Causes of the migrations
The only seemingly incontestable influence on the way people behaved in our period is climate - though it is so general in impact as to explain little or nothing in detail. Every episode of warming can be presumed to have stimulated migration into cooler areas in search of climatic stability or of familiar ecological circumstances in pursuit of relocated food sources.
Every episode of cooling must presumably have had an equal and opposite effect, sending migrants in search of warmth. The alternation of humid phases with periods of desiccation - both of which are impossible to date, in the present state of our knowledge, without margins of error of up to 10,000 years - and the periodic juxtaposition of drying and humidifying zones - can be presumed to account for some shifts of population.[384] Climatic instability generally may have habituated populations to a diversity of conditions and emboldened migrants to penetrate previously untried environments.Perhaps new stresses drove the migrants on. Food shortages or ecological disasters might explain the necessity, but no evidence supports this or fits with the evidence of rising population. In every other case we know of, in all species, population falls when food sources shrink. This point is not decisive, as one can imagine a world in which survivors flee societies in crisis, and rapidly recover demographic buoyancy when they encounter new resources; but such a way of picturing the world of 100,000 years ago is speculative.
Warfare may be a more promising form of stress to consider for present purposes. Among the four horsemen of the Apocalypse - war, plague, famine, and natural disaster - war is the odd one out. The other three tend to inhibit human action, whereas war spurs us to new responses. One of the most fascinating problems of history is how and when war started.
According to one school of thought, war is natural to humankind. Field Marshal Montgomery referred people who asked how he justified war to Maeterlinck's The Life of the Ant.[385] At the time, a number of distinguished anthropologists and zoologists agreed, arguing that evolution implanted aggressive and violent instincts in humans as it did in other animals.[386] Romantics defended the opposite point of view: that human nature is essentially peaceful until competition corrupts it. War, according to Margaret Mead, the great liberal anthropologist of the 1920s and 1930s, was an invention, not a biological necessity.[387]At first, the evidence seemed divided. The earliest archaeological proof we have of large-scale warfare is a battle fought at Jebel Sahaba, near the modern border of Egypt and Sudan, about 11,000 years ago. The victims included women and children. Many were savaged by multiple wounds. One female was stabbed twenty-two times. At the time, agriculture was in its infancy. Today, peoples who practise the simplest agriculture as well as those who supposedly represent modernity and civilization massacre others. These facts have encouraged speculation that warfare began - or, at least, entered a new, more systematic phase - when settled communities started to fight one another to control land and resources.[388] Yet it seems that organized, collaborative violence must really be much older. In the 1970s, the primatologist Jane Goodall observed something like what we would now call gang warfare among chimpanzee communities.[389] When chimpanzee splinter groups secede from their societies, their former fellows try to kill them. Similar conflicts may have made early human splinter groups migrate to safety. It is an intriguing speculation, but, even if it were to prove correct, it poses other problems. What could have caused people to divide and fight each other a hundred thousand years ago? Rising population again? Competition for mates (which is a common reason for secessions from chimpanzee groups)? Or are we driven back to more speculation about increasing competition for supposedly diminishing food stocks, or even to assertions about innate animal aggression?
Whatever migrants' motives were, they needed sufficient technology to multiply and move.
Cooking with fire probably helped to make population growth possible, because it made food easier to digest. Creatures like us, who have short guts, weak jaws, blunt teeth, and only one stomach each, can only chew and digest limited energy sources. As a result, anything that increased the range of foods available to early humans and encouraged and enabled them to eat a lot was a major evolutionary advantage. Still, the chronology of cooking does not seem to fit. The palaeoanthropologist Richard Wrangham has argued brilliantly for a starting date for cooking with fire more than 2 million years ago. His argument is based on inferences from physiology, such as the evolving shape of hominid teeth, which, apparently, got smaller and blunter at that time, presumably in response to food modified by flames; hominids' shortening gut, too, was probably a response to dietary changes in the same period.[390] There is no direct evidence of fire used for cooking at that time, and an increase in the consumption of meat prepared in other ways might explain the physiological evidence. Yet the domestication of fire still seems to have occurred too early to help explain the migrations. Fires that burned in caves between half a million and 1.5 million years ago look as if they were deliberately kindled. An almost irresistible case is that of Zhoukhoudian in China, where the great Jesuit scientist Pierre Teilhard de Chardin (1881-1955) excavated the evidence and the Abbe Henri Breuil (1877-1961), the leading archaeologist of the day, identified it. ‘It's impossible', said the Jesuit, thinking the site was too early for the controlled use of fire, ‘[that] it comes from Zhoukhoudian'. ‘I don't care where it comes from', the Abbe replied. ‘It was made by a human, and that human knew the use of fire.'[391]We are similarly uncertain about when other technologies started, such as drive lanes and corrals, which might have improved diet by enhancing hunting. But the earliest known examples of fire-hardened spears are only 150,000 years or so old, taking us back to a date near the start of the migrations. Presumably, the ‘container revolution' - the beginnings of the technologies that transformed skins or fibres into easily transportable carriers - preceded or accompanied long-range human dispersals; the chronology of these developments cannot be satisfactorily established but they happened early enough to assist the first Homo sapiens migrating out of Africa.[392]