quite dramatically, 'segmenting formerly open ranges into smaller units and arranging the niches for different species by altitude and type of vegetation...Sedentism and the reduction of open range encouraged territoriality. People began to protect and propagate local herds, a pre-domestication practice that can be referred to as food resource management.'11 A further aspect of this set of changes was that the climate became increasingly arid, and the seasons became more pronounced, a circumstance which encouraged the spread of wild cereal grasses and the movement of peoples from one environment to the next, in search of both plants and animal flesh. There was more climatic variety in areas which had mountains, coastal plains, higher plains and rivers. This accounts for the importance of the fertile crescent. Grasses were naturally prevalent in this Near Eastern region (wild stands of emmer and einkorn wheat, and barley, exist there to this day). But it is not difficult to work out what happened. 'The harvested batch of seeds would be selected in favour of non-shattering and uniform maturation. As soon as humans began to sow the seeds they had harvested, they automatically-even if unintentionally- initiated a process of selection in favour of the non-shattering genotype.'12 Mark Nathan Cohen is the most prominent advocate of the theory that there was a population crisis in pre-history and that it was this which precipitated the evolution of agriculture. Among the evidence he marshals to support his argument is the fact that agriculture is not easier than hunter-gathering, that there is a 'global coincidence' in the simultaneous extinction of mega-fauna, the big mammals which provided so much protein for early humans, a further coincidence that domestication emerged at the end of the Pleistocene Age, when the world warmed up and people became much more mobile, and that the cultivation of wild species, before agriculture proper, encouraged the birth of more children. It is well known, for instance, that nomads and hunter-gatherers control the number of children by not weaning them for two years. This limits the size of a group that is continually on the move. After the development of sedentism, however, this was no longer necessary, and resulted, says Cohen, in a major population explosion. Cohen also claims that evidence for a population crisis in antiquity can be inferred from the number of new zones exploited for food, the change in diet, from plants which need less preparation to those which need more, the change in diet from larger animals to smaller (because larger ones were extinct), the increasing proportion of remains of people who are mal-nourished, the specialisation of artefacts which had evolved to deal with rarer and rarer animals and plants, the increased use of fire, for cooking otherwise inedible foodstuffs, the increased use of aquatic resources, the fact that many plants, though available as food in deep antiquity, were not harvested until around 12,000 BP, that grass (cereals) is a low priority in food terms, and so on and so on, all of which Cohen contends is corroborated by archaeological excavation. For him, therefore, the agricultural revolution was not, in and of itself, a liberation for early humans. It was instead a holding action to cope with the crisis of overpopulation. Far from being an inferior form of life, the hunter-gatherers had been so successful they had filled up the world, insofar as their lifestyle allowed, and there was no place to turn.13 It is another attractively simple hypothesis but there are problems with it. One of the strongest criticisms comes from Les Groube, who is the advocate of a rival theory. According to Groube, who is based in France, it is simply not true that the world of deep antiquity was in a population crisis, or certainly not a crisis of overpopulation. His argument is the opposite, that the relatively late colonisation of Europe and the Americas argues for a fairly thinly populated Earth. For Groube, as man moved out of Africa into colder environments, there would have been fewer problems with disease, simply because, from a microbial point of view, the colder regions were safer, healthier. For many thousands of years, therefore, early man would have suffered fewer diseases in such places as Europe and Siberia, as compared with Africa. But then, around 20,000 years ago, an important coincidence took place. The world started to warm up, and man reached the end of the Old World-meaning that, in effect, the known world was 'full' of people. There was still plenty of food but, as the world warmed up, many of the parasites on man were also able to move out of Africa. In short, what had previously been tropical diseases became temperate diseases as well. The diseases Groube mentions include malaria, schistosomiasis and hookworm, 'a terrible trinity'. A second coincidence also occurred. This was the hunting to extinction of the mega-fauna, which were all mammals, and therefore to a large extent biologically similar to man. All of a sudden (sudden in evolutionary terms), there were far fewer mammals for the microbial predators to feast on-and they were driven to man.14 In other words, sometime after 20,000 years ago, there was a health crisis in the world, an explosion of