Jim Daley in Scientific American:
Toward the end of the Stone Age, in a small fishing village in southern Denmark, a dark-skinned woman with brown hair and piercing blue eyes chewed on a sticky piece of hardened birch tar. The village, dubbed Syltholm by modern archaeologists, was near a coastal lagoon that was protected from the Baltic Sea by sandy barrier islands. Behind them, the woman and her kin built weirs to trap fish that they skewered with bone-tipped spears. The woman may have worked the tar until it was pliable enough to repair a piece of pottery or a polished flint tool—birch tar was a common Stone Age adhesive. Or she might have simply been enjoying what amounted to Neolithic chewing gum. In any case, when she discarded the tar, it was sealed away under layers of sand and silt for some 5,700 years until a team of archaeologists found it. Amazingly, they were able to extract the woman’s complete genome from the birch tar, along with her oral microbiome and DNA from food she may have recently eaten.