Archeological evidence indicates that Homo sapiens began migrating out of eastern and southern Africa as early as 200,000 years ago. This expansion took early humans deeper south, west, and north as far as the Mediterranean Sea. Approximately 100,000 years ago, groups of Homo sapiens left the African continent and began a global migration that lasted for tens of thousands of years (Figure 2.6). After crossing the Sinai into southwest Asia, early migrants out of Africa likely followed the coasts of Asia, and by about seventy thousand years ago, they had made their way into India and China. Figure 2.6 Homo sapiens first expanded around south and eastern Africa before embarking on migrations that eventually took them around the world. (credit: modification of work “Spreading of Homo sapiens” by “NordNordWest”/Wikimedia Commons, Public Domain) Some groups continued moving south through Malaysia, into Indonesia and beyond. In places like Papua New Guinea and Australia, there is evidence of settlements at least forty-five thousand years old. Others groups making their way into southwest Asia from northern Africa entered Europe around forty thousand years ago, moving either along the Mediterranean coast or by way of Turkey into the Danube valley. By twenty-five thousand years ago, Homo sapiens had reached Siberia and other parts of northern Asia. And approximately fifteen thousand years ago, some groups in Asia crossed into North America, eventually reaching the tip of South America and settling at various locations in between. This timeline has been pieced together based on the analysis of several archaeological finds. But our knowledge is still limited, and new discoveries frequently require adjustments to the proposed dates and patterns of global human migration. For example, we now know that because the Earth was in its most recent ice age during this period, areas currently covered by water were then dry land. This is true for large portions of maritime Southeast Asia as well as the Bering Strait between Asia and North America. Humans were able to walk as far south as Java and from Asia into Alaska. Yet they also roamed as far as Australia, which was not connected by land to Asia in this period. This means they must have used rafts of some type, probably by crossing short distances between islands. Likewise, discoveries of human habitation dating from fourteen thousand years ago in South America suggest that rafts or boats of some kind may also have been used to skirt the North and South American coasts. No crafts have been or may ever be found, but we must assume they existed. More interesting still, analysis of the remains of the eight-thousand-year-old Kennewick Man discovered in 1996 in Washington State reveals anatomical features more consistent with Southeast Asian populations than with those traditionally assumed to have populated the Americas. This discovery complicates the version of human migration we think we know, and if anything, it suggests there is much about the process that we may never fully understand. But what triggered this migration in the first place? Despite the uncertainties, we can draw some speculative conclusions. We know that around the same time Homo sapiens began leaving Africa, the climate there was becoming increasingly dry. Drier conditions meant fewer of the plants and animals humans needed to survive were available. Modern humans were hunter-gatherers like their evolutionary ancestors, meaning they survived by employing the strategies of hunting animals and gathering wild plants rather than by planting crops and raising livestock. As hunter-gathering societies regularly forage over a large area, any scarcity of resources in some places or abundance in others encourages movement. In the lifetime of a single individual, a large-scale migration would have been barely perceptible, if at all. But over tens of thousands of years, human populations traversed an enormous portion of the globe. Nor did they go in a single direction or all at once. Groups likely moved back and forth over areas, responding to the climatic conditions and availability of resources. There were long periods of relative stasis punctuated by movement, creating waves of migration in various directions. As humans moved into new environments, they adjusted their strategies to be successful under new conditions. This meant learning to gather different types of plants and hunt different types of animals they came into contact with, including mastodons, woolly mammoths and rhinos, various types of grazing animals, and giant sloths and beavers. The arrival of humans who were highly effective at survival occasionally accompanied major transformations in their new environments. Scientists who study now-extinct animals have recognized for some time that human hunting likely contributed to the decline of a number of these species. Before humans arrived approximately forty-five thousand years ago, for example, Australia was home to a number of large reptiles, a marsupial lion (which carried its young in a pouch), and huge wombats and kangaroos (Figure 2.7). These species began to vanish around the same time humans reached Australia and well before the climatic warming that led to the extinction of large animals in other places. Figure 2.7 Tens of thousands of years ago, Australia was home to many large marsupials, such as this marsupial lion. Its fossilized skeleton is shown here in the Victoria Fossil Cave where it was found. (credit: “A skeleton of a Marsupial Lion (Thylacoleo carnifex) in the Victoria Fossil Cave, Naracoorte Caves National Park” by “Karora”/Wikimedia Commons, Public Domain)