Theories of economics and of evolution have obvious ties. Darwin was much influenced by the works of Malthus, who had been disturbed by the new slums of the English cities of the eighteenth century. In his Essay on the Principles of Population Malthus argued that populations will always outgrow resources. That notion led Darwin to the idea of natural selection.

Karl Marx, himself a denizen of one of the most congested of London districts, was just as impressed by the dismal conditions of the new proletariat. He sent Darwin a copy of Das Kapital (which was found unread after his death). Marx, in a letter to Engels three years after The Origin of Species, went so far as to say that 'It is remarkable how Darwin recognises among beasts and plants his English society, with its division of labour, competition, opening up of new markets, inventions, and the Malthusian struggle for existence.¹ Engels took it further. In his essay The Part Played by Labour in the Transitioft from Ape to Man he argued that an economic change, the use of hands to make things, was crucial to the origin of humans. If one substitutes the term 'tools' for 'labour' his views sound rather like those of modern students of fossils.

Genetics shows that much of evolution is, as Engels said, linked to social advance. However, far from society being impelled by its genes, social and economic changes have produced many of the genetic patterns in the world today. Every technical development, from stone tools on, has led to an evolutionary shift and to biological consequences that persist for thousands of years. Society — and most of all the economic pressures that cause people to move — drives genes, rather than genes driving society. Relentless expansion is at the centre of human evolution: in Pascal's pessimistic words, 'AH human troubles arise from an unwillingness to stay where we were born.'

Fossils show that almost as soon as they evolved, humans began to migrate. Why our ancestors were so restless, nobody knows. Technological progress may have been involved, as the emergence of modern humans coincided with improvements in stone axes and the* like (although tools had been made for at least two million years before the great diaspora).

Perhaps climatic change was as important. The Sahara Desert was once a grassy plain and Lake Chad a sea bigger than the present Caspian. Both dried up about a hundred thousand years ago, so that food shortage may have driven man out of Africa. A microcosm of that process is taking place at the southern edge of the Sahara. As the rains fail, the desert has spread into the Sahel and migrants are on the move.

The earliest economies had a simple foundation. People used what nature provided, until it ran out. The world is filled with fossils of large and tasty animals that were driven to extinction soon after humans arrived. In Siberia, so many mammoths were killed that the hunters made villages from their bones. In Australia, too, there was a shift from forests to grasslands as the immigrants burned their way across the continent. The record of destruction is preserved in the Greenland ice-sheet. The snows which fell tens of thousands of years ago retain the soot and ash from gigantic forest fires set by our ancestors.

New Zealand was not colonised until the time of William the Conqueror. For a few years there flourished a culture based on the exploitation of a dozen species of moas, giant flightless birds. The ritual slaughtering grounds where the birds were killed (and where half a million skeletons have been found) are still around. The birds themselves are not. In Europe, too, whole faunas went not long ago. Humans did not reach Crete, Cyprus and Corsica until around ten thousand years before the present. Before then they had some extraordinary inhabitants; pygmy hippos, deer and elephants, and giant dormice, owls and tortoises. Soon after the arrival of the first tourists, all were gone, and the burnt bones of barbecued hippos are scattered among the remnants of the earliest Cypriots.

The common large mammal in Europe and the Near East at the time when modern humans moved from Africa was one of their own relatives, Neanderthal Man. He had lived there quite happily for two hundred thousand years. Many Neanderthals found homes in the dense forests of southern France. Some had an economy based on hunting reindeer, with settlements concentrated around their migration routes. The cave of Combe Grenal in Perigord contains tens of thousands of Neanderthal stone tools. Their culture was, in its own way, sophisticated; but it did not progress and showed no real change for a hundred thousand years. Tools in Britain and the Middle Hast look almost the same. Those who made them had little interest in exploration and never made boats, so that the delights of the Mediterranean islands (hippo-infested though they were) remained unknown. Neanderthals were the first conservatives.

Soon after the invasion of Europe by our own direct ancestors, they disappeared. Why, we can but guess. The guesses range from genocide to interbreeding. The first is unlikely. In France, at the cave of St Cesaire, Neanderthals and moderns lived close to each other for thousands of years. The second is probably wrong. If there had been sex between the indigenous population and the invaders, then modern Europeans would be expected to retain genes from this distinct branch of the human lineage and to have genes distinct from those of today's Chinese or Indians, whose ancestors never met a Neanderthal, let alone mated with one. They do not. Perhaps economic pressure did away with those ancient conformists. For most or history, Africa was the most advanced continent. Africans made sharp blades while Europeans had to manage with blunt axes. There was a period when Neanderthals seemed to pick up some of the new technology, but it did not last. The Hist modern Europeans were found in 1868 during railway work, in the Cro-Magnon shelter at the Perigord village of I.es Kyzics. Cro-Magnons looked much like modern Europeans. They (and their immediate predecessors the Aurigiuicians) had a sophisticated hunter-gatherer economy and made a variety of tools. Their cave art reached its peak around forty thousand years ago. The moderns had tools made of bone and ivory when their relatives sriil were satisfied with stone. They were better at exploiting what was available, so that their populations grew faster. That drove Neanderthals (and their genes) out. The last known skeletons are from St Cesaire. They died more than thirty thousand years ago.

Simple as it was, the Neanderthal economy held our ancestors at bay for a long time. The moderns reached Australia before they filled Europe. Competition from its indigenous inhabitants may have kept them out.

Most of the globe was populated at some speed after humans left their natal continent. The first Australians arrived about sixry thousand years before the present. The earliest remains are in two sites in Arnhem Land, in north Australia, which contain stone tools and ochre paints in a sandy deposit. The sites are close to the shore and perhaps to the point where humans arrived from rhe north. Soon, its inhabitants had complex tools and fishing nets and were economically as well developed as the rest of the world.

For much of its history Australia was joined to what is now New Guinea by a land bridge. It disappeared just seven thousand years ago. Tasmania was also part of Greater Australia. That great continent, Sahul, has always been separated from Asia by a deep trench. The first Australians must have crossed at least ninety kilometres of water to reach their new home.