3D reconstruction of the fossil skull of the sub-adult early Homo from the Dmanisi site in Georgia.
The green, orange, and red colors represent the preserved teeth (imaged respectively with the synchrotron at 5um, with the synchrotron at 47um, and with an industrial scanner at 250um).
The blue teeth are missing ones added by mirroring their symmetrical counterparts.
The purple first lower incisors have not been recovered, and have been extrapolated form the second lower incisor.
Credit: ESRF/Paul Tafforeau, Vincent Beyrand
Recent research challenges the theory that long childhood in humans is due to large brain sizes. Instead, analysis of early Homo fossil teeth suggests that prolonged development was necessary for enhanced cultural learning and knowledge sharing, which later contributed to larger brains and extended lifespans.
Compared to the great apes, humans have an exceptionally long childhood. During this period, parents and other adults contribute to their physical and cognitive development, ensuring they acquire all the cognitive skills necessary for thriving in the complex social environments of human groups.
The prevailing theory has been that the extended growth period of modern humans evolved as a consequence of the increase in brain volume, which requires substantial energy resources to grow. However, a new study on the dental growth of an exceptional fossil suggests the ‘big brain – long childhood’ hypothesis may need to be revised.
Fossil of the near-adult Homo from the Dmanisi site in Georgia, dated to around 1.77 million years ago, scanned at the European synchrotron (ESRF).
Credit: Georgian National Museum
The study, conducted by scientists from the University of Zurich (Switzerland), the European Synchrotron Radiation Facility (ESRF, Grenoble, France), and the Georgian National Museum (Georgia) and published in Nature, 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.
“Childhood and cognition do not fossilize, so we have to 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 Christoph Zollikofer from the University of Zurich and the 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.
https://www.youtube.com/watch?v=Eg3hjfvveuA
3D reconstruction of the fossil skull of the sub-adult early Homo from the Dmanisi site in Georgia.
The green, orange and red colors represent the preserved teeth (imaged respectively with the synchrotron at 5um, with the synchrotron at 47um, and with an industrial scanner at 250um). The blue teeth are missing ones added by mirroring their symmetrical counterparts. The purple first lower incisors have not been recovered, and have been extrapolated form the second lower incisor.
Then the right upper canine is shown at 5um resolution to illustrate the visibility of growth lines on its surface, as well as on virtual slices in its enamel and dentine.
A second level of zoom is done to reach the 0.7umm resolution that shows the daily line increments in enamel.
All the teeth are then virtually extracted from the skull and disposed in order to show the final dentition state at the death of this individual.
Based on the dental increments observed in all the teeth, a virtual growth series has been computed every 6 months from the birth to the death of this sub-adult individual which occurred at 11.42 years. Credit: ESRF/Paul Tafforeau, Vincent Beyrand
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 finalizing 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 Tafforeau.
Over the next few years, five 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,” concludes Tafforeau.
Evolutionary Implications of Dental Development Patterns
“The results showed that this individual died between 11 and 12 years of age, when his wisdom teeth had already erupted, as is the case in great apes at this age,” explains Vincent Beyrand, co-author of the study. However, the team found that this fossil had a surprisingly similar tooth maturation pattern to humans, with the back teeth lagging behind the front teeth for the first five years of their development.
“This suggests that milk teeth were used for longer than in the great apes and that the children of this early Homo species were dependent on adult support for longer than those of the 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 of prolonged childhood.”
Challenging Established Evolutionary Theories
This is where the ‘big brain – long childhood’ hypothesis is tested. Early Homo individuals did not have much bigger brains than great apes or australopithecines, but they possibly lived longer. In fact, one of the skulls discovered at Dmanisi was that of a very old individual with no teeth left during its last few years of life. “The fact that such an old individual was able to survive without any 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 have the greatest experience, so their role in the community likely was to pass on their knowledge to the younger individuals. This three-generation structure is a fundamental aspect of the transmission of culture in humans.
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 information they have to memorize, the longer it takes.
This is where the new hypothesis comes in. Children’s growth would have slowed down at the same time as cultural transmission increased, making the amount of information communicated from old to young increasingly important. This transmission would have enabled them to make better use of available resources while developing more complex behaviors and would thus have given them an evolutionary advantage in favor of a longer childhood (and probably a longer lifespan).
Once this mechanism was in place, natural selection would have acted on cultural transmission and not just on biological traits. Then, as the amount of information to be transmitted increased, evolution would have favored an increase in brain size and a delay in adulthood, allowing us both to learn more in childhood and to have the time to grow a larger brain despite limited food resources.
Therefore, it may not have been the evolutionary increase in brain size that led to the slowdown in human development but the extension of childhood and the three-generation structure that favored bio-cultural evolution. These mechanisms, in turn, led to an increase in brain size, a later adulthood, and a longer life span. Studying the teeth of this exceptional fossil could therefore encourage researchers to reconsider the evolutionary mechanisms that led to our own species, Homo sapiens.
Recommend this post and follow
The birth of modern Man
No comments:
Post a Comment