Select dog characteristics valued as endearing, like floppy ears or white patches of fur, are believed to be the result of what researchers are calling "domestication syndrome."
A study on the theory was published in the journal Genetics this week.
Authors of the paper believe the suite of features are connected to what they've called "domestication syndrome," which can apply to mammals like dogs, foxes, pigs, horses, and even to domesticated fish and birds.
The researchers believe that domestication with tameness as a goal causes genetic alterations that can affect a group of embryonic stem cells known as the neural crest, according to a press release issued by the Humboldt University of Berlin.
Scientists have wondered why domesticated animals seem to have so many behaviors and features in common.
"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," said Adam Wilkins, a researcher from the Humboldt University of Berlin and co-author of a new study on the subject, according to a press release issued by the university.
Wilkins said that the neural crest cells form near the developing spinal cord of early vertebrate embryos. As they mature, the cells migrate to different sections of the body and give rise to many tissue types, according to the release.
These tissues include pigment cells and parts of the jaw, teeth, ears, and skill, along with the adrenal glands, which are the center of the "fight-or-flight" response.
Neural crest cells also indirectly affect brain development.
"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 said. "So, these animals were less fearful."
Since the neural crest influences more than just the adrenal glands, the domestication process could also lead to all of the previously mentioned physical signs of tameness.
They aren't always beneficial however, for example floppy ears may look adorable on rabbits or dogs, but they are actually a result of malformed ear cartilage.
"This interesting idea (the new theory) based in developmental biology brings us closer to solving a riddle that's been with us a long time. It provides a unifying hypothesis to test and brings valuable insight into the biology of domestication," Mark Johnston, Editor-in-Chief of Genetics, said according to the release.
"Animal domestication was a crucial step in the development of human civilizations," Wilkins added. "Without these animals, it's hard to imagine that human societies would have thrived in the way they have."
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