Human migration: the term is vague. What people usually think of is the permanent movement of people from one home to another. More broadly, though, migration means all the waysfrom the seasonal drift of agricultural workers within a country to the relocation of refugees from one country to another. Migration is big, dangerous, compelling. It is 60 million Europeans leaving home from the 16th to the 20th centuries. It is some 15 million Hindus, Sikhs, and Muslims swept up in a tumultuous shuffle of citizens between India and Pakistan after the partition of the subcontinent in 1947. Migration is the dynamic undertow of population change: everyone's solution, everyone's conflict. As the century turns, migration, with its inevitable economic and political turmoil, has been called 'one of the greatest challenges of the coming century.' But it is much more than that. It is, as it has always been, the great adventure of human life. Migration helped create humans, drove us to conquer the planet, shaped our societies, and promises to reshape them again. 'You have a history book written in your genes,' said Spencer Wells. The book he's trying to read goes back to long before even the first word was written, and it is a story of migration. Wells, a tall, blond geneticist at Stanford University, spent the summer of 1998 exploring remote parts of Transcaucasia and Central Asia with three colleagues in a Land Rover, looking for drops of blood. In the blood, donated by the people he met, he will search for the story that genetic markers can tell of the long paths human life has taken across the Earth. Genetic studies are the latest technique in a long effort of modern humans to find out where they have come from. But however the paths are traced, the basic story is simple: people have been moving since they were people. If early humans hadn't moved and intermingled as much as they did, they probably would have continued to evolve into different species. From beginnings in Africa, most researchers agree, groups of hunter- gatherers spread out, driven to the ends of the Earth. To demographer Kingsley Davis, two things made migration happen. First, human beings, with their tools and language, could adapt to different conditions without having to wait for evolution to make them suitable for a new niche. Second, as populations grew, cultures began to differ, and inequalities developed between groups. The first factor gave us the keys to the door of any room on the planet the other gave us reasons to use them. Over the centuries, as agriculture spread across the planet, people moved toward places where metal was found and worked and to centres of commerce that then became cities. Those places were, in turn, invaded and overrun by people later generations called barbarians. In between these storm surges were steadier but similarly profound tides in which people moved out to colonize or were captured and brought in as slaves. For a while the population of Athens, that city of legendary enlightenment, was as much as 35 percent slaves. 'What strikes me is how important migration is as a cause and effect in the great world events,' Mark Miller, co-author of The Age of Migration and a professor of political science at the University of Delaware, told me recently. It is difficult to think of any great events that did not involve migration. Religions spawned pilgrims or settlers wars drove refugees before them and made new land available for the conquerors political upheavals displaced thousands or millions economic innovations drew workers and entrepreneurs like magnets environmental disasters like famine Or disease pushed their bedraggled survivors anywhere they could replant hope. 'It's part of our nature, this movement,' Miller said. 'It's just a fact of the human con