Early hominins may have had a combination of ape-like early maturity and human-like delayed development, according to an analysis of 1.77-million-year-old fossilized hominin teeth from Dmanisi, Georgia, published in Nature.

Compared to great apes, humans have a uniquely extended childhood phase in which parents, grandparents, and other adults work together to provide for our physical and cognitive development. It is thought that this peculiar way of developing evolved as a consequence of our big brains: A child's brain takes a long time to reach adult size, but this provides a unique opportunity to acquire all the cognitive skills needed in the complex social environment of a human group. However, the “big brain – long childhood” hypothesis needs to be revised, as an international team of researchers now shows in the journal Nature.

The research team studied the development of the teeth of a fossilized early Homo individual from the site of Dmanisi in Georgia, dated to about 1.77 million years ago. "Childhood and cognition do not fossilize, so we have to rely on proxy information. Teeth are ideal because they fossilize well and preserve a complete record of their development," explains lead author Christoph Zollikofer, Emeritus Professor of Zurich University (Switzerland) and a Senior Research Affiliate at the IBS Center for Climate Physics (Busan, South Korea).

Just as trees grow in annual rings, teeth grow in daily rings, which can be counted on cross-sections. "Of course, we don't actually cut up the extremely valuable fossil hominin teeth, but we use synchrotron phase-contrast tomography," says Paul Tafforeau, Principal Investigator at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and co-author of the study. "With this non-invasive, high-tech method, we were able to create virtual microscopic slices through all the teeth of the Dmanisi fossil and piece together the development of the entire dentition from birth to death."

The project started in 2005 and produced the first results in 2007. These were so unexpected that the research team decided to validate them further, as they would have far-reaching implications for the “big brain – long childhood” hypothesis. In the years that followed, the team took advantage of new synchrotron methods and technology to carry out four further, completely independent, analyses. All of these gave congruent results, and in the end, the team was confident in their validity.

“The Dmanisi individual died between 11 and 12 years of age,” explains Vincent Beyrand, co-author of the study. “The wisdom teeth were almost fully developed – as they are in great apes at this age.” However, the research team found that this fossil showed a surprisingly human-like way of tooth maturation, with the back teeth lagging behind the front teeth for the first five years of development. "This suggests that the milk teeth were used for longer than in great apes and that this individual was dependent on the support of adults for longer than great ape children," says Marcia Ponce de León, the study's senior author. “We may be seeing the first evolutionary experiment in a prolonged childhood.”

And this is where the “large brain – long childhood” hypothesis is put to the test. The Dmanisi people did not grow much larger brains than the great apes, but they likely grew older. The researchers therefore suggest that longer childhoods may not have evolved because of bigger brains, but because parents, grandparents, and other adults worked together to care for children for longer and thus reproduce more successfully.

David Lordkipanidze, director of the Dmanisi Research Project and co-author of the study, concludes: "Dmanisi remains a treasure trove of new insights into human evolution".