If you notice that domesticated mammals tend to be more tame, have floppier ears, softer jawlines and more “juvenile” faces, you’re not alone – Charles Darwin made the same observation some 140 years ago. It almost seems self-explanatory – of course domesticated animals are tamer, that’s why we domesticate them. Have you ever considered, though, why that’s the case, and why they share so many characteristics? Researchers from Harvard University, the University of Vienna and Humboldt University of Berlin have, and it all has to do with a group of embryonic stem cells called the neural crest.

“Because Darwin made his observations just as the science of genetics was beginning, the domestication syndrome is one of the oldest problems in the field. So it was tremendously exciting when we realized that the neural crest hypothesis neatly ties together this hodge-podge of traits,” says Adam Wilkins, from the Humboldt University of Berlin.

Neural crest cells are formed near the developing spinal cord of early vertebrate embryos. As the embryo matures, the cells migrate to different parts of the body and give rise to many tissue types. Aside from things like teeth and ears, the cells also form the adrenal system – the center of the body’s “flight or fight” response. Wilkins and his colleagues hypothesize that domesticated animals display impaired development in their neural crest cells.

“When humans bred these animals for tameness, they may have inadvertently selected those with mild neural crest deficits, resulting in smaller or slow-maturing adrenal glands,” Wilkins says. “So, these animals were less fearful.”

This is the first theory to connect several components of the so-called “domestication syndrome,” and it might not be a theory for long: Scientists are rapidly mapping the genes that have been altered by domestication in the rat, fox, and dog.

“Animal domestication was a crucial step in the development of human civilizations. Without these animals, it’s hard to imagine that human societies would have thrived in the way they have,” said Wilkins.