As a follow-up of sorts to the
last post on evolution, an excellent article by Steve Jones from the same book discusses the reasons for why evolution results in different skin complexions. It was written around 1996 or so.
----------------------------------------
Everyone knows - do they not? - that many people have black skin. What is more, black people are concentrated in certain places - most notably, in Africa - and, until the upheavals of the past few centuries, they were rare in Europe, Asia, and the Americas. Why should this be so? It seems a simple question. Surely, it we cannot give a simple answer, there is something wrong with our understanding of ourselves. In fact, there is no straightforward explanation of this striking fact about humankind. Its absence says a lot about the strengths and weaknesses of the theory of evolution and of what science can and cannot say about the past. Any anatomy book gives one explanation of why people look different. Doctors love pompous words, particularly if they refer to other doctors who lived long ago. Black people have black skin, their textbooks say, because they have a distinctive Malphigian layer. This is a section of the skin named after the seventeenth-century Italian anatomist Malphigii. It contains lots of cells called
melanocytes. Within them is a dark pigment called melanin. The more there is, the blacker the skin. Malphigii found that African skin had more melanin than did that of Europeans. The question was, it seemed, solved.
This is an example of what I sometimes think of as 'the Piccadilly explanation.' One of the main roads in London is called Piccadilly - an oddly un-English word. I have an amusing book that explains how London's streets got their names. What it says about Piccadilly sums up the weakness of explanations that depend, like the anatomists', only on describing a problem in more detail. The street is named, it says, after the tailors who once lived there and made high collars called
piccadills. Well, fair enough; but surely that leaves the interesting question unanswered. Why call a collar a piccadill in the first place? It is not an obvious word for an everyday piece of clothing. My book is, alas, silent.
Malphigii's explanation may be good enough for doctors, but will not satisfy any thinking person. It answers the question
how but not the more interesting question
why there is more melanin in African skin.
Because the parents, grandparents, and - presumably - distant ancestors of black people are black, and those of white people are white, the solution must lie in the past. And that is a difficulty for the scientific method. It is impossible to check directly just what was going on when the first blacks appeared on earth. Instead, we must rely on indirect evidence. There is one theory that is, if nothing else, simple and consistent. It has been arrived at again and again. It depends solely on belief; and if there is belief, the question of proof does not arise. Because of this, the theory lies outside science.
It is that each group was separately created by divine action. The Judeo-Christian version has it that Adam and Eve were created in the Garden of Eden. Later, there was a gigantic flood; only one couple, the Noahs, survived. They had children: Ham, Shem, and Japheth. Each gave rise to a distinct branch of the human race, Shem to the Semites, for example. The children of Ham had dark skins. From them sprang the peoples of Africa. That, to many people, is enough to answer the question posed in this essay.
The Noah story is just a bald statement about history. Some creation myths are closer to science. They try to
explain why people look different. One African version is that God formed men from clay, breathing life into his creation after it had been baked. Only the Africans were fully cooked - they were black. Europeans were not quite finished and were an unsatisfactory muddy pink. The trouble with such ideas is that they cannot be disproved. I get lots of letters from people who believe passionately that life, in all its diversity, appeared on earth just a few thousand years ago as a direct result of God's intervention. There is no testimony that can persuade the otherwise. Prove that there were dinosaurs millions of years before humans, and they come up with rock 'footprints' showing, they say, that men and dinosaurs lived together as friends. So convinced are they of the truth that they insist that their views appear in school textbooks.
If all evidence, whatever it is, can only be interpreted as supporting one theory, then there is no point in arguing. In fact, if belief in the theory is strong enough, there is no point in looking for evidence in the first place. Certainty is what blocked science for centuries. Scientists are, if nothing else, uncertain. Their ideas must constantly be tested against new knowledge. If they fail the test, they are rejected.
No biologist now believes that humans were created through some miraculous act. All are convinced that they evolved from earlier forms of life. Although the proof of the fact of evolution is overwhelming, there is plenty of room for controversy about how it happened. Nowhere is this clearer than in the debate about skin colour.
Modern evolutionary biology began with the nineteenth-century English biologist Charles Darwin. He formed his ideas after studying geology. In his day, many people assumed that grand features such as mountain ranges or deep valleys could arise only through sudden catastrophes such as earthquakes or volcanic eruptions, which were unlikely to be seen by scientists as they were so rare. Darwin realised that, given enough time, even a small stream can, by gradually wearing away the rocks, carve a deep canyon. The present, he said, is the key to the past. By looking at what is going on in a landscape today. It is possible to infer the events of millions of years ago. In the same way, the study of living creatures can show what happened in evolution.
