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Why has the human brain gotten smaller over time?

Recently, I was lucky enough to attend a live interview with one of my favorite authors, Craig Childs and buy a signed copy of his latest book, Atlas of a Lost World.  The book is about the first humans to arrive in North America, which was also the focus of the interview.  One detail caught me off guard. Just after discussing that the humans to arrive first in North America were in fact modern Homo Sapiens, the interview host Ryan Warner asked if we or those first arrivals had larger brains.  Child’s quick response startled me: They did.  Our ancestors in fact had larger brains than modern humans.  

I wondered initially if it could be our dependence on writing and similar systems that shrank the human brain.  In his book The Dragons of Eden, Carl Sagan discussed Egyptian myth related to the invention of writing and the idea that recording things in text rather than in living memory makes people dumber.  Thamus, an Egyptian god argues against giving writing to humans as Sagan explains,

“I am sure there is some truth to Thamus’ complaint.  In our modern world, illiterates have a different sense of direction, a different sense of self-reliance, and a different sense of reality…”

Sagan goes on to argue the other side, how writing inevitable benefits human society by expanding the effective memory of a species.  Parallels are drawn between the invention of writing and modern computers, calculators, etc. which some think make modern people less intelligent.  

In his book, The World Until Yesterday, Jared Diamond reiterates the importance of memory in older, primary cultures.  Using the example of a traditional society living on the island of Rennell. Rennell periodically experiences cyclones that destroy normal sources of plant food.  The cyclones are widely spaced, so few people have a memory of the cyclones before except for the oldest residences. The older people are then for Rennell, of utmost importance, being the only people to remember vital survival skills from a past cyclone.  Diamond talks about one woman in particular:

“Now, this old woman was the last person alive in her village with that inherited experience and knowledge.  If another big cyclone were to strike Rennell, her encyclopedic memory of which wild fruits to eat would be all that stood between her fellow villagers and starvation.  Such stories about the overwhelming importance of old people’s memories for their relatives’ survival abound for pre-literate societies.”

Diamond is showing how literacy changes the importance of the elderly in a society.  When all knowledge can be accessed from a book or even the internet, long memories become less important.  If not bigger brains, certainly longer memories were more important to our ancestors than to us.

According to Discover Magazine, the male human brain has shrunk by about 150 cubic centimeters, which they say is about the size of a tennis ball.  The female brain has shrunk as well by a proportionally similar amount. This change occurred over the last 20,000 years.  The problem is, as far as we know, writing was invented first in Sumer and Egypt around 3100 BC. The time scale is wrong, there isn’t nearly enough time for the invention of writing to significantly impact the last 20,000 years of human brain evolution.  Still, thinking of how radically the human world has changed, there must have been some reason smaller brains were selected for by evolution. Initially my own mind goes to references of Idiocracy, the cult classic film detailing a future apocalypse brought on by overbreeding idiots.  Are modern humans actually dumber than their ancestors?

Craig Childs didn’t seem to think we are dumber; we just have different needs than our stone spear wielding forebears.  Child’s pointed to the fact that domestic animals have smaller brains than their wild ancestors. A good deal of the brain loss from wild to domestic involves smaller aggression centers.  In the wild, it’s thought that competition and violence is the norm while in captivity it is much more important to be capable of living together with many others. There’s more to domestication than losing violent nature though.  Writing in The Practice of the Wild, Gary Snyder remarks,

“But weeding out the wild from the natures of members of the Bos and Sus clans –cattle and pigs– gradually changed animals which are intelligent and alert in the wild to sluggish meat-making machines.”

It’s hard to be happy about a change in human brains paralleling that in our livestock.  Cows, sheep, chickens are not particularly smart beasts and it’s hard to see any of them surviving long term without human intervention or quick evolution.  There is one important distinction between the brain shrinkage in humans and that in our livestock. Generally animals like cows and pigs are selected to grow as large as possible, supplying more meat.  At least the animals are selected to supply as much meat as practicable (with the exception of dairy or working livestock).

The size of the body relative to the size of the brain is important in both livestock and humans.  Discover Magazine, points out that elephants have a larger brain than humans.  Elephants have larger bodies and thus need more brain power just to control oversized limbs.  The same is true of whales. According to a blog on Scientific American, the largest brain is that of the Sperm Whale at about 8,000 cubic centimeters compared to a human brain averaging 1,300 cubic centimeters.  None of this suggests that whales or elephants are smarter than us, although they are intelligent.  There is the argument that the Cro-Magnons with larger brains also had larger, brawnier cave-people bodies.

The problem is that evidence shows that relatively recently the size of the brain has become smaller disproportionately to shrinking bodies.  Larger brains have some drawbacks, despite our insistence of the importance of them. Humans have prolonged adolescence and our young are relatively helpless compared to say deer fawns.  

Part of why it takes so long for humans to develop has to do with the human brain and the complexity of the society an adult human must navigate.  According to a scientific paper from the journal PNAS, human brains grow by a factor of 3.3 from birth to adulthood compared to chimpanzees whose brains grow by only 2.5 in the same period.  Neanderthals, also had larger brains and their brains seemed to grow even more from birth to adulthood. Perhaps it was simply taking too long for humans to grow into their over-sized brains.  Other scientists suggest that climate change played a role; that smaller bodies and brains work better in hotter climates than in the ice-bound lands of our ancestors.

There is also some evidence that our brains haven’t just become smaller, they’ve been ‘re-wired’ as well.  Discover Magazine sums up the uncertain changes in how our brains work well:

“Over the very period that the brain shrank, our DNA accumulated numerous adaptive mutations related to brain development and neurotransmitter systems—an indication that even as the organ got smaller, its inner workings changed. The impact of these mutations remains uncertain, but many scientists say it is plausible that our temperament or reasoning abilities shifted as a result.”

This brings us back to the hypothesis Craig Childs seemed to be putting forward in the interview I attended.  Perhaps humans domesticated themselves. Perhaps living in large communities together where cooperation and peace were of primary importance changed our brains.  The interesting thing is that morphologically modern humans living in primary cultures still show large amounts of violence while having the same physical brains as Europeans.  

When the Waoroni were first contacted by missionaries, their reaction was to quickly kill them with spears.  Jared Diamond talks about the higher proportion of death in traditional society wars compared to modern wars.  It’s also hard not to note the prevalence of violence in both reality and art in modern society. People like Quentin Tarantino have made careers out of stylizing violence.  Regular school shootings in the US seem now to be the norm. It’s hard for us to think of humans as less violent than our ancestors.

Before we write off the self-domestication hypothesis entirely, we should try and put things into context.  Other animals daily inflict violence on other individuals and other species as the norm. Lions kill to eat regularly.  Wasps parasitize other insects in ways that are horrific by our standards. The important thought isn’t whether we’re violent but if we’re less violent than we once were.  This is a hard question to answer.

So far, there is no consensus as to why human brains have shrunk through our recent evolution.  What’s clear is our world has changed radically from then until now. With the development of more and more technology, our world is changing in an ever increasing manner.  Modern technologies like the internet, smart phones and cars all change what the most important parts of brain function are for evolutionary success. Are we headed towards the horrible synthesis of man and machine causing the functional extinction of our species?  Are we headed towards something resembling Idiocracy?  Are we heading towards a more peaceful world where humans are less violent and more cooperative?  Are smarter or less so than our ancestors? It’s hard to know. It’s unlikely any of us can see very far into the shifting future but the best answers may be rooted deep in the history of the Earth.     

By Zach Fitzner, Contributing Writer

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