Darwin's Island (6 page)

The magic substance also helps to hold diabetes, arthritis, muscular dystrophy and heart disease at bay and protects against the spread of certain cancers, with a higher rate of lung and bowel cancer in cloudy places. Any change in skin colour that helped to generate more of the vitamin must have been most helpful on mankind’s journey into the gloom. Natural selection noticed the new mutations at once and in cloudy places fair skin soon took over.

 

Selection has lead to many other upheavals in human DNA. Many of them emerged from shifts in our habits as we moved from ape to early human, and to modern man. Migration, shifts in diet and the rise of towns and cities all led to genetic change.

For nine-tenths of our history as a species, most people saw fewer people in their lifetimes than an average westerner now does on his way to work. Agriculture led to a population explosion, and
Homo sapiens
is now ten thousand times more abundant than is any other mammal of his size. In a world of pathogens and parasites, abundance is an expensive luxury. Epidemics have often cut our species down to size. They need large populations to sustain themselves, and migrants to spread the infection. The Plague of Justinian, which began in Constantinople in AD 541, put paid to a quarter of the people of the Eastern Mediterranean. The Black Death spread along the Silk Road from China in the fourteenth century and returned again and again to the teeming and filthy cities of the west. Two out of three Europeans died. Sickness is potent fuel for selection and many genes respond to it.

One illness shows its power better than any other. A third of the world’s population is exposed to malaria, half a billion are infected and the disease kills five people a minute. The real attack began about ten thousand years ago, when men moved into - and cut down - tropical forests at a time of warm, wet weather. That helped mosquitoes to breed and the parasite to spread.

In Kenyan families, poor conditions - a marshy spot, too much rain, too many children - explain some of the variation in individual risk of illness, but genetic differences are behind at least a third of the overall chance of ending up in hospital. Some variants have a large influence and are soon picked up by evolution while others are more subtle. The most important involve changes in the red blood pigment, haemoglobin. A quarter of a billion people bear at least a single copy of a mutated version of the molecule. The best known is sickle-cell, a simple change in the DNA alphabet. The haemoglobin of those with two copies forms long crystals in parts of the body low in oxygen. The red cells take up a crescent shape that restricts circulation and causes pain, heart disease and worse. Those with a single version of the altered message are healthy, with half the risk of fever if infected and a ten times lower chance of serious illness. A third of all Africans are in that situation and the gene is common in southern Europe, in the Middle East and in India. It has arisen on at least four different occasions. Other such changes give a lesser protection in countries such as Bangladesh, while deletions of long or short sections of DNA do the same in the Middle East and Oceania. Once again, those who carry two copies of a damaged gene pay a severe price while people with just one are protected.

Many other genetic changes have been pressed into service against that unpleasant illness. The parasite uses a certain red-cell enzyme to fuel its machinery. Hundreds of millions of people bear a defective version, but in return gain a defence. A certain form of the parasite cannot get into cells that lack a particular attachment site. Almost all West Africans have this variant. Elsewhere, a change in the shape of the red cell baffles the agent of infection, while the high salt and iron levels in African blood also fend it off. Dozens of sections of the DNA are implicated in the fight against malaria and many, no doubt, remain to be discovered. Large or small, each has been picked up by the selection, which, just as in the evolution of pale skins in Europe and Asia, has cobbled together a response step by step.

 

Natural selection is always poised to deal with enemies as they arise. Wherever it works, it leaves evidence, often indirect, that it has passed by. Some changes in DNA alter the structure of proteins while others do not. The ratio between the two is a crude test of its actions, for useful sections of the genome are more likely to accumulate change under the influence of selection than are the non-functional parts. On that criterion, our lineage has experienced rather less of its attentions than has that of the chimpanzee.

