Jan Bartek - AncientPages.com - The size of the human brain has long been of interest to scientists. A new study suggests humans have larger brains due to a long childhood.
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.
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 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.
A research team from Switzerland, France and 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.
"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 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.
Paul Tafforeau and Vincent Beyrand at the European Synchrotron (ESRF), scanning teeth. Credit: ESRF/Stef Candé. Credit: ESRF/Stef Candé
Launched in 2005, the project used phase contrast synchrotron tomography to analyze dental microstructures at the ESRF. This technique allowed scientists to create virtual slices of fossil teeth, reconstructing dental growth from birth to death with precision. Essentially, they virtually regrew the hominid's teeth. The project spanned almost 18 years from conception in 2005 to results in 2023. Initial tooth scans occurred in 2006, and age-at-death findings emerged 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.
In the following years, five series of experiments and four analyses using various approaches were conducted as dental synchrotron imaging advanced. All results aligned, potentially impacting the "big brain—long childhood" hypothesis, prompting scientists to think creatively about this fossil.
"It's been a slow maturation, both technically and intellectually, to finally arrive at the hypothesis we are publishing today," concludes Tafforeau.
Milk teeth used for longer
"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".
How teeth can give clues about brain evolution
This is where the "big brain—long childhood" hypothesis is put to the test. 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.
Older individuals, with their wealth of experience, likely played a crucial role in communities by passing down knowledge to younger generations. This three-generation structure is essential for cultural transmission in humans. Young children have the remarkable ability to memorize vast amounts of information due to the plasticity of their developing brains. However, as the volume of information grows, so does the time required for memorization.
A new hypothesis suggests that as cultural transmission increased, children's growth rates slowed down. This allowed more extensive knowledge transfer from older to younger individuals, enhancing resource utilization and fostering complex behaviors—ultimately providing an evolutionary advantage through extended childhood and potentially longer lifespans.
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. Credit: ESRF/Paul Tafforeau, Vincent Beyrand
With this mechanism established, natural selection began acting on cultural transmission alongside biological traits. As more information needed transmitting, evolution favored larger brain sizes and delayed adulthood. This adaptation enabled greater learning during childhood while allowing sufficient time for brain development despite limited food resources.
Thus, it may not have been an increase in brain size that slowed human development but rather the extension of childhood within a three-generation framework that propelled bio-cultural evolution. These processes contributed to larger brains, later adulthood onset, and longer lifespans. Analyzing this exceptional fossil's teeth could prompt researchers to reevaluate evolutionary mechanisms shaping our species' development—Homo sapiens.
The study was published in the journal Nature
Written by Jan Bartek - AncientPages.com Staff Writer
