To live forever was the dream of many emperors in ancient China.People were ordered to call them Wansui,which means 10,000 years,but no human could live that long.How long can humans really live? According to the latest scientific discovery,humans are able to live up to the age of 120 or perhaps even longer.     Several recent studies showed that our life span can be extended far beyond normal limits.Cynthia Kenyon,a professor at the University of California in San Francisco doubled the life of a worm from two weeks to a month.By changing the function of a single gene,known as daf-2,the scientist not only added extra days to the worm's life but also kept it a healthy as worms which were half its age.     There are two similar genes in the bodies of mammals.By changing the function of on gene,scientists extended the life of a mouse by 1 8 per cent(or 4.5 months).Another gene is called IGF-1.Researchers still don't know how it affects our body.But by reducing its functions the life of a mouse increases by 33 per cent for 8.6 months).The average life span of a mouse is 26 months.     In the experiment of extending life,an ancient saying,"you are what you eat." also seems to  be true.Vegetables contain fewer calories than meat,and help to prolong life,scientists say. Research has found that taking in fewer calories affects our lives in a similar way to reducing IGF-1.     US biologist John Phelan suggests low calorie diets could increase human life span,but there is no proof yet.     Researchers are still waiting for the results of a study started in 1987.Monkeys are fed with low calorie food to see what happens.A monkey's typical life span is 40 years,so there is a little while to wait.     In China,average,life expectancy has now reached 70,double that of 1949. Increasing the length of our lives may bring us closer to the dreams of our ancient emperors.But it may also cause social problems such as housing and employment,social scientists have warned. What is the main idea about this passage?
We are able to live longer. The genes affect life span of human beings. A balanced diet prolongs our life. Extending life span has caused terrible social problems.