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Could social bonds be the secret behind the development of large human brains? A recent examination of fossilized teeth from early Homo specimens in Georgia, dating back 1.77 million years, indicates that these early humans had a prolonged childhood, despite having a smaller brain size and an adulthood that resembles that of modern great apes. This finding implies that an extended childhood, along with cultural learning within social groups spanning three generations, may have initiated the evolution of larger brains similar to those of present-day humans, challenging the previous belief that it was the other way around. The results of this research are published in the journal Nature.
Could social bonds be the secret behind the development of large human brains? A recent examination of fossilized teeth from early Homo species in Georgia, which date back 1.77 million years, highlights a longer childhood, despite possessing a small brain and having an adulthood likened to that of great apes. This finding suggests that an extended childhood, coupled with cultural learning within social groups of three generations, may have triggered the evolution of larger brains like those of contemporary humans, rather than the reverse. This study is featured in Nature.
A global team of scientists from the University of Zurich (Switzerland), the European Synchrotron Radiation Facility (ESRF, Grenoble, France), and the Georgian National Museum (Georgia) has contested the assumption that the lengthy childhood of modern humans (Homo sapiens) is directly related to their large brain size. They studied a fossil of early Homo from Georgia, which dates back 1.77 million years. By utilizing synchrotron imaging techniques to analyze dental growth in an almost adult specimen, researchers found that while this species matured as swiftly as great apes (around 12 years), its tooth development pattern was more akin to that of modern humans, indicating a longer period of childhood and greater reliance on adults than observed in great apes.
Since the brain size of early Homo was only marginally larger than that of a chimpanzee, the researchers propose that this prolonged developmental phase was associated with enhanced cultural learning that occurred across generations, where older members shared their wisdom with the younger ones. A longer childhood within a three-generation social framework would have enabled younger group members to absorb an increasing amount of information more effectively. Once this evolutionary tendency began, natural selection would refine the traits that improved cultural learning within social groups. Only in a later phase, with the growth in sharing social knowledge, would evolution have favored the enlargement of brains, eventually leading to the extended adulthood and longevity characteristic of modern humans. This study published in Nature challenges the conventional view that large brains are the primary reason for prolonged childhood, indicating instead that an extended childhood alongside a three-generation social structure may have contributed to the evolution of larger brains.
Humans have an unusually long childhood compared to great apes, during which parental figures, grandparents, and other adults play significant roles in their physical and mental growth. This period is crucial for acquiring the cognitive abilities necessary to navigate the complex social structures found in human groups. The prevalent belief is that the lengthy development of modern humans resulted from increased brain volume, which requires significant energy to grow. However, the hypothesis linking ‘big brains with long childhoods’ may need to be reevaluated, as demonstrated by the findings of this international research team, detailed in the journal Nature, which was based on analyzing the dental growth of a remarkable fossil.
Teeth hold the clues
The research group, comprised of experts from the University of Zurich, the ESRF in Grenoble, and the Georgian National Museum, utilized synchrotron imaging to investigate the dental development of an almost adult fossil from early Homo discovered at the Dmanisi site in Georgia, dated to approximately 1.77 million years ago.
“Childhood and cognitive abilities do not fossilize, so we rely on indirect evidence. Teeth are ideal because they preserve well and exhibit daily growth rings, similar to the annual rings of trees, which record their development,” explains Christoph Zollikofer from the University of Zurich and the lead author of the study. “The development of teeth correlates closely with the growth of the body, including the brain. Thus, insights into a fossil’s dental growth can reveal considerable information about its overall growth,” adds Paul Tafforeau, a scientist at ESRF and co-author of the research.
18 years of research
The research project began in 2005 following promising non-destructive analyses of dental microstructures via phase contrast synchrotron tomography at the ESRF. This advanced technique allowed scientists to create virtual microscopic slices through the fossil’s teeth. The exceptional preservation of the growth structures in this specimen enabled a detailed reconstruction of its dental development phases, from birth to death, with unmatched accuracy. Essentially, the scientists have virtually recreated the teeth of this ancient hominid.
This project spanned nearly 18 years from its inception in 2005 until the findings were finalized in 2023. The initial scans of the teeth occurred in 2006, with preliminary results regarding the individual’s age at death emerging in 2007.
“We anticipated discovering dental growth patterns typical of early hominids, resembling great apes, or those akin to modern humans. However, when we analyzed the initial results, we were astonished as they indicated a unique pattern showing quicker molar crown growth not observed in any other fossil hominin or living great ape,” shares Paul Tafforeau. Over the ensuing years, the research team conducted five experimental series and four comprehensive analyses using varied approaches as technological advancements in dental synchrotron imaging progressed. As all results pointed in a similar direction, suggesting significant implications for the ‘big brain — long childhood’ hypothesis, scientists were pushed to reconsider their understanding of this fossil. “It has been a gradual progression, both technically and intellectually, leading us to the hypothesis we are presenting today,” concludes Paul Tafforeau.
Extended use of milk teeth
“Our findings indicated that this individual passed away around 11 to 12 years old, having already developed wisdom teeth, similar to great apes at that age,” notes Vincent Beyrand, a co-author of the study. However, the research team discovered that this fossil exhibited an unusually similar tooth maturation pattern to humans, with back teeth lagging behind front teeth during the initial five years of development.
“This indicates that milk teeth were retained for a longer duration compared to great apes and that the children of this early Homo species required adult support for an extended period,” explains Marcia Ponce de León from the University of Zurich and co-author of the study. “This might represent the first evolutionary trial of prolonged childhood.”
Teeth reveal clues about brain evolution
This presents an opportunity to test the ‘big brain — long childhood’ hypothesis. Early Homo individuals had brains that were not significantly larger than those of great apes or australopithecines, yet they likely lived longer. One of the skulls found at Dmanisi belonged to a very elderly individual who had no remaining teeth during the last years of life. “The fact that an individual could survive without teeth for several years suggests that the group effectively cared for him,” explains David Lordkipadnize from the National Museum of Georgia and co-author of the study. Older individuals, being the most experienced, likely played a crucial role in the community by sharing knowledge with younger members. This three-generation structure is essential for the cultural knowledge transmission unique to humans.
It’s widely recognized that young children can memorize vast amounts of information due to their brain’s remarkable plasticity. Nonetheless, the more there is to learn, the longer it takes to absorb this knowledge.
This leads to the emergence of a new hypothesis. Children’s growth may have decelerated concurrently with rising cultural transmission, making the amount of information shared from older to younger generations increasingly significant. This transfer of knowledge would have enabled children to better utilize available resources while developing more complex behaviors, consequently offering them an evolutionary benefit favoring longer childhood (and likely a longer lifespan).
Once this mechanism was established, natural selection would focus on enhancing cultural transmission, not just biological traits. As the volume of information to be conveyed increased, evolution would have favored larger brain sizes and delayed adulthood, allowing for enriched learning during childhood and time to develop larger brains despite limited food availability.
Thus, it appears that the evolution of a larger brain may not have been the catalyst for the slow developmental pace in humans, but rather the prolongation of childhood and the three-generation social structure that bolstered bio-cultural evolution. These mechanisms ultimately led to increased brain size, extended adulthood, and longer lifespans. The examination of this extraordinary fossil’s teeth may persuade researchers to rethink the evolutionary processes that shaped our species, Homo sapiens.
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