Past and Present Growth Estimates of world population before 1900 are based on fragmentary (零散的) data, but scholars agree that, for most of human existence, long-run average population growth approached approximately 0.002 percent per year, or 20 per million inhabitants. According to UN estimates, the population of the world was about 300 million in the year AD 1, and it took more than 1,500 years to reach the 500 million mark. Growth was not steady but was marked by oscillations (摆动) dictated by climate, food supply, disease, and war. Starting in the 17th century, great advances in scientific knowledge, agriculture, industry, medicine, and social organization made possible rapid acceleration in population growth. Machines gradually replaced human and animal labor. People slowly acquired the knowledge and means to control disease. By 1900 the world population had reached 1.65 billion, and by 1960 it stood at 3.04 billion. Beginning about 1950, a new phase of population growth was ushered in when famine and disease could be controlled even in areas that had not yet attained a high degree of literacy or a technologically developed industrial society. This happened as a result of the modest cost of importing the vaccines (疫苗), antibiotics, insecticides, and high-yielding varieties of seeds produced since the 1950s. With improvements in water supplies, sewage-disposal facilities, and transportation networks, agricultural yields increased, and deaths from infectious and parasitic diseases greatly declined. Life expectancy at birth in most developing countries increased from about 35～40 years in 1950 to 66 years by 2000. The rapid decline in deaths among people who maintained generally high fertility rates led to annual population growth that exceeded 3.1 percent in many developing nationsa--rate that doubles population size in 23 years.

Regional Distribution As of 2000, 1.2 billion people lived in the developed nations of the world, and 4.9 billion people lived in the less-developed countries. By region, over half the world's population was in East and South Asia China, with 1.3 billion inhabitants, and India, with some 1 billion, were the dominant contributors. Europe and the countries of the former USSR contained 14 percent, North and South America made up 14 percent, Africa had 13 percent, and the Pacific Islands had about 1 percent of world population. Differences in regional growth rates are altering these percentages over time. Africa's share of the world population is expected to more than double by the year 2025. The population of South Asia and Latin America is expected to remain nearly constant in other regions, including East Asia, the population is expected to decline appreciably. The share of the present developed nations in world population--20 percent in 2000--is expected to fall to 15 percent by 2025. Nine out of every ten persons who are now being added to the world's population are living in the less-developed countries.

Urban Concentration As a country develops from primarily an agricultural to an industrial economy, large-scale migration of rural residents to towns and cities takes place. During this process, the growth rate of urban areas is typically double the pace of overall population increase. Some 29 percent of the world population was living in urban areas in 1950 A．Y B．N C．NG