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| The evolution of the human species continues to the present day. Although biological change continues in human populations, the rate of such change is most often exceeded by the rapid rate of culture change. Over the past 12,000 years, the human species has moved from hunting and gathering to agriculture to feed itself, and the total size of the human species has increased roughly a thousandfold. These rapid cultural changes have influenced aspects of human biology by affecting our patterns of health, physical growth, nutrition, mortality, and fertility, among other things. The transition to agriculture represented a major change in human adaptation, and the domestication of plants and animals provided the opportunity to maintain much larger populations, which in turn allowed for the development of complex state-level societies. Larger sedentary populations led to many epidemics of infectious disease. Although more food could be produced, many people in early agricultural societies suffered from nutritional stress. Overall, life expectancy did not increase with the origins of agriculture. The subsequent rise of state-level societies with large urban populations was accompanied by further epidemics and nutritional problems. Exploration, trade, and conquest led to contact between cultures that had been isolated previously, resulting in catastrophic epidemics and social upheaval. By the end of the nineteenth century, modernization had begun the epidemiologic transition, whereby improvements in public health and civil engineering, later supplemented by improvements in medical science, led to a reduction in deaths from infectious diseases and a dramatic increase in life expectancy at birth. As people lived longer on average, the death rate due to noninfectious degenerative diseases increased. In the more developed countries, children mature faster and grow larger because of changes in disease and diet. Recent decades have seen a resurgence in infectious diseases, in part due to the evolution of new diseases and in part due to the reemergence of diseases as microorganisms evolve resistance to antibiotics. The world's population tripled in size during the twentieth century as death rates declined. Many populations have since seen a reduction in fertility rates, such that the rate of population growth has declined somewhat. Even given this trend, most projections suggest a global population of roughly 9 billion by the middle of the twenty-first century. Another consequence of recent demographic change is the changing age structure of the more developed countries. As birth rates decline, populations become older on average. All of these demographic changes are interrelated with many contemporary social, economic, and political problems. |