The brain is the most complex organ in the human body. In our nearly 6 million years of evolutionary history, the human brain has almost quadrupled in size. Previous studies, however, have found that human brain capacity has historically shrunk. What exactly causes the human brain to shrink? When and why did these changes occur? It has long been a mystery that has plagued anthropologists.
Recently, scientists have proposed the hypothesis by looking at ant models that the patterns of human brain evolution are influenced by collective intelligence. In other words, the highly shared society and efficient division of labor have reduced the size of the human brain in order to save metabolic costs.
▲ The paper was published in the journal Frontiers in Ecology and Evolution (Image source: screenshot of the paper page)
"One of the surprising facts about humans today is that our brains are smaller than the brains of our ancestors in the Pleistocene (2.588 million-11,700 years ago). However, we do not know why the human brain mysteriously shrank in size later. Co-author Jeremy Desilva, Ph.D., from Dartmouth College, explains.
To unravel the mystery, an interdisciplinary team of biological anthropologists, behavioral ecologists, and evolutionary neurobiologists set out to explore the evolutionary history of the human brain.
▲ The evolutionary trajectory of the ancient human brain (Source: Reference[1])
First, the researchers performed a variable-point analysis of a dataset of 985 fossils and modern human skulls. The researchers found that the brains of ancient humans enlarged at two points in time during the Pleistocene 2 million years ago and 150 years ago. Over the past 100,000 years, the human brain volume has remained stable until it suddenly shrank 3,000 years ago.
The scientists further found that the time the volume of the human brain increased coincided with the trajectory of early human evolution and technological progress in the archaeological record. For example, during periods when the human brain is enlarged, humans receive better dietary changes (such as cooking food using fire), as well as a larger community size.
So why did the human brain shrink mysteriously later?
Co-author Dr James Traniello of Boston University said: "In our complex evolutionary history, the size of the human brain has increased rapidly. Therefore, the shrinkage of human brain volume 3,000 years ago is even more unexpected. ”
▲Time inference of several points of change in human brain volume during evolution (Image source: Reference[1])
Ants have become a key model for anthropologists to study the role of sociality in brain evolution. Ants are different from human societies, but are similar in many important ways, such as group size, collective intelligence, division of labor, and food production (agriculture), which can help scientists understand the underlying factors influencing the evolution of the human brain. So the interdisciplinary research team found clues through the ant society.
For a variety of workers (oecophylla leaf weaver ants, atta leafcutter ants, or formica common garden ants), the researchers analyzed their computational models and patterns of brain size, structure, and energy use and found that the size of the brain may selectively adapt to the cognition and division of labor at the population level.
Image source: https://commons.wikimedia.org/wiki/file:ant_receives_honeydew_from_aphid.jpg#/media/file:ant_receives_honeydew_from_aphid.jpg
The results of the study analysis mean that in a social group of knowledge sharing, the shrinking capacity of the human brain is due to the increased dependence on collective intelligence. The researchers believe that group cognition reduces the need for neural structures to support certain aspects of an individual's intelligence and decision-making.
In ants, there is evidence that an increase in brain volume increases metabolic costs. Brains with smaller capacity require less energy. Humans began to share knowledge highly 3,000 years ago, social knowledge was externalized, so that individuals did not need to store a lot of information, and the human brain may shrink its brain capacity in order to become more efficient.
Specifically, as the size of the population increases, the complex interaction between humans and the social environment leads to an increase in the demand for the brain. However, there may be limitations in the feedback loop between social network size and brain structure due to the metabolic needs of the brain. If the adaptive group responses, or "collective intelligence," produced by group decisions exceed the cognitive accuracy and speed of individual decisions, then the size of the human brain may shrink in order to save metabolic costs.
Another study, through multidisciplinary modeling methods, does demonstrate that individuals with relatively small brains can achieve behavioral performance levels comparable to those of individuals with large brains but living alone through social interaction.
Scientists look forward to more data coming up in the future to test the hypothesis.
Resources:
[1] desilva,j. m., et, al. (2021). when and why did human brains decrease in size? a new change-point analysis and insights from brain evolution in ants. frontiers in ecology and evolution, 9. https://doi.org/10.3389/fevo.2021.742639
[2] when and why did human brains decrease in size 3,000 years ago? new study may have found clues within ants. retrieved october 22, 2021, from https://www.newswise.com/articles/when-and-why-did-human-brains-decrease-in-size-3-000-years-ago-new-study-may-have-found-clues-within-ants
[3] human brains decreased in size 3,000 years ago, new study says. retrieved oct 25,2021, from http://www.sci-news.com/othersciences/anthropology/human-brain-size-10198.html