"Just 11,000 years ago, there were only roughly 5 million humans who lived on the planet Earth. The initial population growth was slow, due largely to the way humans were living—by hunting. Such lifestyle limited the size of family for practical reasons. A woman on the move cannot carry more than one infant along with her household baggage. When simple birth control means-often abstention from sex failed, a woman may elect abortion or, more commonly, infanticide to limit the family size. Further, a high mortality among the very young, the old, the ill and the disabled acted as a natural resistance to a rapid population growth. Thus it took over one million years for human population to reach the one billion mark. But the second billion was added in about 100 years, the third billion in 50 years, the fourth in 15 years, and the fifth in 12 years. Ever since humans became sedentary, some limits over the family size were lifted. With the development of agriculture, children may have become more of an asset to their families in helping with farming and other chores. By the beginning of the Christian era, human population grew to about 130 million, distributed all over the Earth. By 1650, the world population had reached 500 million. The process of industrialization had begun, bringing about profound changes over the lives of humans and their interactions with the natural world. With improved living standard, lowered death rate and prolonged life expectancy, human population grew exponentially. By 1999 there were about 6 billion people, comparing with 2.5 billion in 1950. The world population is well on its way to 7 billion with an annual growth rate of over 90 million."