The large size of the human brain is what makes us (arguably) the most intelligent creatures on Earth, but new research surprisingly reveals our brains may have been slowly shrinking from about 3,000 years ago. Are we getting less smart? Scientists aren’t ready to declare that yet, but have found a possible answer for the brain size mystery by studying ants and the use of collective intelligence in their social organizations.
Researchers have found that for much of human evolutionary history our brains kept growing. In fact, if you count from our last shared ancestors with chimpanzees six million years ago, the human brain size almost quadrupled. This happened thanks in part to the improving diet and nutrition of early humans. Cro Magnons, the Homo sapiens that had the largest brains in history were alive from 20,000 to 30,000 years ago. But as the recent study from scientists at Dartmouth and Boston Universities points out, around 3,000 years ago, during the current Holocene geological epoch, our brains began to diminish.
"A surprising fact about humans today is that our brains are smaller compared to the brains of our Pleistocene ancestors,” shared the study’s co-author, Dr. Jeremy DeSilva of Dartmouth College, adding, “Why our brains have reduced in size has been a big mystery for anthropologists.”
When did human brains start to shrink?
The scientists discovered the brain reduction trend started much earlier than some previous observers of this phenomenon proposed. By analyzing a dataset of 985 fossil and contemporary human crania, the researchers found brain growth periods at 2.1 and 1.5 million years ago, but also pinpointed the period at about 3,000 years ago as the time when the size of the brain began to decrease. Previous research estimated that, over the past 20,000 years, the average volume of the human brain went from 1,500 cubic centimeters (cc) to 1,350 cc — shrinking by about 150 cc, or 10% (the size of a tennis ball, as commentators have pointed out).
"Most people are aware that humans have unusually large brains—significantly larger than predicted from our body size. In our deep evolutionary history, human brain size dramatically increased," said the study’s other co-author James Traniello, of Boston University. "The reduction in human brain size 3,000 years ago was unexpected."
Why did the brains start shrinking?
The big question of course is, “Why is our evolution going in reverse?". The interdisciplinary research team, which included a biological anthropologist, a behavioral ecologist and an evolutionary neurobiologist, write in the study that their dating techniques do not ”support hypotheses concerning brain size reduction as a by-product of body size reduction, a result of a shift to an agricultural diet, or a consequence of self-domestication.”
Indeed, previous conjectures on the shrinkage did propose that as the average human body declined in size over the millennia, our brains also began to scale down. A larger body would have to support a larger nervous system. Another aspect of this is that diminishing bodies would encourage smaller pelvic sizes in females, as wrote the Scientific American, and selection could consequently promote babies with smaller heads to be born.
But the team behind the new study didn’t find evidence for these explanations. So what did they propose as the reason why did Homo sapiens brains began to shrink?
The scientists believe that the reason we don’t need the same size brains lies in the creation of our social systems, which favor distributed gathering and sharing of knowledge and information while offering advantages in “group-level decision-making.” Since brains use a lot of energy, being able to draw upon the knowledge gathered within a larger society would require less individual brain energy to be expended in order to store and process information. Thus a smaller brain would be able to do the job just as well.
But what happened around 3,000 years ago that specifically impacted our brains? Interestingly, the researchers also suggest that the advent of writing around 5,000 years ago likely had a pronounced effect on the “neural architectures” of individual human brains by increasing the power of group cognition. Decision-making by an increasingly interconnected group could have led to “adaptive group responses exceeding the cognitive accuracy and speed of individual decisions and had a fitness consequence.” This could have led to the human brain size decrease “as a consequence of metabolic cost savings.”
What ants can teach us
Unexpectedly, the researchers arrived at their conclusions by studying ants. As the scientists write in their paper, “Humans live in social groups in which multiple brains contribute to the emergence of collective intelligence.” While understanding the historical forces affecting brain evolution in humans is a complex undertaking, with only the fossil record to use for evidence, the researchers looked at ant communities.
“The remarkable ecological diversity of ants and their species richness encompasses forms convergent in aspects of human sociality, including large group size, agrarian life histories, division of labor, and collective cognition,” explain the scientists in the study. The range of social systems in ant societies allowed them to test out hypotheses and come up with insights that provide a general look at how selective forces can affect brain size.
The research team focused specifically on creating computational models that represent patterns of worker ant brain size and energy use, looking at groups of Oecophylla weaver ants, Atta leafcutter ants, and the common garden ant Formica. What they found is that collective intelligence and division of labor had an effect on the variation in brain sizes. In a group with shareable knowledge and individual specialization in particular tasks, brains were likely to adapt for the sake of efficiency, consequently diminishing in size.
"Ant and human societies are very different and have taken different routes in social evolution," Traniello shared. "Nevertheless, ants also share with humans important aspects of social life such as group decision-making and division of labor, as well as the production of their own food (agriculture). These similarities can broadly inform us of the factors that may influence changes in human brain size."
While this research doesn’t have definitive answers on what affected the changing volume of our brains, the new modeling is a fascinating step forward in getting closer to a fuller picture of our evolutionary transformation.