Emergence of first molars linked to growth and reproduction in apes and humans

In a new research, scientists have determined that the timing of molar development and eruption in apes and human beings is closely related to their growth and reproduction.
 
The research was carried out by Gary Schwartz and Jay Kelley, two scientists at Arizona State University's (ASU) Institute of Human Origins.
 
From the smallest South American monkeys to the largest African apes, the timing of molar development and eruption is closely attuned to many fundamental aspects of a primate's biology, according to Schwartz.
 
"Knowing the age when the first molar appears in the mouths of most primates allows researchers to predict a host of life history attributes, such as gestation length, age at sexual maturity, birth spacing, and overall lifespan," he said.
 
Humans are unique among primates because our life histories are so slow and thus our molars emerge relatively late.
 
"We've known quite a bit about the timing of molar development in chimpanzees, which is important because they are our closest living relative. However, we've known virtually nothing about when this important event occurs in other wild-living ape species - until now," said Kelley.
 
For the research, Kelley found skulls of a wild-shot orangutan and gorilla that preserved emerging first molars, at the Zoologische Staatssammlung museum in Munich.
 
"Like annual growth rings inside trees, the cells that produce teeth leave behind a trace of their presence, not as annual markers, but as growth lines that appear every day," said Kelley.
 
By slicing the teeth in half, he and Schwartz were able to examine these incremental growth lines in ape individuals that died as their first molars were just erupting into their mouths.
 
According to Schwartz, "Because teeth preserve this phenomenal internal chronometer, we were able to count up how many days it took the first molars to form."
 
"In apes and monkeys, first molars start forming very close to the time of birth. As the first molars were still erupting in our specimens, development was incomplete and the final growth line was laid down on the day those animals died," he said.
 
"Therefore, by counting backwards from the final growth line to the day of birth, we determined their age at death and thus the age at which that molar was erupting," he added.
 
Using this novel approach, the two scientists were able to mark the age of the gorilla's first molar emergence at 3.8 years, nearly identical to that of a wild chimpanzee's.
 
The orangutan's age at first molar emergence was surprisingly much later, at 4.6 years, which falls closer to the age of approximately 6 years in modern humans.