According to anthropologists, around 12 000 years ago at the dawn of the Neolithic Revolution, when humans ceased hunting and gathering, there may have been between 2 and 20 million people living on Earth. By about 1750, it would have been around 750 million. A century later, in 1850, the world's population would have been about 1.2 billion. Industrial production of foodstuffs kept creating new room for growth. By 1900, the population was 1.6 billion; in 1950, according to the United Nations, it had reached 2.5 billion, and then soared to 4 billion by 1975. By 2000, the population had reached 6 billion. There are now more than 7.5 billion people living on Earth, and the United Nations expects that figure to rise to 9 billion by 2040.  In the near future, the planet's re...