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why are our brains shrinking?

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my own brain, squeezed of alcohol

my own brain, squeezed of alcohol

Jacinta: So you know that the average human brain mass, or is it volume, has reduced by  – is it 15%, I can’t remember – over the past 20,000 years or so, right? And there’s this theory that it’s somehow related to domestication, because the same thing has happened to domesticated animals…

Canto: How so..?

Jacinta: Well, we don’t know how so, we just know it’s happened.

Canto: How do we know this? Who says?

Jacinta: Well I’ve heard about it from a few sources but most recently from Bruce Hood, the well-known psychologist and skeptic who was talking on the SGU about a recent book of his, The Domesticated Brain. 

Canto: So the idea is that humans have somehow domesticated themselves, in the same way that they’ve domesticated other species, with a corresponding decrease in brain mass in all these species, which signifies – what?

Jacinta: Well it raises questions, dunnit? What’s going on?

Canto: It doesn’t signify dumbing down though – I read in Pinker’s big book about our better angels that our average IQ is rising in quite regular and exemplary fashion.

Jacinta: Yes, the Flynn effect. Though of course what IQ measures has always been controversial. And they do reckon size isn’t the main thing. I mean look at all those small critters that display so many smarts. For example, rats, octopuses and corvids (that’s to say crows, ravens and some magpies). They all seem to be fast learners, within their limited spheres, and very adaptable. But getting back to the human brain, it seems to be something known mainly to palaeontologists, who have a variety of theories about it, including the ‘we’re getting dumber’ theory, but I’m not convinced by that one. It seems more likely that our brains are getting more organised, requiring less mass.

Canto: So this has happened only in the last 20,000 years?

Jacinta: Or perhaps even less – between 10 and 20 thousand.

Canto: Isn’t that a phenomenally short time for such a substantial change?

Jacinta: I really don’t know. They say it might be partly related to a decrease in overall body size, so that the brain to body ratio remains much the same.

Canto: A decrease in body size? What about the obesity epidemic? And I remember way back when I was a kid reading about how we’d been getting taller with each generation since the Great Depression – or was it the Industrial Revolution? Anyway our improved diet, our era of relative abundance, has led to a change in height, and presumably in mass, in only a few generations.

Jacinta: So now you’re saying that substantial changes can occur in a few generations, let alone 10,000 years?

Canto: Uhhh, yeah, okay, but I wasn’t talking about brain size.

Jacinta: Well why not brain size? Anyway, although there have been those recent changes, at least in the west, the story goes that the planet has warmed since the last ice age, favouring less bulky bodies, less fat storage, more gracile frames.

Canto: So what about domestication, why has this led to decreased brain sizes?

Jacinta: Well this is very complex of course…

Canto: I can think of a reason, though it might not be called domestication, more like socialisation, and outsourcing. You can see it in very recent times, with smart phones – it’s even become an already-stale joke, you know phones are getting smarter so we’re getting dumber. But then we always tend to exaggerate the short-term and the present against the longer view. And yet…. I was on the tram the other night, sitting across from this couple, locked into their phone screens, I mean really locked in, earplugs attached, heads bent, utterly fixated on their little screens, completely oblivious, of each other as well as of the outside world. I was reading a book myself, but I became distracted by my irritation with these characters, while wondering why I should be irritated. It just went on so long, this locked-in state. I leaned forward. I waved my hand in front of their bowed heads. I wanted to tell them that the tram had rattled past all the stations and was heading out to sea…

Jacinta: There are some problems with the whole argument. How do we know that domesticated animals have smaller brains? Domesticated cats have a wide range of brain sizes no doubt, but what wild cats are you comparing them with? Even more so with dogs and their immense varieties. Okay they’re descended from wolves so you compare a wolf brain with its modern doggy-wolfy counterpart, but who’s going to agree on type specimens?

Canto: So you brought the subject up just to dismiss it as a load of rubbish?

Jacinta: Well if we shelve the domestication hypothesis for the moment – I’m not dismissing it entirely – we might consider other reasons why human brains are shrinking – if they are.

Canto: So you’re not convinced that they are?

Jacinta: Well let’s be sceptical until we find some solid evidence. In this Scientific American site, from November 2014, palaeontologist Chris Stringer states that ‘skeletal evidence from every inhabited continent’ suggests – only suggests – that our brains have become smaller in the past 10 to 20 thousand years. No references are given, but the article assumes this is a fact. This piece from Discovery channel or something, which dates back to 2010, relies in part on the work of another palaeontologist, John Hawks, whose website we link to here. Hawks also talks about a bucketload of evidence, but again no references. The original research papers would likely be behind a paywall anyway, and barely intelligible to my dilettante brain….

