WASHINGTON (Reuters) - Old people may hold the key to human civilization, U.S. researchers said on Tuesday. They found evidence that, around 30,000 years ago, many more people started living into old age, in turn fueling a population explosion. Rachel Caspari of the University of Michigan and Sang-Hee Lee of the University of California at Riverside believe that groups in which old people survived better were more successful, in turn allowing more people to live into old age. "There has been a lot of speculation about what gave modern humans their evolutionary advantage. This research provides a simple explanation for...