How did humans get big brains? Fossil teeth reveal new clues
The development of a big brain in humans and its connection to social bonding has been a hot topic of debate, and secrets found in fossil teeth may hold the answers.
Researchers have been studying the fossilized teeth of an early Homo from Georgia, dating back approximately 1.77 million years. The research was made possible by the European Synchrotron in Grenoble, France.
The findings suggest that despite having a small brain and reaching adulthood at a similar pace to great apes, these early humans had a prolonged childhood.
This could mean that the large brain of modern humans resulted from an extended childhood along with cultural transmission across generations instead of vice versa.
Long childhood, big brains
A team of researchers – including specialists from Switzerland’s University of Zurich, the European Synchrotron Radiation Facility and Georgia’s National Museum – have challenged the theory that modern humans’ extended childhood is directly linked to their large brains.
The team analyzed dental development in an early Homo that was almost an adult. The results suggest that though these early humans reached adulthood at a similar age as great apes, their tooth development followed a longer timeline similar to modern humans.
This proposes the distinct possibility of a longer dependency on adults and an elongated childhood for these early humans.
Remarkably, the brain size of these early Homo specimens was only marginally larger than that of a chimpanzee. This made the researchers question the link between the prolonged development and cultural transmission across generations.
An extended childhood within a three-generation context might have allowed the immature members of the group to effectively assimilate growing amounts of knowledge.
Eventually, this led to the development of larger brains and longer life spans, traits characteristic of modern humans.
Dental development in early Homo
An enduring assumption has been that humans’ extended growth period is a consequence of increased brain volume since a larger organ requires more energy resources to grow. But analysis of the fossil’s dental growth has caused researchers to question this hypothesis.
The research team 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,” noted study first author Christoph Zollikofer from the University of Zurich.
Unexpected findings and a new perspective
“Dental development is strongly correlated with the development of the rest of the body, including brain development,” explained study co-author Paul Tafforeau, scientist at the ESRF.
“Access to the details of a fossil hominid’s dental growth therefore provides a great deal of information about its general growth.”
After almost two decades of research beginning in 2005, the initial results startled researchers.
“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,” said Tafforeau.
“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.”
In the following years, the team conducted five series of experiments and completed four full analyses, employing various methods as dental synchrotron imaging technology advanced.
As the consistent results suggested significant implications for the “big brain – long childhood” hypothesis, the scientists needed to think creatively to interpret this fossil’s findings.
“It’s been a slow maturation, both technically and intellectually, to finally arrive at the hypothesis we are publishing today,” said Tafforeau.
Longer childhood: Evolution’s early experiment?
The dental development of the studied fossil suggested that milk teeth were used longer than in great apes, implying that early Homo children were dependent on adult care for a longer duration.
This could be the first experiment evolution took towards a prolonged childhood. Despite having brains only slightly larger than australopithecines or great apes, early Homo are believed to have lived longer.
The survival of a very old individual with no teeth for several years indicates that the group sufficiently cared for him. This alludes to a three-generation structure, a key aspect of cultural transmission in humans.
The new hypothesis begins with children’s growth slowing down as cultural transmission increased.
The accumulation of information from older to younger individuals had an advantageous effect on resource utilization and the development of complex behaviors, thus favoring a longer childhood.
Once this mechanism was established, natural selection acted on cultural transmission, not just biological traits.
Big brains and delayed adulthood
As the volume of transferrable information grew, evolution supported a larger brain and delayed adulthood, enabling a longer learning period in childhood and time to grow a bigger brain despite limited food resources.
The large brain of modern humans could be attributed to an extended childhood and a three-generation structure encouraging bio-cultural evolution. These mechanisms may have led to increased brain size, delayed adulthood, and longer lifespan.
The study of this exceptional fossil could motivate researchers to rethink the evolutionary mechanisms that led to Homo sapiens.
The study is published in the journal Nature.
Image & Video Credit: ESRF/Paul Tafforeau, Vincent Beyrand
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