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A long time ago, our ancient animal ancestors had tails, but now, we don't. About 25 million years ago, when our ancestors became different from monkeys, we lost our tails. Scientists have always wondered how and why this happened.  
Recently, scientists found a special change in a gene that helped our ancestors lose their tails. They studied the genes of apes and monkeys and discovered this change. They even did an experiment with mice by changing their genes in the same way, and these mice were born without tails.  
Some people think not having tails helped our ancestors survive better, but others believe it was just by chance. Maybe, not having tails made it easier for some animals to stand up straight and walk on two legs. Monkeys use their tails for balance, but apes like us and some others swing through trees without them. Losing our tails was a big deal, and scientists are still trying to understand all the reasons why it happened.