Inside the tooth is a permanent record of its growth and development,
and that record begins the minute it begins calcifying,
and that actually begins before birth, in utero,
and it continues until the individual typically reaches adulthood throughout the whole dentition.
A single tooth, for example a first molar tooth in a human, begins about a month before birth,
and we have a permanent record as it grows, left behind, of its development.
So we can actually take this record in the form of lines within the tooth
and reconstruct a species’ experience, if you will, by its rate and developmental time.
We also can reconstruct stress from a tooth.
So if an individual experiences an illness while it’s developing its dentition,
there’s oftentimes a record of that illness or stress permanently left behind,
and that doesn’t change for millions of years.