I often wonder about how intelligent the average person of say 500 B.C., or 750 A.D. etc would be compared to the average person of today. Without a formal education system, would a person’s brain develop to its full potential? Were the unwashed masses of the past still intelligent but just less educated? Or were they dullards by today’s standards of intelligence?
Yes, of course there will be outliers who were complete geniuses, I’m more interested in the average.
My teachers always said there were no stupid questions. Unfortunately, they were wrong!
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Happily, yours is not one of those stupid questions. I just wanted to mess wit yas. 
More seriously, no, there’s no reason to think their brains were underdeveloped. They may have been developed more towards different things, as the lack of easy reference materials meant that the educated often developed memory like you wouldn’t believe. But you can be very, very smart, and know all about the intricacies of hunting in your little corner of the world or know all about computers. It’s the same as far as the brain is concerned. If anything, they might have been smarter, because you don’t dodge plague victims from the left and saber-tothed cats from the right without some good brains. 
Now, poor nutrition and disease may well have held some back, as those can affect brain development. That could drag your “average” down. But on the whole you’d expect just about equal intelligence. Certainly nothing like a total difference, there.
It’s assumed that they were no less intellectually capable than today’s humans. The standard exhibit of why this would be so: You can take someone from Papua New Guinea, in a stone age society, whose population has been cut off from the rest of human civilization for tens of thousands of years, and train them to fly an airplane, etc. Unless there was some magical fairy dust in the air that increased the intelligence of all humans worldwide, you wouldn’t expect this sort of thing to happen.
Jared Diamond, in Guns, Germs and Steel, posited that the uneducated and illiterate hunter-gatherers he worked with might have been MORE intelligent than he himself was. They had fantastic memories, as they wrote nothing down. Plus, their very survival was dependent upon their ability to intelligently make use of their environment, whereas a person in a post-industrial society could be well below average in intelligence and still survive to reproduce.
All you have to do is look at the things the ancients built. They were so smart about using manpower, leverage, etc. that you’ve got people of today arguing that aliens must have come along to help. And when you do look at the things the ancients built, you realize that they knew a LOT about things like astronomy. Even flint knives and arrow points are remarkably well made by even the most “primitive” people.
There is, however, one major change that seems to have swept mankind. I wish for the life of me that I could remember enough details to find a cite. Basically, it’s a gene that affect cognitive processing. It seems to be relatively recent (20,000 years old?), yet the majority of modern people have it (implying that it spread so fast by conferring a significant survival/reproduction benefit, possibly relating to allowing us to live in even larger groups, which would have enabled cities, agriculture, etc).
I don’t disbelieve this, but I think it would be fascinating to read about how and when this has been demonstrated.
Ethnographic documentation is awash with descriptions of the mental powers of primitive peoples. Things like knowing the vast foraging lands of yours down to a tee, always knowing where you are relative to base camp in the middle of a jungle, identifying dozens of food and medicinal plants the year round (from withered remains etc.), being able to track man and beast where the European observer sees nothing of help etc. Or take the flint-knapping skills of the ancients. Knapping has been likened to a game of chess (it certainly is the hardest thing I’ve ever tried to learn), and the best pieces done by various groups starting in the Upper Paleolithic are equal to what the best modern knappers are able to achieve after decades of practise, on a good day. One does not need formal education to develop incredible mental skill. Mentors have been the way to go for millennia. And while the skill set has changed, I don’t think the sum of “brain development” for the average adult has increased much, if at all, ever since the modern human brain emerged.
The minimum of ability needed to feed your family has probably never been as low as today. I could do (and have done) that simply by picking up boxes off a conveyor, something a trained ape could handle nicely. Taking my family alive through the boreal winter primitively would require vast amounts of knowledge, skill and the ability to improvise and adapt on a daily basis.
I’m reading a book right now that touches on that issue, actually. This is from “The Brain That Changes Itself” by Norman Doige
In the appendix he mentions a long period of stagnation in prehistoric human culture, which ended with a cultural explosion about 50,000 years ago. It’s possible that what might have caused that cultural explosion was a mutation of this gene which cranked up to human neuron count to just over some necessary threshold. He also references another book The Prehistory of the Mind:The cognitive origins of art,history and science (S. Mithen 1996)
This is an anecdote but in Guns, Germs and Steel, Diamond talks about the Chimbu of New Guinea:
$100,000 in the early sixties, in New Guinea, was likely a lot of money.
