Ants could help explain why our brains mysteriously shrunk thousands of years ago

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In the 6 million years since our ancestors moved away from our ancient primates, the size of the human brain has almost quadrupled.

What many people don’t realize, however, is that sometime after the last Ice Age, that same brain began to shrink.

The result is that our brains today are slightly smaller than those of the first humans living 100,000 years ago, and yet no one really knows when or why this happened.

Today, a biological anthropologist, a behavioral ecologist, and an evolutionary neurobiologist brainstormed together and came up with an intriguing new hypothesis.

It is based on the evolutionary story of a brain a million times smaller than ours: that of the humble ant.

If you are wondering, “What does an ant have in common with a human?” You might be surprised to learn that it’s a lot, in fact.

Although ants and humans are only distantly related, we have both evolved to develop incredibly social lives, forming large, complex, kinship-driven societies. In addition, within these companies, the work is divided among workers of different specialties, some species of ants even producing their own crops as small farmers.

When the researchers analyzed patterns of the brain size, structure, and energy consumption of worker ants, they found evidence that the organ had adapted to become more efficient in social groups.

The authors perhaps suggest that the human brain has been similarly shaped by collective intelligence, where knowledge can be shared and distributed within a colony or community.

At the dawn of human society, they further explain, human knowledge could be externalized and passed on to other group members, distributing information among multiple people instead of storing it all in each person. Cutting off this “intellectual fat” could, theoretically, free the brain to become more efficient in fewer jobs.

“If group decision making generated adaptive group responses beyond cognitive accuracy and speed of individual decisions and impacted fitness,” the authors to suggest, “then the size of the human brain may have decreased due to the savings in metabolic costs.”

In this reasoning, the advent of the written word could also have contributed to greater energy efficiency of the human brain.

Authors acknowledge their hypothesis is based on a “theory of theories” which probably cannot explain all of the changes in the size of our brains throughout our evolutionary history. But the timing certainly makes sense.

When the researchers analyzed nearly a thousand fossilized and modern human skulls, they found that the reduction in the size of the human brain had started quite recently, only about 3,000 years ago.

This is much earlier than previous estimates and several millennia after writing systems began to emerge in the historical record.

This means that the shrinking of our brains could largely parallel the expansion of collective intelligence in human society, adding weight to the new hypothesis.

Other hypotheses have suggested that our noggin began to shrink after the last ice age, either because of a change in our diet or in response to the general reduction in our body size. Yet the new timeline does not support any of these explanations.

Current research won’t solve the mystery surrounding our brain size, but it does offer an intriguing new model for comparing our own evolutionary path.

The authors look forward to testing their hypothesis in the years to come.

The study was published in Frontiers in ecology and evolution.


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