AP Photo

The first thing you have to do to study 4,000-year-old DNA is take off your clothes.  I am standing with Oddný Ósk Sverrisdóttir in the airlock room next to the ancient-DNA laboratory at Uppsala University, in Sweden, preparing to see how she and her colleagues examine the bones of human beings and the animals they domesticated thousands of years ago. These scientists are looking for signs of changes in the genes that allow us to consume dairy products past the age of weaning, when all other mammals lose the ability to digest lactose, the sugar present in milk.