by Steve Jones
It is easy to claim the whole of biology as evidence of its action. Just as Paley interpreted the complexity of plants and animals as an argument for God, a neo-Paleyism plagues evolutionary theory. It claims that all animal structure is well adapted and must hence always reflect the action of selection. This argument is circular, but is hard to disprove. It has led to many disagreements — fascinating to their proponents, tedious to those outside — among biologists. Some feel that the Darwinian machine drives the whole of evolution from the shape of the nose to the order of bases in the DNA. Others see selection as an occasional event that directs some genes while most change at random. The issue is unresolved and some biologists like to spend their time making up stories about how selection has moulded the most improbable characters. Sometimes they even turn out to be right. Anthropologists have even more vivid imaginations and have made some unlikely guesses about how selection may have formed human attributes. Many are fantasy, but because they invoke events that happened long ago are almost impossible to refute.
Differential survival and reproduction are at work in many places without anybody realising. One egg in thousands and one sperm in millions produce an offspring. Do the rest die at random or do they fail for genetic reasons? Nobody knows: but if only the best survive, the Darwinian machine is a more pervasive force than was ever imagined.
Whatever its importance, selection is just a mechanism and not a force for good. Cancer patients are sometimes given a drug which attacks cells as they divide. The treatment often fails because selection is at work. A few cells have undergone a mutation which changes the properties of a certain gene to enable it to break down the drug.
These soon take over, sometimes so effectively that the patient dies. There is not much evidence of a benign designer here.
Humans show as well as anything else its strengths and weaknesses. Homo sapiens has changed as he filled the world over the past 150,000 years. Thar six thousand human generations is the same as the number of generations of mice that separate today's animals from those that infested the brand new Acropolis and ties modern fruit flies to the insects that swarmed over the apples of William the Conqueror. As far as anyone knows, today's mice and fruit flies have scarcely changed over that time, emphasising just how short a period we have had to evolve.
Three ages of history have moulded natural selection. A lengthy Age of Disaster was followed by a shorter Age of Disease and — very recently — by an Age of Decay. For most of the past nearly all those born died by disaster, through cold, hunger or violence. Many individual tragedies acted as agents of progress. This chapter is about how humanity evolved to cope with the changes in climate and diet as we moved from our African home. The second epoch, the Age of Disease (which began only a few thousand years ago), albeit over in the West, is still the rule elsewhere. The Age of Decay (in which most people die of old age) is now upon us. Because most of those who succumb nowadays have passed on their genes it is hard to know what selection will be able to achieve in this, the third age of human evolution.
Our ancestors, our relatives and ourselves are tropical animals. Noel Coward notwithstanding, humans are among the few large mammals able to cope with the African midday sun. Most people, given the choice, prefer a warm place (perhaps only the Costa del Sol for a couple of weeks a year) and many adaptations have evolved to combat heat, rather than cold. Humans are the least hairy of all primates and sweat the most. On a sunny day, the temperature at, or a few inches above, the ground can be as much as twenty decrees higher than that a couple of feet away from the earth, because the surface absorbs and reflects heat from the sun. Our upright posture may have evolved in response. One of the best ways to reduce heat stress is to stand up, out of the layer of hot air. Perhaps, as our distant ancestors moved to the savannah from the forests, they stood up to cool down; opening, literally and figuratively, new horizons for their descendants.
Humans today live in every environment from rainforest to tundra and from sea-level to five thousand metres above it. Culture — fire, clothes and houses — helped us fill the world, but there have been genetic responses to climate as well.
Mankind left Africa more than a hundred thousand years ago and reached New Zealand, the furthest point of his spread, a thousand years before the present. For much of the time, the weather was even worse than it is today. Ancient climates can be inferred from shifts in the chemical composition of water. In the Arctic, this falls as snow and is preserved as ice. A core has been drilled through three thousand metres of Greenland ice to reach the rock below, where the first snows fell two hundred thousand years ago.
The record of the icecap reveals many ice ages during the evolution of Homo sapiens. The last one peaked about eighteen thousand years before the present. It had a drastic effect on the mammals of the world, ourselves included. The giant sloths and native horses went from the Americas, the mammoth from Asia and giant lemurs from Madagascar. Large areas of northern Europe were abandoned. As the climate dried because water was trapped in the ice, parts of Africa became desert and were lost to habitation as well. The level of the sea fell. The Bering Straits and Bass Strait dried out. Broad coastal lowlands emerged in many places. The air filled with dust from the icy deserts. Even if they were cold most of the time, our ancestors had beautiful sunsets.