In
The Origin of Species, published in 1859, Darwin suggested a mechanism whereby new forms of life could evolve.
Descent with modification, as he called it, is a simple piece of machinery, with two main parts. One produces inherited diversity. This process is now known as mutation. In each generation, there is a small but noticeable chance of a mistake in copying genes as sperm or eggs are made. Sometimes we can see the results of mutations in skin colour; one person in several thousand is an albino, lacking all skin pigment. Albinos are found all over the world, including Africa. They descend from sperm or eggs that have suffered damage in the pigment genes. The second piece of the machine is a filter. It separates mutations which are good at coping with what the environment throws at them from those which are not. Most mutations - albinism, for example - are harmful. The people who carry mutant genes are less likely to survive and to have children than do those who do not. Such mutations quickly disappear. Sometimes, though, one turns up which is better at handling life's hardships than what went before. Perhaps the environment is changing, or perhaps the altered gene does its job better. Those who inherit it are more likely to survive; they have more children, and the gene becomes more common. By this simple mechanism, the population has evolved through
natural selection. Evolution, thought Darwin, was a series of successful mistakes.
If Darwin's machine worked for long enough, then new forms of life - new species - would appear. Given enough time, all life's diversity could emerge from simple ancestors. There was no need to conjure up ancient and unique events (such as a single incident of creation) which could neither be studied nor duplicated. Instead, the living world was itself evidence for the workings of evolution. What does Darwin's machine tell us about skin colour? As so often in biology, what we have is a series of intriguing clues, rather than a complete explanation.
There are several kinds of evidence about how things evolve. The best is from fossils; the preserved remnants of ancient times. These contain within themselves a statement of their age. The chemical composition of bones (or of the rocks into which they are transformed) shifts with time. The molecules decay at a known rate, and certain radioactive substances change from one form into another. This gives a clue as to when the original owner of the bones died. It may be possible to trace the history of a family of extinct creatures in the changes that occur as new fossils succeed old.
The human fossil record is not good - much worse, for example, than that of horses. In spite of some enormous gaps, enough survives to make it clear that creatures looking not too different from ourselves first appeared around a hundred and fifty thousand years ago. Long before that, there were apelike animals which looked noticeably human but would not be accepted as belonging to our own species if they were alive today. No one has traced an uninterrupted connection between these extinct animals and ourselves. Nevertheless, the evidence for ancient creatures that changed into modern humans is overwhelming. As there are no fossilised human skins, fossils say nothing directly about skin colour. They do show that the first modern humans appeared in Africa. Modern Africans are black. Perhaps, then, black skin evolved before white. Those parts of the world in which people have light skins - northern Europe, for example - were not populated until about a hundred thousand years ago, so that white skin evolved quite quickly. Darwin suggested another way of inferring what happened in the past: to compare creatures living today. If two species share a similar anatomy, they probably split from a common ancestor more recently than did another which has a different body plan. Sometimes it is possible to guess at the structure of an extinct creature by looking at its living descendants. This approach can be used not just for bones but for molecules such as DNA. Many biologists believe that DNA evolves at a regular rate; that in each generation, a small but predictable proportion of its subunits changes from one form into another. If this is true (and often it is), then counting the changes between two species reveals how closely they are related. What is more, if they share an ancestor that has been dated using fossils, it allows DNA to be used as a 'molecular clock,' timing the speed of evolution. The rate at which the clock ticks can then be used to work out when other species split by comparing their DNA, even if no fossils are available.
Chimpanzees and gorillas seem, from their body plan, to be our relatives. Their genes suggest the same thing. In fact, each shares 98 percent of its DNA with ourselves, showing just how recently we separated. The clock suggests that the split was about six million years ago. Both chimp and gorilla have black skins. This, too, suggests that the first humans were black and that white skin evolved later. However, it does not explain why white skin evolved. The only hint from fossils and chimps is that the change took place when humans moved away from the tropics. We are, without doubt, basically tropical animals. It is much harder for men and women to deal with cold than with heat. Perhaps climate has something to do with skin colour. To check this idea, we must, like Darwin, look at living creatures. Why should black skin be favoured in hot and sunny places and white where it is cool and cloudy? It is easy to come up with theories, some of which sound pretty convincing. However, it is much harder to test them.