Another clue to the action of Darwin’s agent comes from the blocks of genetic variants packed close to each other along each chromosome. As a favoured gene - a new anti-malaria mutation, perhaps, or a change in skin colour - is picked up and becomes more common, it will drag along sections of DNA on either side. The stronger and more recent the selection, the longer the segment that accompanies it. In Africa, both the gene for black skin colour and that for sickle-cell sit in the middle of great sections of double helix that vary scarcely at all from person to person. That pattern hints that in each case the new mutation was seized upon at once and spread fast.

In time such uniform blocks of DNA are broken up by the random reshuffling of genes that takes place when sperm and egg are formed, but the process can take a long time. A length of DNA that is identical from person to person within the generally diverse genome is hence evidence that selection is, or has been, at work. The human and chimp genomes each have thousands of such segments. One gene in sixty among the chimps bears that Darwinian mark but only half as many in humans, as proof that we have coped with new challenges in a manner that our close relative cannot. Man’s ability to modify the environment to suit his needs has weakened the hammer blows of nature. Anti-malaria drugs now do what could be achieved only by expensive mutations. Thousands of years ago, our skin responded fast to a shift in climate, with a genetic change; but most people, black or white, now protect themselves against the sun in quite a different way, with clothes.

The loss of our native nudity was an early hint of the evolutionary talent that made us unique - the ability to respond to a challenge not with bodies but with brains. Clothing allowed us to spread across the world, for with its help we take the tropics with us wherever we go.

Adam and Eve, in their sultry paradise, were unashamed, but after the first (and least original) of all sins they made aprons to hide their nether parts. When did they first put them on? Lice hint at when garments were invented. Chimps and gorillas have lots and spend many hours grooming as a result. When humans emerged on to the sunny savannahs they lost their hair. The lice had a hard time and evolved to live in the few patches of habitat left. We now have three kinds, the head and the body louse, plus the pubic louse. The body louse is the only one that hangs on to clothing. The pubic version is closest to the lice found on gorillas and may have joined us from there. DNA shows that the other two evolved from a chimpanzee parasite which began its intimate acquaintance with our own bodies six million years ago. The body and head forms, in contrast, separated more recently - perhaps no more than fifty thousand years before the present. That may mark the moment when we first donned our vestments and gave a resourceful louse a new place to live. Men, their parasites prove, dressed themselves as they took their first steps towards the icy north.

Since then we have learned to cope with external parasites with insecticides, with cold with central heating and with noxious foods with kitchens. Each talent is a product of the contents of the skull, which are - like Adam’s underpants - unique. Darwin noted that ‘There can be no doubt that the difference between the mind of the lowest man and that of the highest animal is immense’, and he was right. To understand human evolution we need to know how and why our brain, the most human of organs, is so different from that of any other primate and why and how our behaviour is even more so.

The structure is three times as big, and the cortex, the thoughtful bit, five times larger than that of the chimpanzee and the modern skull is several times roomier than that of three million years ago. Chimps are born with a brain almost as big as that of an adult animal while babies, whose brains are already larger than that of a chimpanzee, continue to invest in grey matter until they are two. Genes active within the human cranium have multiplied themselves when compared with those in other primates and one such, which when it goes wrong leads to the birth of infants with tiny heads, has evolved particularly fast. The nerves within the human skull are more connected to each other, and their junctions more sophisticated, than are those of the chimp and the structure is also busier at the molecular level. Even so, much of the DNA most active in that part of the body has changed no more rapidly than that at work in liver, muscle or scrotum.

The brain is expensive, for by weight it uses about sixteen times more energy than does muscle. That represents a quarter of the entire budget of the body at rest and means that we expend twice as much effort on the intellect as do chimpanzees. How can we afford such a luxurious appendage? Humans eat no more than other primates of comparable size but have a richer diet, with more meat and fewer roots and leaves, than do our relatives. As a result we need smaller intestines to soak it up. We also invest less in muscle than other apes and the enzymes that burn food are more efficient than theirs. All this began, like black skin, a million and more years ago, when people moved from forests to savannahs, travelled in larger groups and became better hunters with a meatier diet. The way to man’s brain was through his guts.

