It may seem unremarkable to assert that we've got such a thing as a cognitive surplus, but just in case, I'd hate for anyone to assume that it's because we're fundamentally different than our ancestors in much more than lifestyle technology. From Jared Diamond's Guns, Germs, and Steel:
... [A]n entire field of science, termed ethnobiology, studies peoples' knowledge of the wild plants and animals in their environment. Such studies have concentrated especially on the world's few surviving hunting-gathering peoples, and on farming peoples who still depend heavily on wild foods and natural products. The studies generally show that such peoples are walking encyclopedias of natural history, with individual names (in their local language) for as many as a thousand or more plant and animal species, and with detailed knowledge of those species' biological characteristics, distribution, and potential uses. As people become increasingly dependent on domesticated plants and animals, this traditional knowledge gradually loses its value and becomes lost, until one arrives at modern supermarket shoppers who could not distinguish a wild grass from a wild pulse. ...
We didn't necessarily get smarter, as such, we made our environment simpler. It might not always be optimally healthy, but in general, all you need to do to survive food-wise in our society is get money (which may itself be quite a complicated procedure) and exchange it for things labeled as food in a clearly marked grocery or convenience store. That leaves a lot of mental labor free for other activities.
So while our base capacity for intelligence is unlikely to have changed much, and has likely just been devoted to other things, I'm going to turn around and suggest that we may actually be somewhat smarter than past generations. It's been long and well documented that malnutrition damages brain development (pdf), particularly protein malnutrition, and that this is a lifelong, permanent effect for the individual, even though it has no bearing on one's genetic makeup.
The appropriate scientific concept is genotype vs. phenotype. The genotype means the available genetic instructions for building your body and operating it throughout your life. The phenotype is what you actually get out of that range of possibilities. Genotypes can be diverse and surprising, with a lot of room for variation, but by certain stages of development, much of that variant possibility has been set in bone. As it were.
Hence, it's entirely possible for people who were never able to live up to their own genetic potential, for any of a host of environmental factors, to have children who are more intelligent in practice but, and this is key, not necessarily any more intelligent than their parents could have been with a different upbringing. Height works the same way, though we obsess over it a bit less.
... While farmers concentrate on high-carbohydrate crops like rice and potatoes, the mix of wild plants and animals in the diets of surviving hunter-gatherers provides more protein and a bettter balance of other nutrients. In one study, the Bushmen's average daily food intake (during a month when food was plentiful) was 2,140 calories and 93 grams of protein, considerably greater than the recommended daily allowance for people of their size. It's almost inconceivable that Bushmen, who eat 75 or so wild plants, could die of starvation the way hundreds of thousands of Irish farmers and their families did during the potato famine of the 1840s.
... Skeletons from Greece and Turkey show that the average height of hunger-gatherers toward the end of the ice ages was a generous 5' 9" for men, 5' 5" for women. With the adoption of agriculture, height crashed, and by 3000 B. C. had reached a low of only 5' 3" for men, 5' for women.
... Another example of paleopathology at work is the study of Indian skeletons from burial mounds in the Illinois and Ohio river valleys. At Dickson Mounds, located near the confluence of the Spoon and Illinois rivers, archaeologists have excavated some 800 skeletons that paint a picture of the health changes that occurred when a hunter-gatherer culture gave way to intensive maize farming around A. D. 1150. Studies by George Armelagos and his colleagues then at the University of Massachusetts show these early farmers paid a price for their new-found livelihood. Compared to the hunter-gatherers who preceded them, the farmers had a nearly 50 per cent increase in enamel defects indicative of malnutrition, a fourfold increase in iron-deficiency anemia (evidenced bya bone condition called porotic hyperostosis), a theefold rise in bone lesions reflecting infectious disease in general, and an increase in degenerative conditions of the spine, probably reflecting a lot of hard physical labor. "Life expectancy at birth in the pre-agricultural community was bout twenty-six years," says Armelagos, "but in the post-agricultural community it was nineteen years. So these episodes of nutritional stress and infectious disease were seriously affecting their ability to survive." ...
It was a very long time indeed before the agricultural gamble paid off with a better average quality of life and nutrition for the masses than hunter-gathering lifestyles. Pay off, it eventually did, even if there are serious questions about how to extend those benefits more equitably within developed societies and around the globe.
I guess what I'm suggesting in a roundabout way is that it's possible what we're experiencing is a liberation of the full potential of a lot of hunter-gatherer intellects that were previously stifled by poor development environments.
Just imagine what we could do as a species if we really put an end to poor development environments.
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