless skin of emerging humans vulnerable to a harsh environment, including the ravages of the sun’s radiation.

UV radiation has a malign influence on humans, destroying DNA and cell membranes, amongst others. In addition to the skin cancers caused by exposure of light-coloured skin to UV, Jablonski’s work has shown that in unprotected skin solar radiation causes the breakdown in folate, a B Vitamin that’s critical in making DNA and cell division. A lack of folate during gestation can result in birth defects such as spina bifida.

Over time, as our ancestors shed their hair and exposed their fair skin to the sun, the lighter skinned of their kind would have fallen to skin cancers and birth defects while slightly darker skinned individuals would have survived.

“Here is the mechanism for understanding the evolution of skin pigmentation – if you could evolve a mechanism to protect yourself from the breakdown of an essential molecule like folate, then you got something important going for you in evolutionary terms. (Natural selection) led to the permanent fixation of dark melanin pigmentation in our skin, a natural sunscreen that projects our skin and our bodies from the harmful effects of UV radiation.”

By about a million years ago, early humans had darkly pigmented skin covered with virtually no hair whatsoever. So if we started out dark, she poses, where did the incredible range of colour come from, from the darkest skin colour to the very lightest and every shade in between?

It has to do with what modern science has revealed about the skin’s response to UV radiation, along with modern human’s migration out of Africa and colonisation of parts of the world which experience much less intense sunlight.

One positive effect of UV radiation on the skin, Jablonski explains, is that it triggers the production of Vitamin D in the epidermis – a vitamin that’s essential for building strong skeletons as well as for bolstering the immune system.

Most of Homo sapiens evolution took place in Africa, where these dark skinned people spent most of their days outdoors, many thousands of years before the earliest civilisations started constructing buildings in which people could escape from the sun.

“It didn’t matter that there was such a dense and excellent complement of sunscreen in the skin because enough UV could penetrate through the melanin to be able to make Vitamin D in the skin.” But Homo sapiens eventually began migrating out of Africa into the Middle East, Europe, Asia, India, the Americas – all diverse places, far away from its equatorial homeland and often with remarkably different solar regimes.

“People with strong melanin complements were able to do very well under intense solar regimes. But as soon as humans ventured outside of high UV areas, a heavy coat of melanin became a liability biologically. The superior sunscreen suddenly doesn’t work to our advantage because it takes so much longer to make Vitamin D in the skin.”

Darker skinned people moving into higher latitudes would have begun to suffer from Vitamin D deficiencies such as rickets (where skeletal disfigurement takes place during childhood, especially when Vitamin D deficiency is combined with a shortage of calcium). Jablonski demonstrates how this would allow the sword of natural selection to fall once again: a woman who suffered from rickets as a child would have a pelvis shape that was too narrow to allow for safe childbirth. A faltering immune system would leave people vulnerable to disease such as the common cold, flu or even chronic illnesses like tuberculosis.

“We have an interesting problem in human evolution. We have an African-based species that starts dispersing to low UV regions, people with dark pigmentation experience slow Vitamin D production, because of a high complement of melanin, leading to serious health problems including problems in childbirth and natural selection occurring for less pigmentation. We start out as genetically dark and some of our ancestors undergo mutations to become lighter.”

The higher in latitude different groups settled, the lighter their skin became. But should those lighter skinned people move back into areas with high UV levels, as has happened in recent centuries with modern travel and globalisation, they become susceptible once again to skin cancer and folate loss.

Sadly, though, this understanding of the physical and evolutionary processes behind skin colour comes after centuries of misinterpretation of this visual code.

“Skin pigmentation is the most obvious product of evolution on the human body. But think about what this means for our social concepts of race. What do these colours mean? How much energy and acrimony has been invested into the labels that we affix to colours, the labels that we have come to call ‘races’ of people?”

Jablonski’s call is for people to recognise that the very basis of these labels, the colours that distinguish us, are the product of evolution. Yet the labels themselves have maintained a reality and a durability within the modern world, even though they are a social construct and do not equate with biology.

“We must use this knowledge in our everyday lives to help us understand our own diversity. We have to deal with the heritage of labels and value-laden labels. But we can begin to do that every day by recognising the deep history of those labels, the biology behind them, and begin the process of education.”

Understanding skin colour from the perspective of evolutionary time unlocks “a huge feast of diversity of appearance and opportunity in humanity”.

“This is tremendously important for our futures, the recognition that these differences between us are only skin deep. We are united by our skin colour, not divided.”

Professor Nina Jablonski, head of anthropology at PennState University, delivered the lecture Why human skin comes in colours as part of the 2010 Darwin Seminars hosted by the African Genome Education Institute.