During the first 2 million or so years of its history the human population was a minor element in the world ecosystem, with at most 10 million members. In the New Stone Age, less than 10,000 years ago, the number of humans began to increase more rapidly. The rough equilibrium maintained before Neolithic times gave way when the human population developed agriculture and animal husbandry and no longer had to spread out in search of game. With the abandonment of a hunting-gathering way of life and the rise of permanent settlements and eventually cities, the human population underwent dramatic growth. By the beginning of the Christian era it had reached 250 million, and by 1650, half a billion. Growth of population during 20th century was very rapid. In 1994 the total world population was estimated at about 5.6 billion people. It increased nearly by 4 billions of people during past 100 years. The most significant world trend is that death rates are currently falling in poor and rich countries alike, while birthrates remain high in most poor countries and low in most rich ones. Exceptions are the generally higher death rates of Africa and the high birthrates of the rich oil-producing countries. The most populous countries, in descending order, are China, India, the United States, Indonesia, Brazil, and Russia. The U.S. population totaled about 260.8 million in 1994. In the 1990s about 4.1 million children were born annually in the United States, and more than 2.1 million persons died yearly. The greater number of births is due in part to a fertility rate that has increased by nearly 20 percent since the mid-1980s. International immigration, both legal and illegal, is another major element in U.S. population growth. Legal immigration has recently amounted to about 1 million per year; illegal immigration is thought to be several hundred thousand. In China, the world's most populous country, the 1994 population was estimated at nearly 1.2 bil...
