Steve Jones - The Language of the Genes

But why black skin? Patients treated with UV to combat skin diseases show a sudden drop in vitamin and antibody levels and perhaps black skin protects against this. Another idea is that that black skin allows the peoples of the tropics to warm up at dawn as the sun rises, even if they have to shelter from the heat of the day — when it may act as camouflage in shady places. It is easy to make up stories about how selection may favour certain genes, but none can be taken seriously without experiments.

In fruit flies heat has many genetic effects. It acts on inherited differences in enzyme structure, increases the mutation rate, and even causes 'selfish DNA' to hop around. Humans can regulate their internal temperature quite well so that differences in climate must have a less direct effect. Even so, blood groups and the alternative forms of certain enzymes show north-south trends. Whether these are due to climatic selection is not known.

Humans, like most animals, live on a thermal tightrope. If our body temperature goes up by a few degrees, we die. Molecular biology has illuminated the imminence of thermal disaster. In snails and fruit flies heat shock proteins are switched on when life gets too hot. Sometimes, most of the cell's machinery is devoted to the job. During a fever, our own cells make such proteins. They cluster around delicate enzymes which might be damaged by high temperature. Even a rise of a couple of degrees sets the protective machinery into action. Perhaps people from tropical and temperate climes differ in the sensitivity of the heat shock system. As yet, nobody knows.

Lower animals were once described dismissively as 'cold-blooded'. They lack the machinery which keeps mammals warm, but many hold their temperature stable by the way they behave. One species of lizard thrives from the deserts of California to the ice caps of the Andes. It keeps its temperature almost the same across this vast range just by moving in and out of the sun. I once invented a paint which fades at a measurable rate when exposed to daylight. To put spots of this onto snail shells shows how long each animal has spent in the sun over a month or so. Snails from hot and cold places behave differently and within a population dark- and light-coloured individuals (which differ in the extent to which they soak up solar energy) also differ in exposure to sunshine. Perhaps the method could be used to study dark- and light-skinned people, too.

Behaviour can be crucial in the control of temperature. Desert lizards cannot stray more than a couple of yards from shade before they die of heat stroke, but are obliged to venture into the sun every few minutes to feed. Some spiders spend half their energies in shuttling between sun and shade. A spider in a place with the right balance of shady and sunny patches can produce far more eggs than one whose home has plenty of food but not enough sunshine. It is easy to forget the importance of behaviour in our own thermal lives. A quick estimate of how much a choice of the right temperature costs the average Briton (or, even more so, the average inhabitant of Chicago) — a bill which includes houses, clothes, central heating, air-conditioning, food and holidays in Marbella or Florida — shows that the spiders are modest in what they spend on keeping comfortable. Warm-blooded we may be, but evolution has forced us into some cold-blooded decisions about how to stay alive in the move from the tropical climates in which our ancestors evolved.

Humans, like most mammals, are adapted to lowlands. They cannot survive for long at over five thousand metres as the amount of oxygen in the air is half that lower down. In the Andes people live at this height. The children of Andean Indian are better able to cope with such conditions than are those of immigrants from the plains. Native highlanders brought up at sea-level are better at extracting oxygen from the mountain air. Perhaps there has been an evolved response to oxygen starvation.

Diet, too, has been an agent of change. In the world as a whole only a minority of adults (the population of western Europe included) can digest cows' milk. Most animals (humans before agriculture included) never have the chance to drink milk of any kind after they have been weaned. Its digestion depends on an enzyme that allows the milk sugars to be broken down. If it stays active until adulthood, cows' milk is a useful food. If it does not, milk loses much of its value and an adult who drinks it suffers from wind and indigestion. The relevant gene is rare in much of Africa and in the Far East (which means that the dried milk once sent as food aid to these places was largely-wasted). It is much more common in western Europe and in some Africans such as the Fulani of northern Nigeria who herd cattle. Which is the evolutionary chicken and which the egg is not certain. Perhaps the gene was favoured in desert peoples as it allowed them to drink camels' milk to get water. In Europe it may be advantageous because those who have it can extract calcium from cows' milk and avoid rickets. Again, it is easy for imagination to take precedence over experiment.

The best-understood force of selection in humans comes from inherited differences in resistance to disease. Disease is an unavoidable part of existence; and even creatures preserved from the dawn of existence show signs of infection. The games of computer 'life' based on an analogy of natural selection have their sicknesses in the form of computer viruses. Disease has a history and a geography: people have faced different plagues at different times and in different places. Infection is a relentless enemy. It involves creatures who themselves must change in response to the body's defences, or die out, in an evolutionary arms race between ourselves and our diseases. To see what natural selection can and cannot do in response is the task of the next chapter.

A fifteenth-century chronicle by the Portuguese explorers of West Africa expresses a bitter complaint: 'It seems that for our sins, or for some inscrutable judgement of God, in all that we navigate along He has placed a striking angel with a flaming sword of deadly fevers. 1Three hundred years later, half the Englishmen who went to that part the world died within a year. When Europeans and their African slaves first went to South America, it was the natives' turn to suffer. The population of Mexico dropped from twenty-five million to one million between 1500 and 1600. Some tribes disappeared. The number of Quimbaya in Colombia who paid tribute to the Spaniards was fifteen thousand in 1539, but sixty-nine in 1628. Everywhere, the great killer was infection: malaria, smallpox and typhus. In both New and Old Worlds those who had lived with a disease for many generations survived better than those who experienced it for the first time. There seemed to be inborn differences in resistance between people from different places that at the time seemed almost miraculous. Now, the evolution of defences against disease is the finest example of natural selection in action. The Age of Disease might be (at least for the moment) over, but its genetic consequences will persist for many years to come.

Western society has won a respite in the battle, but throughout recent evolutionary history pestilence has been the greatest killer and the greatest agent of evolution. In the fourteenth century — thirty generations ago — the population of England was halved by the Black Death. Death from cold or starvation may be brutal, but at least the enemy is predictable. Bacteria and viruses are themselves alive. They have an ecology, as they need a constant supply of new victims. They can evolve, which leads to a race between natural selection on our survival and that on their ability to infect us. It is an implacable and endless relay race. As soon as one opponent is defeated, another comes along.