A long childhood was a prelude to the evolution of a large brain

Nov. 15 () –

A study of fossil teeth of early Homo found in Georgia reveals a long childhood despite having a small brain and an adulthood comparable to that of great apes.

This discovery suggests that a prolonged childhood, combined with cultural transmission in three-generation social groups, may have triggered the evolution of a large brain like that of modern humans, which tests the ‘big brain, long childhood’ hypothesis. The study is published in Nature.

The research team, made up of scientists from the University of Zurich, the European Synchrotron Radiation Facility (ESRF) in France, and the National Museum of Georgia, Georgia, used synchrotron imaging to study the dental development of a nearly adult Homo fossil. primitive site of Dmanisi in Georgia, dated around 1.77 million years ago.

“Childhood and cognition do not fossilize, so we must rely on indirect information. Teeth are ideal because they fossilize well and produce daily rings, in the same way that trees produce annual rings, which record their development,” explains in a statement Christoph Zollikofer, from the University of Zurich and first author of the research, published in Nature.

“Dental development is strongly correlated with the development of the rest of the body, including brain development. Access to the details of dental growth of a fossil hominin therefore provides a lot of information about your general growth“adds Paul Tafforeau, ESRF scientist and co-author of the study.

The project was launched in 2005, following the initial success of non-destructive analyzes of dental microstructures using phase-contrast synchrotron tomography at the ESRF. This technique allowed scientists create virtual microscopic cuts through the teeth of this fossil.

The exceptional quality of preservation of the growth structures in this specimen has allowed us to reconstruct all phases of its dental growth, from birth to death, with unprecedented precision. In a way, scientists have managed to make this hominid’s teeth grow back.

This project lasted almost 18 years from its initial conception in 2005 to the completion of the results in 2023. Scientists scanned the teeth for the first time in 2006, and the first results on the age of the fossil at death were obtained in 2007.

“We expected to find dental development typical of early hominids, close to that of great apes, or dental development close to that of modern humans. When we got the first results, we couldn’t believe what we saw, because it was something different that involved a molar crown growth faster than any other fossil hominid or extant great ape“explains Taffordeau.

In the following years, five sets of experiments and four complete analyzes were carried out using different approaches as technical advances were made in synchrotron dental imaging. The results point in the same direction and could have a big impact on the “big brain, long childhood” hypothesis, so scientists had to think outside the box to understand this fossil.

“It has been a slow process of maturation, both technical and intellectual, to finally arrive at the hypothesis that we publish today“concludes Taffordeau.

WISDOM TEETH AT AGE ELEVEN

“The results showed that this individual died between 11 and 12 years of age, when his wisdom teeth had already emerged, as happens in great apes at this age”, explains Vincent Beyrand, co-author of the study.

However, the team found that this fossil had a strikingly similar dental maturation pattern to humans, with the back teeth lagging behind the front teeth during the first five years of their development.

“This suggests that baby teeth were used for longer than in great apes and that the children of this early species of Homo depended on the support of adults for longer than those of great apes,” explains Marcia Ponce de León. , from the University of Zurich and co-author of the study. “This could be the first evolutionary experiment in extended childhood.”

Here the ‘big brain, long childhood’ hypothesis is tested. Early Homo individuals did not have much larger brains than great apes or australopithecines, but they likely lived longer. In fact, one of the skulls discovered in Dmanisi was that of a very old individual who had teeth left during his last years of life.

“The fact that such an old individual being able to survive without teeth for several years indicates that the rest of the group took good care of him,” comments David Lordkipadnize of the National Museum of Georgia and co-author of the study.

The older individuals are the ones with the most experience, so it is likely that their role in the community was to transmit their knowledge to the younger individuals. This three-generation structure is a fundamental aspect of the transmission of culture in human beings.

It is well known that young children can memorize an enormous amount of information thanks to the plasticity of their immature brains. However, the more you have to memorize, the longer it takes to do so.

SLOWED GROWTH

This is where the new hypothesis comes into play. Children’s growth would have slowed down while cultural transmission increased, which would make the amount of information communicated from adults to young people increasingly important.

This transmission would have allowed them to make better use of available resources while developing more complex behaviors, thus giving them an evolutionary advantage in favor of a longer childhood (and probably a longer life).

Once this mechanism was established, natural selection would have acted on cultural transmission and not only on biological traits. Then, by increasing the amount of information to be transmitted, evolution would have favored an increase in brain size and a delay in adulthood, which It would have allowed us to learn more in childhood and have time to develop a larger brain despite limited food resources.

Therefore, it is possible that it was not the evolutionary increase in brain size that caused the slowdown in human development, but rather the prolongation of childhood and the three-generation structure that favored biocultural evolution. These mechanisms, in turn, led to increased brain size, later adulthood, and longer life expectancy.. Therefore, studying the teeth of this exceptional fossil could encourage researchers to reconsider the evolutionary mechanisms that gave rise to our own species, Homo sapiens.