Neolithic people seem to have enjoyed chewing gum just as much as a bored kid in calculus. Their discarded wads are even revealing surprising details on human life as far back as 6,000 years ago. Tar brewed from the bark of a birch tree is the world's oldest-known synthetic material . Neolithic communities in the European Alps used this malleable, tacky substance to attach handles to stone blades, repair pottery, and gnaw on while they worked. "The precise reason for chewing tar remains unclear,