Chew on This.

• Position: ANCIENT HISTORY

When hunter-gatherers living in what is now Denmark popped sticky pieces of birch tar into their mouths about 5,700 years ago, they didn't realize they were leaving future scientists their DNA. Ancient people used the sticky residue that comes out of birch bark to fix tools, and when it started to solidify, they rolled it in their mouths and chewed on it, like primitive bubble gum. Researchers recently found a wad of the gum, and the DNA in the saliva provided many details about who had chewed it: a woman with a dark complexion, dark hair, and blue eyes. (They could also tell she'd eaten duck and hazelnuts and couldn't digest milk.) This...