Human brain size nearly quadrupled in 6 million years since Homo last shared a common ancestor with chimpanzees, but human brains are thought to have decreased in volume since the end of the latest Ice Age; the timing and reason for this decrease is enigmatic. Using change-point analysis, a team of researchers from Dartmouth College and elsewhere has estimated the timing of changes in the rate of hominin brain evolution. The authors have found that hominin brains experienced positive rate changes at 2.1 and 1.5 million years ago, coincident with the early evolution of Homo and technological innovations evident in the archeological record; but they’ve also found that human brain size reduction was surprisingly recent, occurring in the last 3,000 years.
DeSilva et al. suggest that patterns of human brain evolution were influenced by collective intelligence, a convergent characteristic of diverse group-living animals. Image credit: Gerd Altmann.
“A surprising fact about humans today is that our brains are smaller compared to the brains of our Pleistocene ancestors,” said Dr. Jeremy DeSilva, a researcher in the Department of Anthropology at Dartmouth College.
“Why our brains have reduced in size has been a big mystery for anthropologists.”
To disentangle this mystery, Dr. DeSilva and colleagues set out to study the historical patterns of human brain evolution, comparing their findings with what is known in ant societies to offer broad insights.
“A biological anthropologist and a behavioral ecologist and evolutionary neurobiologist began sharing their thoughts on brain evolution and found bridging research on humans and ants might help identify what is possible in nature,” said Dr. James Traniello, a researcher in the Department of Biology at Boston University.
In the study, the researchers applied a change-point analysis to a dataset of 985 fossil and modern human skulls.
The dataset represents brain evolution over the last 10 million years of hominid and hominin evolution and includes Rudapithecus, Sahelanthropus, Ardipithecus, Australopithecus (including Paranthropus), Early Pleistocene Homo, Middle Pleistocene Homo, Late Pleistocene Homo, and Holocene Homo sapiens.
They found that human brains increased in size 2.1 and 1.5 million years ago, during the Pleistocene, but decreased in size around 3,000 years ago, which is more recent than previous estimates.
“Most people are aware that humans have unusually large brains — significantly larger than predicted from our body size,” Dr. Traniello said.
“In our deep evolutionary history, human brain size dramatically increased.”
“The reduction in human brain size 3,000 years ago was unexpected.”
The timing of size increase coincides with what is previously known about the early evolution of Homo and the technical advancements that led to, for example, better diet and nutrition and larger social groups.
As for the decrease in brain size, the team proposes a new hypothesis, finding clues within ant societies.
“We propose that ants can provide diverse models to understand why brains may increase or decrease in size due to social life,” Dr. Traniello said.
“Understanding why brains increase or decrease is difficult to study using only fossils.”
The scientists studied computational models and patterns of worker ant brain size, structure, and energy use in some ant clades, such as the Oecophylla weaver ant, Atta leafcutter ants, or the common garden ant Formica.
Their results show that group-level cognition and division of labor may select for adaptive brain size variation.
This means that within a social group where knowledge is shared or individuals are specialists at certain tasks, brains may adapt to become more efficient, such as decreasing in size.
“Ant and human societies are very different and have taken different routes in social evolution,” Dr. Traniello said.
“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.”
Brains use up a lot of energy, and smaller brains use less energy. The externalization of knowledge in human societies, thus needing less energy to store a lot of information as individuals, may have favored a decrease in brain size.
“We propose that this decrease was due to increased reliance on collective intelligence, the idea that a group of people is smarter than the smartest person in the group, often called the ‘wisdom of the crowds’,” Dr. Traniello said.
“We look forward to having our hypothesis tested as additional data become available,” Dr. DeSilva added.
The study was published in the journal Frontiers in Ecology and Evolution.
Jeremy M. DeSilva et al. When and Why Did Human Brains Decrease in Size? A New Change-Point Analysis and Insights from Brain Evolution in Ants. Front. Ecol. Evol, published online October 22, 2021; doi: 10.3389/fevo.2021.742639