Biologists have been puzzled by the evolutionary adaptation behind a common tooth trait of northern Asians and Native Americans: shovel-shaped incisors. An analysis of archeological specimens shows that nearly 100 percent of early Native Americans had shoveled incisors, and genetic evidence pinpoints the selection to the Beringian standstill 20,000 years ago. One researcher proposes that a trait linked to shoveling, mammary duct growth, was selected to provide vitamin D and fat to infants.