In the 6 million years since our ancestors first branched off from our ancient primate relatives, the volume of the human brain has nearly quadrupled.

What many people don't realize, however, is that sometime after the last ice age, that very brain actually began to shrink.

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

Now, a biological anthropologist, a behavioral ecologist, and an evolutionary neurobiologist have put their heads together and offered up an intriguing new hypothesis.

It's based on the evolutionary history of a brain a million times smaller than our own: that of the humble ant.

If you're thinking, 'What does an ant have in common with a human?', you might be surprised to learn it's quite a lot, actually.

Although ants and humans are only distantly related, we have both evolved to develop incredibly social lives, forming large, complex, kin-oriented societies. What's more, within these societies, labor is divided among workers of different specialties, with some ant species even producing their own crops like little farmers.

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

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

With the dawn of human society, they further explain, human knowledge could be externalized and passed on to others in the group, distributing information among multiple people instead of storing it all in each person. The cutting of this 'intellectual fat' could, theoretically, free up the brain to become more efficient at a smaller number of jobs.

"If group decision-making generated adaptive group responses exceeding the cognitive accuracy and speed of individual decisions and had a fitness consequence," the authors suggest, "then human brain size may have decreased as a consequence of metabolic cost savings."

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

The authors acknowledge their hypothesis is based on a "theory of theories" that probably can't explain all the size changes in our brains throughout our evolutionary history. But the timing certainly makes sense.

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

That's far earlier than previous estimates and several millennia after writing systems first began to emerge in the historical record.

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

Other hypotheses have suggested our noggins began to shrink after the last ice age, either from a shift in our diets, or in response to the general size reduction of our bodies. Yet the new timeline does not support either of these explanations.

The current research won't solve the mystery surrounding the volume of our brains, but it does offer an intriguing new model by which to compare our own evolutionary path.

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

The study was published in Frontiers in Ecology and Evolution.