There doesn’t need to be DNA change to make people smarter. Just having a few generations in a row with adequate diet, not breathing smoke from a wood fire every day, less damage from childhood disease, etc. could make a huge difference.
There is a fundamental mistake in assuming a "couple generation" linear trend will continue to infinity providing us with 30 foot tall men with IQs of 200+. We are probably at or around peak height and peak IQ right now for various rather obvious income inequality reasons, and we can expect both to decline in the future.
There is a coupling between womens pelvis size and newborn head size and medically assisted survival during childbirth which could provide some multigenerational effect to growth.
There have been a lot of arguments relying on generational inheritance of intelligence / IQ, however:
"As Flynn pointed out in his Ted Talk on the Flynn Effect, in 1900 only 3% of Americans performed "cognitively demanding" jobs - now the figure is 35%"
That would imply the percentage of humanity where cognition has no effect on their job, and presumably lifestyle, is at least 65% and has always historically been higher, as high as 97%, so an evolutionary argument for IQ growth seems extremely weak. It would be as if doctrine were peacock females select their mates based on tail feather size leading to large tail feathers, yet actual data shows 97% to 65% of actual living peacocks come from bald ancestors.
On the contrary, the evolutionary argument here is that a few generations under these conditions would allow more biologically "stupider" humans to survive (since intelligence isn't being selected for) and pass on their genes, creating future generations of dumber humans.
Jared Diamond makes a similar argument in Guns, Germs, and Steel for why the people of New Guinea are smarter than their western counterparts.
There has been shrinkage in the average brain size over the last five thousand years. Intelligence is a very complicated thing and brain size might not be the only factor—that shrinkage might not even be relevant. But that does not mean that evolutionary change cannot take place on thousand year timelines.
Current theory is skull size was adapting based on temperature not intelligence. http://en.wikipedia.org/wiki/Brain_size#mediaviewer/File:Bra... Also, there are likely more people alive now with larger brain volumes vs five thousand years ago and the same apples to just about any trait you can name due to the rapid population boom.
The important thing to note is the human population covers the globe. There are plenty of sifts in populations like Lactose tolerance becoming widespread over the last 7,500 years, but even that is a long way from becoming effectively universal.
I think that was my point. You said that ten thousand years of civilization was too short for meaningful evolution. At least that was my understanding.
10,000 years is long enough for significant beneficial traits to spread widely. It's not enough time to Change neutral traits that used to be beneficial.
That's generally a vary slow change consider most animals don't need dietary vitamin C even if there diets provide plenty of the stuff.
However, you used the term "a few generations" as in below 1,000 years and that's rediculusly fast for a species that takes as long as we do to reproduce and have so few children.
I didn't use the term "a few generations", that was another commenter. :)
That said, you mentioned a beneficial trait becoming neutral, and that's making the assumption that greater intelligence is neutral. It could be that larger social organizations reward slightly less intelligent critters. That would do the trick rather quickly. I'm not making that claim, but am saying that discussions around evolution and traits that we have bias toward seeing as "good" are fraught.
There's also the further issue of intelligence being a very nebulous term that encompasses tons of cognitive traits, which makes any discussion of intelligence in evolutionary terms even more difficult.
It's also possible that some of what we call intelligence is epigenetic.
I don't think we have good data on people’s intelligence 10,000 years ago. But, civilization as we think of it only impacted a fairly small chunk of humanity for a relatively small time period. So, at best 'larger social organizations' was rather localized.
Just 500 years ago lots of people in North America where living nomadic hunter gather lifestyles. Even just 100 years ago you could find tribes in many parts of the world still living hunter gather lifestyles little changed over the last 15,000+ years and probably similar to how people lived 100,000+ thousand years ago.
As to epigenetic factors, the classic hunter gather lifestyle could be very healthy as long as population numbers stayed low. Farming actually lowered many heath indicators even as it allowed for massive increases in population sizes.
PS: Anyway, evolution is only 'fast' when there are significant benefits or harms. If a single mutation or environment change increases survival chances by say +/- 1% it takes exponentially longer to spread than a +/- 10% change.
I don't disagree with any of those points. My original post just stated that I couldn't imagine any fitness pressure that would be making humans smarter and, if anything, we were probably going in the other direction. My secondary point was that change can happen on short timelines.
I do want to mention that larger social organizations were quite widespread in the Americas, despite the existence of hunter-gatherer tribes. The new world certainly wasn't an uncivilized wasteland when Europeans arrived.
Epigenetic factors can be triggered buy group size (domestication) as well. So those hunter-gatherer tribes might share the same genetic traits but express them differently based on their environment.
any fitness pressure that would be making humans smarter
I don’t recall the study, it was either men in the US or Britton. But, income was positively correlated with number of children. Income and intelligence are also linked so that's evidence that right now intelligence is positively correlated with number of children. This may be strongly linked with prison time but again that links with intelligence.
The important thing to note is men often have children with more than one person and can have children vary late in life.
Deny. There's no reason dying of wood smoke or disease selects for the smart or the dumb. Removing those pressures has no obvious effect on the intelligence of the survivors.
