Compared to the great apes, humans have an exceptionally long childhood, during which parents, grandparents and other adults contribute to their physical and cognitive development. This is a key developmental period for acquiring all the cognitive skills needed in the complex social environment of a human group. The current consensus is that the very long growth of modern humans has evolved as a consequence of the increase in brain volume, since such an organ requires significant energy resources to grow.
However, the ‘big brain – long childhood’ hypothesis may need to be revised, as shown by an international team of researchers in the journal Nature, based on an analysis of the dental growth of an exceptional fossil.
The research team, made up of scientists from the University of Zurich (Switzerland), the ESRF and the Georgian National Museum (Georgia), used synchrotron imaging to study the dental development of a near-adult fossil of early Homo from the Dmanisi site in Georgia, dated to around 1.77 million years ago.
Teeth are the key
“Childhood and cognition do not fossilise, so we have to rely on indirect information. Teeth are ideal because they fossilise well and produce daily rings, in the same way that trees produce annual rings, which record their development”, explains Christoph Zollikofer from the University of Zurich and first author of the publication. “Dental development is strongly correlated with the development of the rest of the body, including brain development. Access to the details of a fossil hominid’s dental growth therefore provides a great deal of information about its general growth”, adds Paul Tafforeau, scientist at the ESRF and co- author of the study.
18 years of research
The project was launched in 2005, following the initial success of non-destructive analyses of dental microstructures using phase contrast synchrotron tomography at the ESRF. This technique enabled scientists to create virtual microscopic slices through the teeth of this fossil. The exceptional quality of preservation of the growth structures in this specimen has made it possible to reconstruct all the phases of its dental growth, from birth to death, with unprecedented precision. In a way, the scientists have virtually regrown the teeth of this hominid.
This project took almost 18 years from its initial conception in 2005 to the finalisation of the results in 2023. The scientists scanned the teeth for the first time in 2006, and the first results on the fossil’s age at death were obtained in 2007. “We expected to find either dental development typical of early hominids, close to that of the great apes, or dental development close to that of modern humans. When we obtained the first results, we couldn’t believe what we saw, because it was something different that implied faster molar crown growth than in any other fossil hominin or living great ape”, explains Paul Tafforeau.
Over the following few years, five series of experiments and four complete analyses using different approaches were carried out as technical advances were made in dental synchrotron imaging. With the results all pointing in the same direction, and potentially having a strong impact on the ‘big brain – long childhood’ hypothesis, the scientists had to think outside the box to understand this fossil. “It’s been a slow maturation, both technically and intellectually, to finally arrive at the hypothesis we are publishing today”
Read more on ESRF website