Scientists have created the world’s first monkey embryos containing human cells in an attempt to investigate how the two types of cell develop alongside each other. The embryos, which were derived from a macaque and then injected with human stem cells in the lab, were allowed to grow for 20 days before being destroyed. We have a term for this type of life form: a chimera, named after the fire-breathing monster of Greek mythology that was part lion, part goat and part snake. It’s hoped that part-human chimeras – essentially animal bodies with some human organs or other characteristics – might one day offer clues to help us treat human diseases, as well as providing organs to transplant to humans. But for these purposes, part-human chimeras will first have to be born, and this research takes us one step closer to that eventuality.