The human brain's volume has almost quadrupled in the six million years since our ancestors separated from their primate relatives.
Many people are unaware that the brain began to shrink sometime after the last Ice Age.
The result is that our brains today are smaller than those of early human beings 100,000 years ago. Yet, no one knows why or when.
A biological anthropologist and a behavioral economist have come together to form an intriguing new hypothesis.
It is based on the evolutionary history a brain that is a million times smaller then ours: the one of the humble Ant.
You might be wondering, "What do ants and humans have in common?" You might be surprised to find out that there are a lot of similarities between ants and humans.
Although we are distantly related to ants, both humans and ants have evolved to live complex social lives. They form large, complex, kin-oriented communities. These societies also have labor division among different specialties. Some ants even produce their own crops, just like small farmers.
Researchers analyzed brain structure and energy use models of worker ants to find evidence that the organ had evolved to be more efficient in social settings.
The authors speculate that the collective intelligence of humans has shaped the brain in a similar way. Knowledge can be shared among communities or colonies.
They explain that human knowledge was possible to be shared and transferred among the group at the dawn of human society. This allows information to be distributed across multiple people rather than being stored in one person. This 'intellectual fat' could theoretically be cut to allow the brain to perform better at a lesser number of jobs.
The authors suggested that "if group decision-making produced adaptive group responses exceeding cognitive accuracy and speed, and had a fitness consequence", then "human brain size might have decreased as a result of metabolic cost savings."
This reasoning suggests that the invention of the written word may have also contributed to greater energy efficiency for the human brain.
Although the authors admit that their hypothesis is based upon a "theory" of theories, it doesn't account for all the changes in brain size throughout evolutionary history. The timing makes sense.
Researchers analyzed almost a thousand skulls from fossilized and modern human beings to discover that the decline in brain size occurred only 3,000 years ago.
This is far more than was previously thought and comes several millennia later than when writing systems first appeared in the historical record.
This suggests that the shrinkage in our brains could be broadly correlated with the growth of collective intelligence within human society, which adds weight to the new hypothesis.
Others have speculated that our noggins shrank after the last Ice Age, either due to a change in our diets or because of the general shrinkage in our bodies. The new timeline doesn't support either of these explanations.
Although the current research doesn't resolve the mystery of the volume in our brains, it does provide a fascinating new model to help us compare our evolutionary paths.
The authors are looking forward to seeing their hypothesis tested over the next few years.
Frontiers in Ecology and Evolution published the study.