In 2013, Dunning-Hotopp showed that these short-lived unions are surprisingly common (Riley et al., 2013). She analysed hundreds of human genomes that had been sequenced from body cells – the ones from kidneys or skin or livers, none of which get passed on to offspring. She found traces of bacterial DNA in around a third of them. They were especially common in cancer cells; an intriguing result with unclear implications. It might be that tumours are especially prone to genetic intrusions, or that bacterial genes help to transform healthy cells into cancerous ones.
