The question as to why many domestic species share similar features and behaviors has long troubled scientists. Even Darwin, the father of evolutionary biology, was puzzled.

"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.

Wilkins' new study, published this week in the journal Genetics, suggests traits like floppy ears and splotches of white hair or fur are byproducts of domestication.

As the new research explains it, the majority of the characteristics that can be traced from one domesticated species to another are traits largely controlled by what's known as the neural crest. The neural crest is a group cells containing instructions for how different parts of the body and different tissue types form and mature -- like the color of fur and skin or the structure of ear cartilage.

Located at the top of spinal cord, the cells migrate throughout the body to communicate these instructions, their coding dictating the tissue traits of pigment cells and parts of the skull, jaws, teeth, and ears. The neural crest also contains genetic instructions for the body's adrenal system which controls the "fight-or-flight" response.

"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 explained. "So, these animals were less fearful."

In the the process of domestication and manipulation of the adrenal glands -- from generation to generation -- humans seem to have caused a series of common genetic mutations in domesticated species, byproducts of the quest for tamer animals.

"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," said Mark Johnston, Genetics' editor-in-chief.