Even so, today’s organ of thought is no bigger than that of the Neanderthals. Fossils of their newborns show that they were born with a brain as large as our own, which grew even faster during infancy, but those creatures acted far more like apes than we do. Something more than an extra dose of grey matter has made us what we are. To quote Darwin: ‘of all the differences between man and the lower animals, the moral sense or conscience is by far the most important’. A glance at our relatives shows how right he was.

Chimpanzees are nastier than many people like to think. They kill monkeys and are pretty unpleasant to each other too. Their sex lives would shock Queen Victoria and their ethical universe, if they have such a thing, is far darker than our own. They live in groups, but the groups break and reform as their members quarrel. Terror makes the world go round. Set up a task in which two chimps need to pull a rope to get a tray of food. They will, but only if they are out of reach of each other. Otherwise, the dominant animal attacks its subordinate even if neither then gets anything. Anger and greed destroy the hope of reward. What makes humans different is a loss of fear, odd as that sounds in a world where that emotion seems to be everywhere. When anxiety goes, society can emerge.

Our social skills begin early. A group of two-year-olds asked to find a piece of food after they saw it moved to a new place or turned to a new position or put in a box with a beep were pretty good at each job - but no better than adult chimpanzees, for both babies and chimps succeeded at about two trials out of three. When it came to the need to learn from others the babies won hands down. They became far better at each problem when they saw someone else solve it, or when an adult pointed to or gazed at where the food was hidden or made noises that told them they were getting warm. Each response demands an insight into another’s inner sentiments. We have a lot more of that talent than do our relatives. The chimps took no notice of those who tried to help.

Chimpanzees can learn, but do not teach: like all apes, they ape but do not educate. In some places, adults fish for insects with a stick or bash nuts with a stone, and the young emulate them. Even so, the grown-ups make no effort to show the infants how to do the job, do not change their ways when youngsters are around and never check to see how well they are doing. Birds, with their bird brains, can do what a chimp does, for a budgie will pull out the stopper of a bottle of food if it sees another do the job, and crows are even smarter.

Real education asks for more. A good pedagogue can teach almost any subject as long as he keeps a few pages ahead of his charges and they respond to his efforts. Teachers also have insight into the mental lives of their pupils, into who understands the lesson and who does not, and know how to encourage them without their becoming bored.

The chimp’s negligence about the next generation is a reminder that the minds of our hairy relatives are not much like our own. A competent teacher needs to understand what his students are thinking - and chimps do not: they have no more than a rudimentary ‘theory of mind’, as psychologists put it. We have lots, and it helps those on both sides of the lectern. Teenagers might doubt the fact, but no ape could ever become a schoolmaster.

 

The best way of reading a mind is to chat to it. Thomas Love Peacock invented a character called Sir Oran Haut-Ton, who learns to play the French horn but not to speak (he is elected to Parliament, where his silence gives him an air of wisdom).
Homo sapiens
is the eloquent ape. Even deaf children left in groups babble with their hands. Speech is the scaffold upon which society is built. No other primate can speak and all attempts to persuade them to do so have failed (Noam Chomsky, the theoretician of language, noted that it was ‘about as likely that an ape will prove to have a language ability as there is an island somewhere with flightless birds waiting for humans to teach them to fly’).

The origin of language is a cause of endless dispute which, given that just one creature can speak, may never be resolved. Darwin thought that perhaps it began with imitation: that ‘some unusually wise ape-like animal should have thought of imitating the growl of a beast of prey, so as to indicate to his fellow monkeys the nature of the expected danger’.
The Descent of Man
also suggests that it could have started with love songs, and that speech was in part a side-effect of sexual selection. Perhaps it was; or perhaps it grew instead from the simple fact that we are social animals. Apes groom each other because the constant pacification calms them all down and cuts down the conflict that is never far from the surface. Big groups demand too much scratching time but reassuring sounds can placate lots of individuals at once. The savage breast might first have been charmed in that way; possibly, indeed, with song - which could be why some stutterers can sing a sentence when they cannot say it.