MATCH diseases such as leptospirosis and salmonella have their origin in domestic animal hosts. Often these viruses are endemic in animal and bird populations, where they may produce no symptoms. But the pandemics of flu in 1918, 1957, and 1968 show how devastating they can be once they have found a way of infecting humans and evading our immune defences. The pandemic of 1918 is estimated to have killed between 20 and 40 million people worldwide, far more than the First World War it so closely followed. The recent cases of avian flu in humans, caused by the H5N1 strain of the virus, first arose in China where humans live in close contact with many species of domesticated and wild birds, and they show how important this threat remains. They are the most recent in a long line of such infections reflecting the constant battle between the influenza virus and humans for reproductive fitness. And there is another side to this story too. Because we became adapted to living with high levels of some forms of bacterial pathogens, it is now suggested that our immune systems suffer from the lack of such threats in our modern, cleaner world. This may partly explain the increasing incidence of asthma and allergies, especially in children.28
Villages and towns The development of agriculture brought very drastic changes in social organization. Villages appeared, followed by towns and then cities. Some people now lived in contact with much wider networks of others than they would have had if they had maintained the hunter-gatherer or pastoralist way of life. Echoes of the latter persist today. Much of the so-called developing world persisted with a hunter-gatherer/pastoralist approach until colonization. The strength of colonizing powers was based on their technological advantage arising from denser populations, networked collective learning, and highly differentiated skills. It enabled them to achieve technological dominance over people they generally viewed as more ‘primitive’. We will first discuss those peoples for whom settlement became the dominant way of life. It necessitated the construction of more permanent dwellings. It also led to groups of people living together, and as these groups got larger, skills started to be separated within society. Not everyone needed to do everything—perhaps some wove and some made tools and some tended the crops. Populations grew rapidly when times were good but settlement made it harder to move in years when the harvest was poor.
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