Betel nuts on Sulawesi are 10 to 20 millimeters in diameter.

The pattern appeared in both individuals, separated by more than 10,000 years.

That separation is itself significant: the same distinctive, unusual wear pattern appearing in two individuals at the same general location but separated by a vast span of time points to a persistent, culturally transmitted behavior, not a random idiosyncrasy.

Second, chemical analysis.

Matheson and colleagues extracted dental tissue from the ancient teeth and subjected it to chemical analysis looking for traces of arecoline.

This is technically demanding work: arecoline is a small, volatile molecule that might not be expected to survive in fossilized dental tissue over tens of thousands of years.