Human Brains Shrank 3,000 Years Ago—Ants Could Tell Us Why

New research could explain why human brains have reduced in size since the last ice age, using an example from an unexpected source: ants.

The team behind the research, published in the journal Frontiers in Ecology and Evolution, suggests that studying ants could help illustrate why brains both increase and decrease in size.

The team chose ants to study because, across their species, the insects demonstrate a wide range of social systems to test hypotheses concerning brain size enlargement or reduction and measure against patterns of brain evolution identified in humans.

"Ant and human societies are very different and have taken different routes in social evolution," Boston University researcher Dr. James 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.

"These similarities can broadly inform us of the factors that may influence changes in human brain size."

Scientists have a good handle on why human ancestors' brains expanded during our early evolution, around 2.5 million years ago, as social groups increased and diets and nutrition improved. What is less certain is why this organ began to shrink.

The authors of this research discovered that this brain shrinking may have peaked much more recently than previously believed, during the Holocene, just 3,000 years ago, rather than since the Pleistocene, around 1.5 million years ago.

They also believe that decreasing brain size could be linked to the development of collective intelligence in human societies and the externalization of information.

"A surprising fact about humans today is that our brains are smaller compared to the brains of our Pleistocene ancestors," explained Dartmouth College researcher and paper co-author Dr. Jeremy DeSilva. "Why our brains have reduced in size has been a big mystery for anthropologists."

To solve this mystery, researchers from across a broad range of scientific disciplines came together to examine historical evidence of human brain evolution from 985 fossils and the modern human cranium. They then compared their findings to evidence collected from ant societies.

"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 that might help identify what is possible in nature," Traniello said.

Tranielo explained that to solve this mystery the team studied patterns of worker ant brain size, structure and energy use in some ant clades, such as the Oecophylla weaver ant, Atta leafcutter ants and the common garden ant Formica.

These models showed that in ant groups where knowledge was shared or there existed individuals with highly specialized skills, brains may have adapted to become more efficient. This seemed to involve shrinking rather than growing.

This is because brain function takes up a lot of energy, and smaller brains use less energy. Externalizing knowledge in human societies meant that less information needed to be stored by individuals. And storing less information required less energy.

Traniello concluded: "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."