Fossil teeth hint at a surprisingly early start to humans’ long childhood

An extended childhood, a hallmark of human development, may have had an ancient and unusual beginning.

One of the earliest known members of the Homo genus experienced delayed human-like tooth development during childhood before undergoing a more chimpanzee-like dental growth, a new study concludes.

Fossil teeth of an approximately 11-year-old individual reveal slowed development of premolar and molar teeth until about 5 years of age, followed by accelerated development of the same teeth. This slower start represented an initial evolutionary drive toward prolonged growth during childhood, say University of Zurich paleoanthropologist Christoph Zollikofer and colleagues.

Their results, based on X-ray imaging technology that examined the microscopic growth lines inside the fossil teeth, appear Nov. 13 in Nature.

The young man’s skull, along with four others discovered at the Dmanisi site in Georgia, date to between 1.77 million and 1.85 million years ago (SN: 4/8/21). While some researchers classify these fossils as Homo erectusZollikofer’s group considers the Dmanisi specimens to be indeterminate Homo species. Homo sapiens it began much later, about 300,000 years ago.

Although a popular idea holds that a long childhood, slow dental development, and an extended lifespan evolved along with brain expansion in H. sapiens“This may not have been the case at first Homo“, says Zollikofer. Homo The individuals in Dmanisi had brains only slightly larger than those of modern chimpanzees.

Zollikofer’s team provides the first “fairly complete” reconstruction of the dental development of an ancient hominid, says paleoanthropologist Kevin Kuykendall of the University of Sheffield in England, who was not involved in the new study. Previous studies of ancient hominid tooth development have focused on fossil individuals no older than about 4 years of age, Kuykendall says.

The X-ray images allowed the researchers to estimate the growth rate of teeth at different ages during their lifetime Homo young, who died shortly before reaching dental maturity between the ages of 12 and 13.5 years.

In contrast, dental maturity in humans today occurs between the ages of 18 and 22. Chimpanzees reach dental maturity between the ages of 11 and 13.

If ancient Dmanisi individuals were our direct ancestors, then shared childcare, involving grandmothers and unrelated helpers, may have fueled the initial evolution of a longer childhood, Zollikofer suspects. Much later, childhood growth slowed further as H. sapiens the brain grew.

If early Homo in Dmanisi belonged to a dead-end lineage, “then Dmanisi looks like a first evolutionary experiment with extended childhood,” Zollikofer says.

Those scenarios are possible, Kuykendall says. But the finding that a slow onset of tooth growth did not fundamentally delay dental maturity may instead indicate one of the many ways in which tooth development evolved among ancient hominids, including early Homo species that were introduced into different habitats, he says (SN: 18.2.15).

For example, variations in available foods or age at weaning, rather than shared childcare, may have shaped early dental development. Homo groups, says Kuykendall.

Paleoanthropologist Tanya Smith of Griffith University in Southport, Australia, says the new study fails to show that young Dmanisi had a prolonged childhood. She points to the study’s estimate that Dmanisi’s first molar erupted around the age of 3.5 years, closer to that of chimpanzees than humans. Previous studies show that the timing of first molar eruption strongly predicts many aspects of dental and physical development, placing Dmanisi on a chimpanzee-like trajectory, Smith says.

The chimpanzee-like dental features of young Dmanisi are consistent with that individual’s small brain size, an even stronger indicator of rapid overall development, Smith says.