A few weeks back, I was in a waiting room, poss. to a doctor’s office. And I began perusing an interesting magazine article. It said (due to evolution, of course) human heads are gradually getting smaller!
Anyways, I had to leave in a hurry, so I didn’t get to read the whole article. I did pick up a couple of interesting points. For example, in our highly technological world, we just don’t need that much intelligence than we did in, say, the Middle Ages. Or conversely, consider computers. Every year, they get smaller and smaller. But they are in fact getting more, not less advanced.
BTW, I am unable to provide a cite yet. I looked briefly at Wikipedia, but so far found nothing.
But putting that aside for the moment, what do the rest of you think? Is it true? And if it is, what could be causing it?
Head or brain size has little to no correspondence with intelligence within a species. You will find that people with absurdly small brains often have mental retardation, but then abnormally large brains have the same problem. As long as you are within normal range, your intelligence is more likely than not to be normal and not related to absolute size.
The rest of it seems suspect. In the Middle Ages, most people were uneducated farmers and such, so it is difficult to assess absolute intelligence. Also, newer computers are smaller because they use new technologies. Humans and their ancestors have been using the same “technology,” neuron based gray matter, at least since the spinal cord was developed.
The upper limit of the human head (as a collective species, ignoring individual outliers) is determined by the opening in the pelvis. Bipedal locomotion would tend to favor a smaller pelvic opening, which would mean a selective pressure for smaller heads, but I can’t find any references to back this up without going into research literature.
We studied just 4 ancestors in my high school biology class, but I’m pretty sure there was a definite trend of increasing cranial capacity. I guess if you’re talking about external dimensions, later sub/species didn’t have as large jaws and brow ridges (iirc from my fuzzy memories of high school biology).
Then also we have to consider that even though pelvis size was once a limiting factor to head size now, with Cesarean sections being available, this becomes less of a factor.
The most obvious candidate explanation is that people today are much more likely to suffer from disease and malnutrition than ancient people. Agriculture means people live very close together, and alongside animals. This means contagious diseases spread extremely rapidly. And agricultural people tend to suffer from chronic malnutrition. They get enough calories from their staple crops, but often not enough protein, vitamins, and minerals.
Doesn’t become less of a factor at all. This is all perfectly obvious and it is indeed an evolutionary mechanism. The fact is that women got all stroppy when they had a choice about reproduction and they decided they weren’t going to have so many babies. Since evolution is a bit thick and a bit basic it thought to itself, “Well that must be because having babies hurts, and so the thing to do is make it hurt less. I will reduce the size of the head”. And so it did.
And that, children, is not so very far away from how some people see evolution working
Thank gods for the smilie, 'cause that post made *my *head hurt! :smack:
There’s a big jump there from 12,000 years ago to Today, so I’d be interested in some numbers in between. At a rough guess, I’d go along with the pelvic outlet theory: babies with smaller heads were more likely to survive childbirth and childhood (being as they were less likely to cause their mothers to tear and die of hemmorhage) and pass on their “smaller head” genes.
Now, of course, c-sections may make that form of selection less impactfu. The number of women having big headed babies and surviving the experience may give less advantage to having a small head, and perhaps our heads will begin increasing in size again. We haven’t been doing routine c-sections long enough to really know - it’s just not an evolutionary timeframe yet.
I’ve seen it pointed out that Cro-Magnon’s brains were bigger. The theory I’ve heard for it is that over time the brain has evolved to be set up more efficiently, requiring less size for the same capacity. The driving evolutionary pressure for this would of course be the fact that the bigger the heads of babies are, the larger the chance of childbirth killing the mother.
Heads in the industrialized world, especially Europe (America has fallen behind) have been increasing in recent decades though, probably because of better maternal health & nutrition.
What about the evolutionary pressure of world wars? People with bigger heads poking up out of trenches and foxholes having a slightly higher statistical probability of getting shot in those big brains?
As a notoriously big-headed person myself, I’d like to take comfort from this, but it’s the first modern assertion I’ve heard that makes this claim – is there consensus on this assertion yet?
My big brain feels more comfortable inside a nice roomy scientific consensus.
There is certainly a reasonable “just so” explanation. I can’t imagine how you would test it. Many women died in childbirth. But fewer women who produced small-headed infants died in childbirth. This pretty clearly does explain why we are born so infantile, compared to many, even most, other placentals. But I guess there is a tradeoff. Smaller head size at birth may lead to smaller grown head size.
Possibly, but that would work to prevent heads from getting bigger in the first place. Hominids have given birth through the pelvis, well, always, and have been more-or-less bipedal since long before current brain sizes. It’s not like evolution has momentum and can ‘overshoot’ and have to come back.
It believe it’s pretty well accepted that limited childhood nutrition leads to smaller adult body sizes, and that agriculture (before the last 100 years or so) often, but not always, leads to a lot of limited childhood nutrition.
So it’s quite possible that this is completely an effect of the shift to agricultural societies and the subsequent tendency for somewhat stunted growth. It’s possible that the climate may have an effect, though it’s less clear how.
I have read that this effect is evidence that we are “domesticating” ourselves. Domestic animals have smaller brains than their wild (and smarter?) cousins.
Googling, I came across a link to an NPR program that aired today on this very topic:
What about the fact that brains are expensive organs? They’re almost all fat, so are expensive to grow in the first place. And they consume an inordinate amount of calories (in glucose only) to keep running well.
This produces a pressure for smaller brains that couldn’t be solved by having a smaller brain at birth or a bigger pelvis.