Canto: Your diminishing brain.

Jacinta: Okay I’m prepared to believe Hawks about our incredible shrinking brains, but is domestication the cause, and what exactly is domestication anyway? Hawks doesn’t go with the domestication hypothesis. In fact the Discovery article usefully covers a number of alternative hypotheses, and of course the shrinking may be due to a combo. In fact that’s more than likely.

Canto: So what’s Hawks’ hypothesis, since we’re supposedly admirers of his?

Jacinta: Well Hawks decided to look more closely at this brain contraction – which is interesting because I was thinking along the same lines as he was, i.e. has it been a uniform contraction, or was there a sudden, quick development, followed by a stagnant period, as you would expect?

Canto: Anyway isn’t brain organisation more important than brain mass? Sorry to interrupt, but haven’t we already established that?

Jacinta: We haven’t established anything, we’re just effing dilettantes remember. Hawks started looking at more recent data, over the past 4000 years or so, to see if he could detect any difference in the encephalisation quotient (EQ) – the ratio of brain volume to body mass – over that time. He found that indeed there has. The picture is complicated, but overall there has been a reduction in the brain compared to the body. His explanation for this though is quite different. He reckons that a series of mutations over recent history have resulted in the brain producing more out of less…

Canto: Right, just as a series of modifications have allowed us to produce smaller but more powerful and fuel-efficient cars.

Jacinta: Uhh, yeah, something like that.

Canto: But we know what those modifications were, we can name them. Can we name the mutations?

Jacinta: Clever question, but we know about cars, we built them and they’ve only been around for a bit more than a century. We know vastly less about the brain and we’re still getting our heads around natural selection, give us a break. Hawks points out that it’s a rule about population genetics well-known in principle to Darwin, that the larger the population the more numerous the mutations, and there was a surge in the human population back when agriculture was developed and large settlements began to form. So a number of brain-related mutations led to streamlining and, as you suggest, fuel efficiency.

Canto: But isn’t this compatible with the domestication hypothesis? I imagine that, if there really is a brain reduction for domesticated animals, it’s because they don’t have to rely on their brains so much for survival, and we don’t either, the collective has sort of magically taken care of it through farming and infrastructure and supermarkets.

Jacinta: Yes but they all have their own complicated networks and issues we have to wrap our brains around. The domestication hypothesis is really about aggression apparently. The argument goes that all animals under domestication become more varied in size, coloration and general build, with a tendency to become more gracile over all. Selection against aggression, according to the primatologist Richard Wrangham, favours a slowly developing brain – one that is, in a sense, in a perpetually juvenile state (think of cute cat and dog videos). Of course, all this assumes that juvenile brains are less aggressive than adult brains, which some might see as a dubious assumption.

Canto: Yes, think of school bullying, Lord of the Flies, youth gangs, the adolescent tendency to extremes…

Jacinta: Well, both Wrangham and Hood offer a particularly interesting example of ‘super-fast’ domestication to illustrate their hypothesis:

In 1958 the Russian geneticist Dmitri Belyaev started raising silver foxes in captivity, initially selecting to breed only the animals that were the slowest to snarl when a human approached their cage. After about 12 generations, the animals evidenced the first appearance of physical traits associated with domestication, notably a white patch on the forehead. Their tameness increased over time, and a few generations later they were much more like domesticated dogs. They had developed smaller skeletons, white spots on their fur, floppy ears, and curlier tails; their craniums had also changed shape, resulting in less sexual dimorphism, and they had lower levels of aggression overall.

Now, how does this relate to juvenilism? Well, in the wild, offspring grow up quickly and have to fend for themselves, which requires a certain ruthless degree of aggression. Cats and dogs, yes, they abandon their offspring soon enough, but those offspring continue to be tutored, tamed, domesticated under their human owners. We hear a lot about school bullying and gangs of youths, but they’re actually the exception rather than the rule, or a last ditch rebellion against the domestication pressure that’s exerted by the whole of society, and they’ll either succumb to that pressure or end up in jail, or worse. It’s a bit like the Freudian concept of sublimation, you channel your aggressive energies into creativity, competitive problem-solving, sports achievements and the like.

Canto: So you’re in favour of the domestication hypothesis?

Jacinta: Well, I’m not against it. It sounds plausible to me. Human domestication, or self-domestication if you want to call it that, is a social-contract sort of thing. You agree to outsource and comply with certain arrangements – laws, government, taxation and so forth, in return for certain benefits in terms of security and resources. So you don’t have to fend for yourself. And that affects the brain, obviously. Though it might not be the whole story.

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Written by stewart henderson

April 15, 2016 at 8:42 am

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