Of all the mammals, humans experience the longest period of childhood. Compared to other species, humans continue to grow and rely on their parents for much longer.
The length of a human childhood even surpasses that of other primates. A great ape would become an adult before a human child reaches the first grade. The leading theory as to why this is suggests that the development of bigger brains is the cause.
Now, some answers to our extended childhood have been found in 1.77 million-year-old teeth. In a new study, researchers analyzed the teeth of a prehistoric child who lived in present-day Dmanisi, Georgia.
The 11-year-old individual belonged to our genus, Homo. The fossil teeth showed signs of delayed development, just like those of modern human children. But after the first several years of life, the teeth started growing in a more ape-like way.
To determine when the pattern of slow development first emerged in our genus, the research team measured the growth lines in the prehistoric child's molars. The growth lines act a lot like tree rings.
"You can cut the tooth and see the growth rings and see how it grew. It can be like a movie of how the tooth developed from birth to death," said Christoph Zollikofer, the lead author of the study and a paleoanthropologist at the University of Zurich in Switzerland.
Previous research was limited to the fossil teeth of individuals up to four years old. It is the first time that a reconstruction of an ancient hominid's dental development has been relatively completed.
The recently examined teeth were discovered in 2001. The team was able to X-ray images of the growth lines due to their excellent preservation.
Then, they used the images to investigate how the teeth formed and developed throughout the 11-year-old's lifetime.
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The prehistoric youth's teeth grew slowly until about four years of age. Around age eight, the teeth began to speed up and grow at the rate of a chimpanzee's. By 12 to 13.5 years old, the youth would've achieved dental maturity.
The researchers pointed out that the slowdown started before brain size saw a significant increase. Overall, the findings suggest that Homo species evolved to have longer childhoods to spend more time learning about social behavior before brain development got more advanced.
Alternatively, slow dental development could also be linked to diet or the age of weaning instead of social learning and brain size. It's hard to say for sure.
There is still a lot to learn about the evolution of humans. Perhaps more hints will be revealed by studying chemical signatures in fossil tooth enamel or other similar approaches.
The details of the study were published in the journal Nature.