Back in old times, people had little knowledge about the universe and nature. Things we now consider to be common sense were mysteries to our ancestors. Over the years, major breakthroughs have been made in science and many phenomena have been explained. But still, there are always questions we can't yet answer, and The Guardian has listed some of them. 1. What makes us human? Just looking at your DNA won't tell you - human DNA is 99 percent _ to that of the chimpanzee and, believe it or not, 50 percent identical to a banana's! A lot of the things we once thought were unique about us - language and tool use, recognizing ourselves in the mirror and so on - have since been seen in other animals. Perhaps it's our culture that makes the difference or maybe our ability to use fire. It's also possible that our capacity   for co-operation and our trading skills are what make us unique. 2. Why do we dream? Given the fact that we spend around a third of our lives sleeping, shouldn't we know everything about it? Unfortunately, scientists are still searching for a complete explanation of what happens when we sleep and why we dream. Austrian psychologist Sigmund Freud believed dreams were the expressions of wishes that we can't fulfill in our real lives. Others have wondered whether dreams are just random "noise" coming from a sleeping brain. 3. Could we someday live forever? Apart from accidents, most people die because of diseases that can be treated and aging. And since many diseases, such as diabetes  and cancer, are diseases of aging, treating aging itself could be the key to extending our lives. Our knowledge of what causes us to age - and what allows some animals to live longer than others - is expanding rapidly. And though we haven't quite worked out all the details, we've worked out some pieces of the puzzles such as DNA damage and metabolism  , which are all leading to the invention of drugs that can slow down the aging process. If we're lucky enough to lengthen our lives, we might even get to see the day when all of these questions are answered. The main point of the passage is   _  .
to inform people of the knowledge about the universe and nature to introduce major breakthroughs that have been made in science to present some questions we can't yet answer to explain what were once mysteries to our ancestors