It is well documented that human brains have increased in size over the course of history, evolving to be much larger than the brains of Neanderthals. What is often forgotten in the narrative is that human brains actually decreased in size during the last Ice Age.
“Our brains are smaller compared to the brains of our Pleistocene ancestors. Why our brains have reduced in size has been a big mystery for anthropologists,” said Jeremy DeSilva, associate professor of anthropology at Dartmouth College, and co-author of a new study on the topic.
In hopes of solving said mystery, DeSilva teamed with a behavioral ecologist and evolutionary neurobiologist to study the historical patterns of human brain evolution and compare them with what is known about ant societies to offer broad insights.
The researchers first studied a dataset of 985 fossil and modern human crania. Applying a change-point analysis, they found that human brains increased in size 2.1 million years ago and 1.5 million years ago during the Pleistocene; but, they decreased in size around 3,000 years ago (Holocene)—which is more recent than previous estimates.
While ant and human societies took different routes in social evolution, they do share some common aspects, including group-level cognition and division of labor. This, in combination with the fact that 3,000 years ago was the beginning of technical advancements for Homo sapiens, led the researchers to look to the insects as an unusual model.
“Understanding why brains increase or decrease is difficult to study using only fossils,” said co-author James Traniello, behavioral ecologist and professor at Boston University.
Studying computational models and patterns of worker ant brain size, structure and energy use in three different ant species—Oecophylla, Atta and Formica (common garden ant)—the researchers found that group-level cognition and division of labor may allow for adaptive brain size variation. In practice, 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.
“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,” said Traniello. “We propose that the 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.”
While it is just a working hypothesis at the moment, the timing of the decreased size lines up with previous literature and what we know about shifts in culture around 3,000 years ago in the Late Pleistocene/Early Holocene eras. Specifically, humans at the time—much like ants—were making the shift to growing and producing their own food.
In what is now Northern Peru, for example, a team of researchers found tools without fishhooks and harpoons, but with remains of avocado, bean, squash and chile pepper. The authors say their study shows “diverse food procurement strategies” at the time. Additionally, numerous other studies of the time period have found a shift in tool-making toward bone-based technologies. In the Late Pleistocene/Early Holocene, humans began shaping tools into microblades, milling stones and pottery, further alluding to the increased role of agriculture in society.