Nature and scope of the migrations
The first great problems early humans' dispersal poses for us are why they moved, and what made them so adaptable to different environments. These are big, perplexing problems because most species stay in the environments where they are best adapted.
Even human populations rarely, if ever, seek new environments willingly, or adjust easily.Yet when groups of Homo sapiens migrated out of Africa to people the world, they often relocated in challengingly different places: deep forests, where grassland habits were of limited use; extremely cold climates, to which they were physically ill-suited; deserts and seas, which demanded technologies they had not yet developed. Some of these new habitats bred unfamiliar diseases. Still, people kept on moving, through them and into them. We are still struggling to understand how it happened. In these pages, I assume that ‘migration' - rather than ‘dispersion', a term that specialists in the movements of other species tend to prefer in disavowal of the implication of a conscious process - is the right term to designate human movements, even as long ago as 100,000 years. The equipage of migrants of that era contained artefacts associable with thoughts and sensibilities akin to our own: shell jewellery, incised slabs of ochre, and, in Blombos Cave in South Africa, in an area settled by migrants from East Africa about 100,000 years ago, shell-crucibles and spatulas for mixing pigments.[378] [379] [380] Investigators have perceived objects of art and the remains of musical instruments in stratigraphic layers up to 100,000 years old. That our ancestors then had ‘a theory of mind' - consciousness of their own consciousness - is a proposition hard to resist in the presence of so much evidence of imaginations so creative and so constructive. They had the mental equipment necessary to be able to imagine themselves in changed circumstances and new environments, and to attempt to realize the changes.To some extent, we can reconstruct where and when Homo sapiens travelled while peopling the Earth, even though the archaeological evidence is patchy, by measuring differences in blood type, genetic make-up, and language among populations in different parts of the world. 14 The greater the differences, the longer the ancestors of the people concerned are likely to have been out of touch with the rest of humankind. The science is inexact, because people are rarely isolated for long. Over most of Eurasia and Africa, populations have moved about tremendously in recorded history. Groups of people have frequently been mixed and restirred. There are, moreover, no agreed ways to measure the differences among languages. Readers should be aware that genetic and linguistic work has yielded few, if any, uncontested conclusions.
Still, for what it is worth, the best-informed research puts Homo sapiens in the Middle East by about 100,000 years ago. The colony failed, but newcomers reappeared about 60,000 years ago. Settlement then proceeded along the coasts of Africa and Asia, probably, at least in part, by sea.15 The earliest agreed-upon archaeological evidence of Homo sapiens in China is about 67,000 years old (although some digs have yielded puzzlingly earlier dates for remains that seem like those of Homo sapiens). It may seem surprising that humans developed nautical technology so early. Yet some evidence supports or, at least, invites inferences that even Homo erectus may have travelled short distances by sea, and the first colonizers of Australia arrived over 50,000 years ago and must have used boats, because at that time, water already separated what are now Australia and New Guinea from Asia.[381] [382]
Homo sapiens reached Europe only a little later. Northern Asia - isolated by daunting screens of cold climate - was probably colonized about 30,000 years ago. The New World was settled from there in, for scholars, the most contested phase of the story.
According to the formerly dominant theory, a gap opened between glaciers towards the end of the Ice Age. A race of hunters crossed the land link between Asia and North America, where the Bering Strait now flows, to enter a paradise where no human hunter had ever trod before. The abundance was so great and the animals so unwary that the invaders ate enormously and multiplied greatly. They spread rapidly over the hemisphere, hunting the great game to extinction as they went. The Clovis people, as these hunters were dubbed after an early archaeological site in New Mexico, seemed to resemble modern American pioneers. They exhibited quick-fire locomotion, hustle and bustle, technical prowess, big appetites, irrepressible strength, enormous cultural reach, and a talent for reforging the environment.By comparison, the truth about the peopling of the hemisphere is disappointingly undramatic. These first great American superheroes - like most of their successors - did not really exist. Although archaeologists have excavated too few sites for a complete and reliable picture to emerge, a new theory dominates. We have evidence of early human settlement scattered from the Yukon to Uruguay and from near the Bering Strait to the edge of the Beagle Channel. This evidence is so widespread, over so long a period, in so many different geological layers, and with such a vast range of cultural diversity that one conclusion is inescapable - colonists came at different times, bringing different cultures with them.17
Some came by sea and continued to come after the land bridge was submerged. Around 10,000 years ago, a catastrophic cluster of extinctions wiped out the mammoth, mastodon, horse, giant sloth, sabre-toothed tiger, and at least thirty-five other large species in the Americas. New hunting techniques and perhaps new hunting peoples were probably partly responsible. But we can only explain the events in the context of vast climatic changes that affected habitats and the whole ecology on which these animals depended.[383]
In any case, by the end of the era of climatic fluctuation, humans occupied almost all the habitats their descendants occupy today, with the exception of relatively remote parts of the Pacific, accessible only by high-seas navigation and unsettled, as far as we know, for many millennia more.
A handful of Eve's children had multiplied to the point where they could colonize most of the habitable Old World in less than 100,000 years.The expansion of Homo sapiens implies an astonishing rate of population growth. We have no idea - beyond guesswork - of the actual numbers that migrated, but we can estimate a figure in millions by the end of the process. As far as we know, everyone at the time lived by foraging and moved on foot. Because mothers cannot easily carry more than one or two infants, large numbers of children are unsuited to foraging life. Consequently, foragers usually limit their families, either by strictly regulating who can mate with whom or by practising other forms of population control. Their main contraceptive method is a long period of lactation. Breast-feeding mothers are relatively infertile. The demographic growth that peopled the Earth is surprising, therefore, because it breaks the normal pattern of population stability in foraging communities. How can we explain it? Was the increase in population cause or effect of the migrations? And how did it relate to the other changes migration brought? Migrating groups were doubly dynamic: not just mobile, but also subject to huge social changes.