Hormones control growth, metabolism, reproduction and many other important biological processes. In humans, and all other vertebrates, the chemical signals are produced by specialised brain centres such as the hypothalamus and secreted into the blood stream that distributes them around the body.

“This suggested that hormone-secreting brain centres have arisen after the evolution of vertebrates and invertebrates had split,” says Detlev Arendt, whose group studies development and evolution of the brain at EMBL. “But then vertebrate–type hormones were found in annelid worms and molluscs, indicating that these centres might be much older than expected.”

“These findings revolutionise the way we see the brain,” says Tessmar-Raible. “So far we have always understood it as a processing unit, a bit like a computer that integrates and interprets incoming sensory information. Now we know that the brain is itself a sensory organ and has been so since very ancient times.”