Lydia Fairchild was confused. She had applied for state benefits to look after her three children, but according to DNA tests, she was not their mother. It was ridiculous – she knew full well that the children were hers, but she was being taken to court nonetheless.

This happened in 2002, but Fairchild’s case has striking parallels with one that cropped up just this year, involving a Mediterranean sponge called Scopalina lophyropoda. French scientists Andrea Blanquer and Maria-J Uriz found that around a quarter of the sponge’s larvae are genetically distinct from the parents that they come from. Somehow, they had inherited genes from a different source.

Sponges are about as far away from humans as you could imagine an animal being – their bodies are just two layers of cells, curved and folded into tubes and chambers. But even though their bodies are worlds apart, the mysteries of both Lydia Fairchild and S.lophyropoda had the same answer.

Both of them are chimeras, living things that are formed when two or more fertilised eggs fuse together. The chimeras of myth were monsters that combined parts of lion, snake and goat. Real ...