There was a theory kicking around for a while that communication between the two halves of the brain had not fully developed in ancient times, such that ancient people perceived insights from the right side of the brain as coming from a separate being, i.e., the voices of gods.
The theory shows up in science fiction now and then (Marvel Comics, Snow Crash), but I assume it’s junk speculation. IIRC, it did not propose a specific mechanism by which human brains suddenly developed into the unified entity they are today. Besides, how could we possibly know that this was how the ancients thought?
Insert Flynn Effect controversy here.
The problem with this hypothesis is that this is not what happens to people who had a corpus callosotomy. Under laboratory settings, it’s clear that some information isn’t making it from one hemisphere to another (for example they can’t draw with their right hand something they can only see in their left eye). Further, the corpus callosum is found in most mammals, so it would be very, very surprising that it would have appeared in humans so recently.
See also:
It strikes me as plausible that the effect of never having a connection between the hemispheres might be different from the effect of losing connection that one had had since birth. Think of the differences in dreaming between people born blind and people who lost their sight later.
The evolutionary point, though, seems conclusive. Like I said, I think this theory was essentially just far-fetched speculation about the ancient world – a sort of Velikovsky of the brain. 
See the link I posted at the end of my post. Some people are born that way and they have a bunch of problems as a result.
Besides, you don’t need to go back millenia to find people who claim that supernatural beings talk to them.
On a related note, I wonder about fetal alcohol syndrome, especially after the introduction of cheap distilled liquor (gin) in the 18th C. Perhaps when people were drinking mostly small beer in the pre-modern age, not enough alcohol was consumed to really be an issue, but people – pregnant people–drank like fish in the tenements (and, in many cases, townhouses) of 18th and 19th C cities. So was a larger percentage of the population mentally retarded/hyperactive etc in drinking countries, when compared to places without the abundance of cheap grain spirits?
See, when I posted the OP I was posting under the assumption that the answer would have been ancient people were less intelligent, not more. I had two reasons for this:
Lack of intellectual stimulation. I know after a day of monotonous labor my brain feels much less stimulated than after a rousing debate in class. I assume that an ancient farmer being out hoeing fields all day, or other such tasks as the masses had to accomplish, would have not given sufficient intellectual stimulation for the brain to develop.
Lack of proper nutrition. It has been proven that we as a species are getting taller, mostly due to proper nutrition as we age. It seems common sense that the brain would be undergoing a similar change.
This is interesting.
When I was a teacher and I interviewed…I would get this question. Of course the answer is “Oh no! There are no stupid questions!”
I took the opposite approach…I would give the interviewer one 
I taught a Business Calculus course…and on the day before the final exam week when I was doing a review couple of hours one student who I didn’t recognise (meaning he rarely showed up to class) asked:
“I’ve been meaning to ask…what is that S thing you put up in front of your equations?”
Huh? S thing?
“You know that S thing right there”
You mean the integral sign?
Yes, there are stupid questions
A place I answered this way did hire me
But going back to the Ancient Greeks, people preferred diluted wine and other alcoholic drinks to raw water because of the disease problem. So I don’t know how it would’ve changed in 18th century.
I do wonder about all those drunken men in relation to the aggression common then.
But farming came late, after hunting/gathering, which required a lot of skill.
Second, while it’s true today that a lot of people farming in the country are dumb rednecks, farming in itself offers a lot to learn by observing nature, season and plants, and caring for the animals. You can either plow the field, or you can watch which plants are growing better on what field, whether you should plant the wheat early or late in the sandy or the loam field, whether you should combine beans with maize so you don’t need sticks (the Native Americans did that, whites used sticks) plus squash to cover the earth so it doesn’t dry out, and did you notice that if you put plant A next to flower B, there will be less insects eating A and B? And don’t pull that burning nettle out, it’s a healing plant. And if you rotate your crops every three years, you will get better results.
And in the evening you have to repair and build your tools and card and spin the wool from your sheep and slaughter a pig and prepare the meet and …
Officially, Farming is an apprentice job which takes three years to learn with school, or you can study it at university.
Yes, the brain is impacted if there is lack of nutrients and protein during development. However, I wonder if that applied during daily life or only during catastrophes like wars, pestilence, little ice age or the year without summer.
Sir, that is not only ignorant, it’s downright foolish. Youn don’t live by farming unless you know a lot. Modern farmers are well-educated half-scientist half-businessmen who use capital markets to invest in efficient agriculture. I’ve never met a “dumb redneck farmer” in my life.