Evolution of human longevity, population pressure and the origins of warfare.

In a protected environment, humans have the longest lifespan of all primates. However, during the emergence of Homo sapiens from pre-hominids, the expectation of life at birth would have been quite low. On the basis of reasonable assumptions, an average expectation of life of less than 20 years is sufficient to maintain a population of hunter-gatherers. As individuals became better adapted to their environment, the mortality rate would gradually decrease, and this would result in the survival of more offspring to adulthood. Thus, the population will increase, and one of the consequences in human evolution is the migration of human communities to many new habitats. The development of agriculture provided a more reliable source of food, and stimulated further the increase in population size. Villages became towns, and then cities, states and empires arose which had very large populations, and competed for land and other resources. Armies were raised and were often at war. All this was due to population pressure, as Malthus had realised more than 200 years ago. However, neither he, nor any of the others who discussed warfare, understood that the demographic changes that produced large human populations was a steady increase in the expectation of life at birth. This inevitably occurred at the same time as man gradually gained more control over his environment, and achieved far more reproductive success than is seen in hunter-gatherers living in a harsh, stressful environment.