Once upon a time, your origins were easy to understand. Your dad met your mum, they had some fun, and from a tiny fertilised egg you emerged kicking and screaming into the world. You are half your mum, half your dad – and 100% yourself. Except, that simple tale has now become a lot more complicated. Besides your genes from parents, you are a mosaic of viruses, bacteria – and potentially, other humans. Indeed, if you are a twin, you are particularly likely to be carrying bits of your sibling within your body and brain. Stranger still, they may be...