Steve Jones - The Language of the Genes

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 anddeformed 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.