Ants provide new clues as to why the human brain got smaller 3,000 years ago
It is well documented that human brains have increased in size throughout history, evolving to become much larger than the brains of Neanderthals. What is often overlooked in the story is that the size of the human brain actually shrank during the last ice age.
“Our brains are smaller than those of our Pleistocene ancestors. Why our brains have been reduced in size is a big mystery to anthropologists, ”said Jeremy DeSilva, associate professor of anthropology at Dartmouth College and co-author of a new study on the subject.
In the hope of solving this mystery, DeSilva has teamed up with a behavioral ecologist and an evolutionary neurobiologist to study historical models of human brain evolution and compare them with what is known of the human brain. ant societies to provide a general overview.
The researchers first studied a data set of 985 fossil and modern human skulls. Applying shift point analysis, they found that the size of the human brain increased 2.1 million years ago and 1.5 million years ago during the Pleistocene; but, their size decreased about 3000 years ago (Holocene), which is more recent than previous estimates.
While ant and human societies have taken different paths in social evolution, they share some common aspects, including group-level cognition and division of labor. This, in combination with the fact that 3000 years ago was the start of technical progress for Homo sapiens, has led researchers to consider insects as an unusual pattern.
“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.
To study computer models and patterns of the size, structure, and energy use of worker ants’ brains in three different ant species—Oecophylla, Atta and Formica (common garden ant) - researchers have found that cognition and division of labor at the group level can allow adaptive variation in brain size. In practice, this means that within a social group where knowledge is shared or where individuals are specialized in certain tasks, brains can adapt to become more efficient, for example by decreasing in size.
“The externalization of knowledge in human societies, thus requiring less energy to store a lot of information as individuals, may have promoted a decrease in brain size,” said Traniello. “We propose that the decrease is due to an increased reliance on collective intelligence – the idea that a group of people is smarter than the smartest person in the group.”
Although this is only a working hypothesis for the moment, the timing of the decrease in size is consistent with previous literature and what we know about changes in culture around 3,000 years ago to the end of the Pleistocene and the beginning of the Holocene. Specifically, humans back then, much like ants, used to make shit to grow and produce their own food.
In what is now northern Peru, for example, a team of researchers found tools without hooks or harpoons, but with leftover avocado, beans, squash and chili. The authors say their study shows “diverse food supply strategies” at the time. Additionally, many other studies from the period have revealed an evolution in tool making towards bone-based technologies. During the late Pleistocene and early Holocene, humans began to fashion microblade, millstone, and pottery tools, further hinting at the increased role of agriculture in society.