The extremely slow rate of tooth growth in our 1.8 million-year-old ancestors suggests that the lengthening of our childhoods may have begun before our brains grew.
We all know that humans, unlike many other creatures, take a long and complicated path to adulthood—about twice as long as chimpanzees. Scientists see this delay as an opportunity to develop larger brains and learn skills that help us survive and reproduce.
But now, a study examining ancient juvenile teeth suggests that the slow growth process may have begun at least as early as 1.8 million years ago, about half a million years earlier than any other evidence.
In a new study published in the journal Nature, researchers used advanced X-ray imaging to examine the growth lines of the molars of an ancient extinct member of the Homo genus that lived 1.77 million years ago in what is now Georgia.
The findings show that although Dmanisi’s teeth grew more slowly than modern children, his molars grew slowly in the first five years of life, just like modern humans. The discovery, in a group of humans with brains roughly the size of chimpanzees, could reveal secrets about the beginning of the long childhood of humans.
“One of the big questions in paleoanthropology is when the slow growth pattern emerged in the human species. Now we have an important clue,” says Alicia Nava, a bioarchaeologist at Sapienza University in Rome.
However, other researchers caution that while Dmanisi’s teeth grew slowly, other people, including our direct ancestors, may have grown faster.
Traces of the Past in Teeth
Since the 1930s, research has shown that humans have a longer period of adolescence than other members of the hominidae family. There are also theories that our ancestors evolved this delay to give them more time to develop larger brains or learn to adapt to complex social interactions. Teeth, especially permanent molars, are key to understanding this process, because their growth lines, like tree rings, record the exact details of the growth process.
Research has shown that early human ancestors, like other members of the hominin family, grew rapidly. For example, the teeth of a 2.4-million-year-old Australopithecus afarensis, a species that lived in Ethiopia and was the same species as Lucy, grew at the same rate as chimpanzees. But by 1.2 million years ago, the Homo antecessor from Spain showed signs of slower growth, still longer than other hominids and not yet as slow as our own species, Homo sapiens.
The fossil of Hominid Damiani, discovered in 2001, consisted of a perfectly preserved skull, jaw, and teeth. Christoph Zollikofer and Marcia Ponce de León, paleoanthropologists at the University of Zurich, and Paul Tafuro, a paleoanthropologist at the European Synchrotron Radiation Facility, used the synchrotron radiation force (a ring-shaped particle accelerator) to take high-resolution X-ray images to count the growth lines on the fossil’s teeth. These lines are formed by the accumulation of layers of dentin and occur approximately every eight days in modern humans. By counting these lines, the team determined that Hominid Damiani was about 11.4 years old at the time of his death.
The researchers aligned and reconstructed the growth paths of all the teeth by analyzing the dark stress lines on the teeth, which are caused by disease or poor diet. They created a virtual video of how the teeth in this ancient child’s mouth grew over six-month periods. “The results show how each tooth grew throughout the individual’s life,” says Ponce de Leon.
A Look at Modern Humans
A comparison of the growth trajectory of Hominid teeth with modern humans, chimpanzees, and other hominids showed that for the first five years of life, the child’s molars grew slowly, just like modern humans. But from the ages of 6 to 11, tooth growth accelerated, more like chimpanzees. This suggests that although Hominid matured faster than modern humans, the initial slowdown in growth was the first step toward our delayed puberty and late growth.
Ponce de Leon believes the child was likely dependent on its elders for food and care long after it was weaned. Previous findings suggest that Demanesian adults used stone tools, cut meat to help a toothless individual, and even cared for the elderly in addition to children.
Our Demanesian ancestors were able to make such advances with brains that were only slightly larger than those of chimpanzees. The slow growth of teeth seems to have occurred before the brains grew, perhaps because tool use, meat consumption, and social changes allowed children to remain dependent on their elders for longer.
“To me, this is very exciting because it shows that the slow growth of teeth began before the dramatic increase in brain size,” says Debbie Guatley Steinberg of Ohio State University.
However, even if a particular Dhamnic youngster grew slowly in early childhood, not all members of the species may have done so, because growth rates always vary between individuals and perhaps even at different times in an individual’s life. Leslia Hlucoescu, a paleoanthropologist at the National Center for Research on Human Evolution in Spain, says she would like to get more data on how the development of teeth in hominids and humans changed over the course of their lives on an individual scale.
Some researchers remain skeptical. Did the slow rate of tooth growth directly lead to our extended childhood? Others warn that it may just have been a characteristic of Dhamnic societies. “There is a slow growth in some aspects of Dhamnic hominin dental development,” says Kevin Kuykendall of the University of Sheffield. “But did this slow growth translate into a longer growth period and a longer childhood in modern humans? We don’t have enough evidence yet to confirm that.”
Ultimately, the research findings open a new window into the distant past, suggesting that the human journey toward a long childhood may have begun much earlier than we thought.