In the Russian Plain settlements flourished within a hundred and fifty miles of the icecap. The Frenchmen who painted the cave at Lascaux could not relax by basking in the sunshine in a pavement cafe. The arctic tec was three hundred miles away and they had to stay warm to stay alive. Perhaps the need to keep under cover fostered artistic endeavour. The explosions of artistic and technological style all happened on the northern edges of humanity's range. Humans survived the harsh new climate and at the peak of the last glaciarion were the most widely distributed mammal in the world — a status they have retained ever since.
Not all was gloom at the time of the global spread. In brief periods, up to a couple of thousand years long, the temperature rose by as much as seven degrees in just a few decades; a change equivalent to the Scottish climate shifting to that of southern Spain within a lifetime. Perhaps these sudden strange warmings impelled the colonists on their way.
As in the sparrows, differential survival and reproduction favoured those best adapted to climate. The Neanderthals, our extinct cousins, who had lived in a chilly Europe long before the modern upstarts arrived, were short, squat and heavy. Rather like today's Eskimos they were adapted to the cold (so much so, indeed, that the average Neanderthal was more thickset than all but a tenth of modern Eskimos). Most people would change seats if Cro-Magnon, an early European, sat next to them on the tube, but would change trains if a Neanderthal did the same.
Modern humans show geographical trends in body build that reflect the action of climatic selection. Eskimos are about.1 third heavier for a given height than the world average, while men from parts of East Africa are much slimmer thnn others, at about three quarters the weight expivk'd lor their height. Much of the difference arises from changes in body proportions. Most peoples from the tropics are tall, thin and have long arms and legs. Those from the north tend to be more heavily built. For unknown reasons, the trends are stronger in men than in women. The same is true for the changes in shape in sparrows, perhaps because larger males are more aggressive in the struggle for food in winter. Although little is known about the inheritance of such characters (and environmental effects are without doubt involved) the differences across the globe are at least in part genetic.
The short, fat peoples of the north are better at keeping heat in the body core. Those with more graceful figures from hotter climes cool down better through their long arms and legs. Most of the body's excess heat is lost from the skin and the amount of surface per unit of volume is greater in thin and spindly individuals.
Some populations are even able to regulate the amount of heat that gets to the arms and legs. If a European or an African puts a finger into icy water
, its temperature drops to a level low enough to damage the flesh. When an Eskimo does the same his finger stays warm. Again, it is not clear how much the effect is genetic, but among North Atlantic fishermen those of European origin are worse at keeping their hands warm than are the natives of the North. Australian Aborigines have another defence against a climate hot during the day but cold at night. They close down blood vessels near the surface on cold nights, so that their skin temperature falls to well below that of a European in the same conditions, saving heat in the body core. Aborigines are also better able to handle cold without shivering and can sleep in the open without too many problems. Even the rate at which the body uses energy is lower in those who evolved in the tropics.
Other patterns might also be due to climate. The woolly hair of Africans is said to help sweat to evaporate and cool the head down. The long fine noses of peoples from the Middle East might help to moisten the desert air before it reaches the lungs and the narrow eyes of Chinese to protect against the icy winds of the Asian plains. All this is guesswork.
The globe has one anomaly in its climatic trends. In the Old World at least, most tropical peoples have darker skins than do those from cooler climes. As anyone who has sat on an iron park bench on a sunny day knows, black objects heat up more in the sun than do white, so that black skin, far from protecting against the sun's heat, soaks it up. None of the theories that try to explain why humans evolved light skins as they migrated to the dismal climates of the north is altogether satisfactory. Skin cancer is found among people with light skins who expose themselves to ultraviolet by sunbathing. Black people almost never get the disease, but that illness has probably not produced the global trend. First, it is rare even in whites, with about one case per ten thousand people per year. More important, it is a disease of the old. Those who die from it have already passed on their genes, those for colour included.
Without vitamin D, children get rickets, soft and deformed bones. Vitamin D can be made in the skin by the action of ultraviolet light. Under a UV light, whites synthesise a useful dose in half an hour, while blacks take six times as long to do so. Even a few hours in sunshine allows a fight-skinned baby to avoid rickets and it is no accident that African babies are lighter than are adults. Perhaps natural selection favoured light skins as man began his long walk from the tropics to the gloom of northern Europe.
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.
Chapter Thirteen THE DEADLY FEVERS
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.1 Three 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.