The cells in question are choanoflagellates – the closest living relatives of all animals. They’re not our direct ancestors, but they give us clues about what those ancestors were like when they were still swimming around as single cells. Choanoflagellates normally live in solitude, moving about with sperm-like tails and voraciously eating bacteria. But they can also form big colonies. If we can understand why this happens, we might get hints as to why our single-celled ancestors did the same.

King has now found the answer, and it’s a tantalising one. The solitary cells become sociable after being exposed a molecule called RIF-1 that’s produced by some of the bacteria that they eat. When they divide in two, the daughters normally go their separate ways; add a splash of RIF-1, and they stick together instead.

This raises an obvious question: did bacteria also help the single-celled ancestors of animals to band together? Did they contribute to the evolutionary foundation of every ant and elephant, every fish and finch? “That’s my favourite hypothesis,” says Rosie Alegado, the lead author on the new study. “Animals evolved in seas teaming with bacteria and have been passively exposed to bacterial chemical cues, intended and unintended.” But she cautions that this is still an open question.

Her research has certainly excited Michael Hadfield from the University of Hawaii at Manoa, who studies the development of animals. “This discovery is one of the most exciting for evolutionary biology in many years,” he says. In his view, King’s bacteria are inducing the creation of a primitive embryo and “hinting at a very important bacterial role in the evolution of animals.”