This research is of utmost importance because apes are our closest relatives, and thus understanding of the host history attributes such as sexual maturity, birth spacing, and overall lifespan in apes helps to predict those attributes in humans.
The research mainly involves obtaining tooth emergence ages from animals in the wild. Because there exist so many difficulties in choosing wild animals, he opted skulls of a wild-shot orangutan (Pongo pygmaeus pygmaeus) and gorilla (Gorilla gorilla gorilla) that preserved emerging first molars from Zoologische Staatssammlung(ZSM), a research institution for zoological nomenclature and their applications.
Researchers says that like annual rings in trees, slicing teeth cells in half also leave a trace of their presence as growth lines that appear everyday. Such traces indicate new growth. Accordingly the teeth preserve a internal chromometer, which helps to count up how many days it took the first molar to form.
Kelly and Schwartz were successful in examining these increment growth lines in ape individuals that died as their first molars were just errupted in their mouth.
They determined their age at death by counting backwards from the final growth line to the day of birth. Thus they marked the age of of the gorilla’s first molar emergence at 3.8 years and that of orangutan’s was surprisingly much later, at 4.6 years, which falls closer to the age of approximately 6 years in modern humans.
“We were excited to discover this much older age for the orangutan, since orangutans have much slower life histories than the other two great apes,” says Kelly.