It is believed that it took about 800 thousand to 1 million years for the earth's population to reach the 250 million total which existed at the end of the first century AD. For some time after that, disease, famine(¼¢¶ö) and war kept the population increase down to a rate of 1 percent a year so that more than 15 centuries passed before the population reached 500 million. But in the next 250 years, up to 1850, the population of the world shot to 1 billion and today it has reached 3.5 billion. Scientists say that in the next 35 years the population of the world will double, reaching almost 7 billion by the year 2010.