The most obvious idea is wrong. It is that black skin protects against heat. Anyone who sits on a black iron bench on a hot sunny day soon discovers that black objects heat up
more than white ones do when exposed to the sun. This is because they absorb more solar energy. The sun rules the lives of many creatures. Lizards shuttle back and forth between sun and shade. In the California desert, if they stray more than six feet from shelter on a hot day, they die of heat stroke before they can get back. African savannahs are dead places at noon, when most animals are hiding in the shade because they cannot cope with the sun. In many creatures, populations from hot places are lighter - not darker - in colour to reduce the absorption of solar energy. People, too, find it hard to tolerate full sunshine - blacks more so than whites. Black skin does not protect those who bear it from the sun's heat. Instead, it makes the problem worse. However, with a bit of ingenuity, it is possible to bend the theory slightly to make it fit. Perhaps it pays to have black skin in the chill of the African dawn, when people begin to warm up after a night's sleep. In the blaze of noon, one can always find shelter under a tree.
The sun's rays are powerful things. They damage the skin. Melanin helps to combat this. One of the first signs of injury is an unhealthy tan. The skin is laying down an emergency layer of melanin pigment. Those with fair skin are at much greater risk from skin cancer than are those with dark. The disease reaches its peak in Queensland, in Australia, where fair-skinned people expose themselves to a powerful sun by lying on the beach. Surely, this is why black skin is common in sunny places - but, once again, a little thought shows that it probably is not. Malignant melanoma, the most dangerous skin cancer, may be a vicious disease, but it is an affliction of middle age. It kills its victims after they have passed on their skin-colour genes to their children. Natural selection is much more effective if it kills early in life. If children fail the survival test, then their genes perish with their carriers. The death of an old person is irrelevant, as their genes (for skin colour or anything else) have already been handed on to the next generation.
The skin is an organ in its own right, doing many surprising things. One is to synthesise vitamin D. Without this, children suffer from rickets: soft, flexible bones. We get most vitamins (essential chemicals needed in minute amounts) from food. Vitamin D is unusual. It can be made in the skin by the action of sunlight on a natural body chemical. To do this, the sun must get into the body. Black people in sunshine hence make much less vitamin D than do those with fair skins. Vitamin D is particularly important for children, which is why babies (African or European) are lighter in colour than are adults. Presumably, then, genes for relatively light skin were favoured during the spread from Africa into the cloud and rain of the north. That might explain why Europeans are white - but does it reveal why Africans are black? Too much vitamin D is dangerous (as some people who take vitamin pills discover to their cost). However, even the fairest skin cannot make enough to cause harm. The role of black skin is not to protect against excess vitamin D.
It may, though, be important in preserving other vitamins. The blood travels around the body every few minutes. On the way, it passes near the surface of the skin through fine blood vessels. There, it is exposed to the damaging effects of the sun. The rays destroy vitamins - so much so, that a keen blond sunbather is in danger of vitamin deficiency. Even worse, the penetrating sunlight damages antibodies, the defensive proteins made by the immune system. In Africa, where infections are common and, sometimes, food is short, vitamin balance and the immune system are already under strain. The burden imposed by penetrating sunlight may be enough to tip the balance between health and disease. Dark skin pigmentation may be essential for survival. No one has yet shown directly whether this is true.
There are plenty of other theories as to why some people are black. For an African escaping from the sun under a tree, black skin is a perfect camouflage. Sexual preference might even have something to do with the evolution of skin colour. If, for one reasons or another, people choose their partners on the basis of colour, then the most attractive genes will be passed on more effectively. A slight (and perhaps quite accidental) preference for dark skin in Africa and light in Europe would be enough to do the job, This kind of thing certainly goes on with peacocks - in which females prefer males with brightly patterned tails - but there is no evidence that it happens in humans. Accident might be important in another way, too. Probably only a few people escaped from Africa a hundred thousand years and more ago. If, by chance, some of them carried genes for relatively light skins, then part of the difference in appearance between Africans and their northern descendants results from a simple fluke. There is a village of North American Indians today where albinos are common. By chance, one of the small number of people who founded the community long ago carried the albino mutation and it is still abundant there.
All this apparent confusion shows how difficult it is for science to reconstruct history. Science is supposed to be about testing, and perhaps disproving, hypotheses. As we have seen, there is no shortage of ideas about why people differ in skin colour. Perhaps none of the theories is correct, or perhaps one, two, or all of them are. Because whatever gave rise to the differences in skin colour in different parts of the world happened long ago, no one can check directly. But science does not always need direct experimental tests. A series of indirect clues may be almost as good. The hints that humans evolved from simpler predecessors and are related to other creatures alive today are so persuasive that it is impossible to ignore them. So far, we have too few facts and too many opinions to be certain of all the details of our own evolutionary past. However, the history of the study of evolution makes me confident that, some day, the series of hints outlined in this essay will suddenly turn into a convincing proof of just why some people are black and some white.
An article in a recent issue of New Scientist about what is responsible for fear of spiders led me to slightly disagree with the explanations afforded by the researchers. Have a quick read:
