In contrast, from 1955 to 1963, atmospheric radiocarbon levels almost doubled.
Since then they have been dropping back toward natural levels.
The result is the oldest well-dated evidence of , suggest that our species came into the world face-first, evolving modern facial traits while the back of the skull remained elongated like those of archaic humans.
Thus, pupal case radiocarbon content would serve as a decay-resistant proxy for the tissues, yielding the year of death.
The spike in atmospheric carbon-14 levels during the 1950s and early 1960s makes this approach possible, but it also means it will have a limited period of utility because the amount of carbon-14 in the atmosphere is slowly returning to its natural level.
The researchers found that if they assumed tooth enamel radiocarbon content to be determined by the atmospheric level at the time the tooth was formed, then they could deduce the year of birth.
They found that for teeth formed after 1965, enamel radiocarbon content predicted year of birth within 1.5 years.
Archaeologists have long used carbon-14 dating (also known as radiocarbon dating) to estimate the age of certain objects.