The Human Brain Shrank 3,000 Years Ago. Now We Know Why

Studying the evolution of the brain in the ant, of all things, may have, shed light on why our brains mysteriously contracted after the Stone Age

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A transparent human head with a brain in 3-D space: downsized 3,000 years ago.
A transparent human head with a brain in 3-D space: downsized 3,000 years ago.Credit: Adrian Grosu/Shutterstock
Ruth Schuster
Ruth Schuster

We say we are the smartest animal on the planet, and maybe we are. Our brains have quadrupled in size since we split from the chimp millions of years ago. But it hasn’t been a linear journey and, it turns out, after the Stone Age our brains shrank.

They did? By how much? “Brain size in humans today averages 100 to 150 cubic centimeters smaller than at the end of the Pleistocene – a reduction that is equivalent to the size of a lemon,” answers Jereme DeSilva of Dartmouth, co-author of the paper published this week in Frontiers in Ecology and Evolution.

Nor is this some artifact of male-female or region; it impacted humans globally, he adds.

The Great Post-Pleistocene Brain Contraction is agreed on. One question is when exactly this happened. Another is why. Now the multidisciplinary team of researchers believes they have solved both conundrums. With the help of ants, of all things.

“We used patterns of ant brain evolution to help interpret the decrease in human brain size we observed,” explains co-author James Traniello of Boston University. “Ants are exceptionally diverse socially and their evolutionary history is well understood. In ants and humans, cooperating groups may be more intelligent than individual members of a group. Some ants and humans are also both ultra-social – they have evolved agriculture and produce their own food crops. Their brains are highly diverse and can be comparatively studied.”

So, the way ant social life impacts ant brain evolution can “broadly inform” our understanding of brain evolution in humans, Traniello sums up. OK.

We were all Goliaths

The next shocker is when this happened; science had agreed that it did, but was divided on tens of thousands of years or a bit less.

This trend reversal happened just 3,000 years ago, the researchers deduce based on change-point analysis (analysis of time-ordered data to see when change happened, if any).

In other words, our brains today are a lemon’s worth smaller than that of people living in the First Temple period, when David became king of Israel and reportedly vanquished “the giant” Goliath using a slingshot; and begat his brainy son Solomon, according to tradition.

The timing is all the more intriguing when we learn why the team thinks this happened.

Their work refutes other hypotheses: that our brains shrank because of the Neolithic transition from hunting and gathering to farming, or because humans all shrank in average body size between the Pleistocene and the Holocene (which they did). Nor was the post-Pleistocene cranial contraction a consequence of self-domestication, the team says.

Asked what that means, DeSilva explains that self-domestication is a process by which an organism (humans in this case) selects against the more aggressive individuals in a group, thereby promoting more cooperative behavior. “In domesticated dogs and other domesticated animals, the brain size is reduced compared with the wild counterpart,” he says.

The key, they believe, lies in the hive mind. “Humans live in social groups in which multiple brains contribute to the emergence of collective intelligence,” they explain.

They do acknowledge there are a few differences between us and the ant, but support the comparison by our common predilection for super-organismic decentralized “brain” networks.

“Collective intelligence may reduce brain size in both clades,” they write, i.e., a hive mind doesn’t have to consist of individual geniuses. But there is a snag in the theory.

Coming down from the trees

All this begs the question of how our brains evolved from the get-go. So let us peer into the dimmest reaches of our history, when we split from the chimp about 6 to 7 million years ago.

Bear in mind that there is sadly little fossil evidence, and that our family tree got messy following the post-chimp split. Until the last alt-human species went extinct, which they all have, there were numerous human variants and species – we don’t know how many – at various times. Small-brained types could coexist in time and apparently in space as well with larger-brained ones. For example, wee-brained Homo naledi in southern Africa lived at the same time as much bigger-brained types.

So: we split from said chimp and nothing much happened upstairs for millions of years. “Brains remained quite ape-like, with perhaps some subtle reorganization, between 7 and 3.5 million years ago,” DeSilva explains. This applied to the earliest members of the future human line, the likes of Sahelanthropus and Ardipithecus, for instance.

And then, around 3.5 million years ago, the time of Lucy and her kind, brains increased in volume about 20 percent.

“This genus Australopithecus had gorilla-sized brains in chimpanzee-sized bodies. But our statistical methods did not detect a major shift in brain trends until a rapid increase in volume 2.1 million years ago, coinciding with the evolution of our own genus, Homo,” DeSilva says – by whom he is referring to the likes of Homo erectus.

The team also deduced that there had been another brain-size leap 1.5 million years ago, and we toddled along thusly until about 3,000 years ago when David allegedly slew Goliath, and all our brains shrank.

The snag is that if we say groupthink allowed our brains to contract, theoretically it should have happened earlier, no? We were thoroughly socialized well over 3,000 years ago.

We add that mutations are chance; beneficial ones tend to survive and spread in the population, neutral ones either survive or don’t, and bad ones kill the bearer or at least don’t spread as much as good and neutral ones. So the human race might have been able to afford to let our brains shrivel but had to wait for the right mutation to come along.  

But by 3,000 years ago, humans had spread to all the continents, so how it is possible that brains shrank equally everywhere? DeSilva suggests that humans “have always been connected by gene flow (neighbors mating with neighbors across the globe). It is a misunderstanding that we have existed as isolated populations once we spread to the different parts of the globe. And even in places that might have been a bit more isolated geographically, the same processes may have unfolded – population increase, sedentary behavior, occupational specialization, collective intelligence – that favored reduced brain size.”

Separate work, including by a team headed by Tel Aviv University’s Prof. Ran Barkai, looking deep into prehistory, posits that our brains had to adapt to our prey environment. As we ate all the easily-caught giant animals, we had to get cleverer, from the stage of Homo erectus, in order to catch the fleeter, smaller meals on feet.

It begs adding that our gigantic brains are extremely “expensive” to maintain: they are only 2 percent of our body mass, but they take 20 percent of our energy to maintain, DeSilva points out.

All this could explain our brain growth over the eons, but it hardly helps understand why our brains contracted only after the Ice Age. We were social beings before 3,000 years ago, and before the Holocene, and discovered cooking maybe 400,000 if not a million years ago (according to Israeli researchers). As for settlement and agriculture, that began in the early Holocene. What happened 3,000 years ago?

“We wonder if this is related to overall population and density in which humans reached a critical threshold of individuals, and externalization of information tipped the scales toward collective intelligence in a slightly smaller brained species,” DeSilva suggests. What does this mean for the internet hive-mind generation? Stay tuned.

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