Cuvier also pointed to a deeper truth about teeth, albeit unintentionally, since the details of an individual’s early life are registered in perpetuity. Teeth are unlike any other body part, recording physiological rhythms as often as every 8–12 hours, and as infrequently as every season or year. Kids today learn about annual tree rings in elementary school, yet many dentists and oral health specialists are surprisingly unaware that the focus of their livelihoods is a sophisticated time machine — one that, amazingly, goes all the way back to before birth.

Baby teeth begin calcifying when we are still in the uterus, as does our first permanent molar. These teeth preserve lines that indicate the temporary position of enamel- and dentine-forming cells during birth. Scientists discovered these neonatal lines by examining hundreds of baby teeth from human children, finding dark, accentuated lines in a similar position in most teeth. Dental researcher Isaac Schour was the first to suggest that this microscopic disruption is caused by the physiological transition of birth. Others have since counted subsequent daily growth lines in young individuals, finding close agreement with their age and confirming that this line is indeed formed at birth. Under the microscope one can often see that enamel prisms deviate slightly from their straight course as they pass through the neonatal line. The adjacent enamel also shows a change in mineralization, producing a darkly contrasted line. Some hypothesize that these lines appear in infants’ teeth because of changing calcium levels during or just after birth.