How would you feel if you gave birth to a beautiful newborn, only to clock they have a tail? It sounds outlandish, but it can happen in very rare cases when humans can be born with these boneless rear-end appendages, which are sometimes up to 18 centimetres long. To date, official records have tallied about 40 babies born with these 'tails' which are easily removed through surgery. So how does it happen? Experts used to think the tails were evolutionary accidents, leftovers from our primate ancestors. Sign up to our free Indy100 weekly newsletter But thanks to research, experts have realised they probably come from an incomplete fusion of the spinal column, or what's known as a spinal dysraphism. Human babies that are born with tails tend to have serious associated neurological defects. In 2008, a paper argued that "true vestigial tails are not benign" because they may be associated with underlying dysraphism. Roughly half of the cases reviewed were associated with either meningocele or spina bifida occulta. So it is not just a harmless evolutionary quirk after all, but more research needs to be done, scientists say. Have your say in our news democracy. Click the upvote icon at the top of the page to help raise this article through the indy100 rankings.