The passage may have been difficult, but the genetics of today's aboriginals suggest that it was made by many people. Native Australian DNA, like that of Papuans, is quite diverse. There must have been many founders, with several incursions into the continent. Once they got there, the new inhabitants found their home congenial and, at least in the tropical north, tended to stay put. As a result, in today's Papua New Guinea, local populations are quite different from each other, with distinct 'clans' of mitochondria! lineages, each limited to a few remote mountain valleys. Their denizens stayed isolated until the first Europeans reached the interior half a century ago. They were in their own way advanced, and cut down trees to allow the tastier plants beneath them to grow. Hidden in their fastnesses for tens of thousands of years they remained insulated from the economic strife and the waves of movement that affected the rest of the world.

At the other end of Sahul, rising sea-levels soon marooned the inhabitants of Tasmania. They remained in ignorance of the world outside until the arrival of Europeans, in the eighteenth century. Nothing is known of the Tasmanians' genes, for a simple reason. They were driven to extinction (and sometimes hunted down) by ambassadors of the modern world's economy. There was a sordid episode in anthropology when the Tasmanians were regarded — absurdly — as the elusive 'missing link' between humans and apes and the museums of the world quarrelled over the bones of the last survivors.

Human traces show that even remote Pacific islands (such as Manus Island in the Admiralty group, three hundred and fifty kilometres from the nearest land mass) were occupied twenty-eight thousand years ago, so that by then it was possible to make substantial voyages. The genes of present day Melanesians, those from the islands north and east of Australia, still resemble those of the ancient populations in the Papuan highlands. They are the descendants of these ancient voyagers.

The Polynesians who occupy the rest of the Pacific are quite different and got there much more recently. Hawaii and Easter Island were reached only a couple of centuries after the birth of Christ. In the far Pacific, islands separated by thousands of miles of ocean are not at all distinct in their genes, proof that water is a less effective barrier to movement than is land.

Almost all the peoples of the distant Pacific carry a small change in their mitochondria! DNA. Nine letters of the message are missing. This deletion has spread through the whole of Polynesia from Fiji to New Zealand. In some places it is so common as to suggest that most of the present population descends from a single female who was the ancestor of almost all the inhabitants. It is shared with the populations of Taiwan and the Japanese, and shows that the Polynesians spread across the Pacific from Asia and not from Australia. Australian aboriginals and the Highlanders of Papua New Guinea do not have this genetic signature. They descend from a migration which began thousands of years before that of the Polynesian arrivistes.

One thing is clear: the inhabitants of the Pacific and those of South America have few genetic links. Thor Heyer-dahPs book of his intrepid voyage in a balsa raft across eight thousand miles of Pacific from Peru has sold twenty million copies, more than all other anthropology books put together. Unfortunately, his view that to reconstruct the past it is necessary only to relive it is wrong. Population genetics has sunk the Kon-Tiki.

Twenty thousand years ago, much of the Pacific had a dense population and a prosperous economy. In Europe, too, trade was well advanced. Flint for stone tools was transported for many miles and Baltic amber reached the Mediterranean. There was a brief rise of art, perhaps a mere couple of centuries long, which tilled the caves at Lascaux and Altamira with images.

While the world economy boomed the Americas were empty. They were at last reached from Siberia. Many of the inhabitants of that icy land, which was even colder than it is today, lived by hunting mammoths. As they spread they destroyed their food sources. At last, they came to the Bering Land Bridge which joined Asia to Alaska. It emerged from the sea, as did thousands of square miles of coastal plains all over the world, as water was locked into the ice. At the end of the ice age the water rose and twelve thousand years ago the bridge between Old and New Worlds was breached. Just before it disappeared, a few pioneers made their way across. If their experiences were like those of what we know of the nineteenth-century Inuit who made long voyages across such barren landscapes they had a grim time. Many must have starved. Nevertheless, some reached the broad plains of North America and soon spread to the continent's southern point, reaching it within a couple of thousand years. This seems like a rapid expansion but is, after all, less than ten miles a year into a deserted land. The journey was helped by a brief warming which meant that, even in Alaska, a few trees appeared in the bitter landscape.

Once again, the edible inhabitants suffered. Mammoths, sloths, giant tapirs and camels followed each other into extinction. Each was large, tasty, naive and tame. They reproduced slowly. Once humans had arrived their fate was certain. The wave of destruction tempted the first Americans south until, in Patagonia, they could go no further.

The date of the American invasion is not certain. The oldest traces of occupation in North America are in a rock shelter in Pennsylvania. They date from about twelve thousand years ago. Soon, members of the 'Clovis culture', in what is now the United States, could produce sharp and effective arrowheads. The first art in the Americas is at the cave of Pedra Furada — the Perforated Rock — in Brazil, which has twelve-thousand year-old imam-s of birds, deer and armadillos, together with human stick figures. Some claim that charcoal from nearby caves dates back for fifty thousand years, but few anthropologists accept (his as evidence of human occupation. Most believe thai ilu* first Americans arrived less than twenty-five thousand years before the present.

The genes of Native Americans fit the idea of a small band that filled a new-found land. Americans as a whole are less diverse and more uniform than arc the peoples of highland Papua New Guinea (who fill a tiny proportion of the space). The mitochondria! genes of all Native Americans fall into four major lineages as a hint that just a small group managed to complete rhe hazardous traverse of the Bering Bridge. The same ones are found in some three-thousand year-old Chilean mummies, implying that there were not many bottlenecks on the way through the Americas from north to south. The mitochondria of South American Indians resemble those of north-east Asia, supporting the idea that their ancestors, like those of Polynesians, came from that part of the world (although there is a hint of an ancient link with Europe in a few Northern tribes, suggesting that a more distant traveller across the land-bridge helped found some American groups).

By ten thousand years before the present, humans had filled the whole habitable world, apart from some remote islands. Everywhere they lived in small bands. Every Englishman needed ten square miles of land to feed himself.

The global spread was accompanied by technical advances in axes, arrowheads and nets as the animals easiest to exploit — reindeer, mammoths, giant kangaroos or emus — disappeared and the hunters were forced to move to less easy prey.