范文：
 　　It has been estimated that a thousand years ago there were less than 400 million people on earth. However, in the recent thousand years human population has increased nearly 20 times. Nowadays the world population has reached more than 7000 million. The population explosion has caused many problems. Generally, they come down to four major ones.
　　First, a lot of people can't get employment, which is a universal problem in the world. Second, in developing countries there are not enough shelters for too many people and so people are not properly housed and even are exposed to the elements. Third, in underdeveloped countries a lot of people suffer from hunger because of  food shortage, which in turn causes many other problems, malnutrition, disease, etc. Fourth, there are not enough schools for so many people and so a lot of people are illiterate.
　　Therefore, to eliminate these problems, human beings should take effective measures to control population.