Most likely cause of brain size variation in human history?

[JeuneKoq]

Some researchers suggest that homo's brain size increased when we started eating more energy-efficient foods like animal meat, and that higher intelligence was useful in getting hold of some.
On this website*, they say "[Researchers] point out that there are two essential fatty acids, docsahexaenoic and arachidonic, that are essential to brain development in modern humans, and the best sources of these two fatty acids are bone marrow, and particularly ruminant brains. Therefore, the consumption of these animal products would have facilitated expansion in brain size and increased cranial capacity over the long term.".

Others suggest that meat was not the main cause of our brain development: "They argue, by analogy, that the majority of living primates are largely vegetarian, and that we, as primates, are best adapted to a mainly vegetarian diet. Milton (1993) writes that mandible size decreased due to the increased consumption of energy-rich plant foods such as fruits, and not necessarily meat. The complex skills required to harvest these energy-rich plants would also result in a selection for more intelligent hominids, with resulting increased brain size through time."

As most of us know, our modern human brain is on average 10% less voluminous than our Paleo ancestors. However, I remember someone -I think it's you, Tyler- stating that brain size decrease has been uniform in all humans of the planet. Which means people who still ate plenty of bone marrow and ruminant brain until recently, like Inuits I presume, still saw their average brain size decrease. Is this confirmed and documented? If so, what is the most likely cause of this brain shrinkage?

* http://www.nature.com/ejcn/journal/v56/n12/full/1601646a.html

[TylerDurden]

Well, Paleophil and I found out that the Inuit have the largest skull size on average among ethnic groups. Maybe that means something. Yet that would mean that vegan Hindus had the smallest average brain-size, but that does not seem to be the case, last I checked.  There have also been claims that the very last spurt in brain-size ocurred with the advent of cooking  c.400,000 years ago, yet other researcher point to 250,000 to 300,000 years ago as the more likely advent of cooking due to the appearance of hearths.Plus, in the neolithic era, Mankind ate far more cooked foods as a proportion of the diet yet developed reduced brain-size.

From the evidence of anthropologists, the evidence seems to be that the decrease in brain-size was uniform across Mankind(I have only seen assertions by anthropologists, I have not seen any  actual pubmed article, afaik). What I find interesting is that the Neanderthals had far bigger brains than humans yet had the same body-mass(ie a higher encephalisation quotient)(ie stockier body but shorter heights on average).

My view is that diet is not that essential. I mean it is true that carnivores generally are more intelligent than herbivores but that is because  higher intelligence is needed in order to hunt prey than to eat plants.However, Wrangham's notion sounds better :- namely, that when wild animals are domesticated, then after several generations, the descendants have 10% smaller brains on average than their wild ancestors and also have lower testosterone levels. The idea being that higher intelligence is exchanged, to some extent,  for increased social cooperation.

Another theory is the natural selection theory, that increased social cooperation in the Neolithic era allowed those with mediocre IQ to survive and breed  when they would otherwise have died out.If that is correct, then the relevant scientists claim that in 10,000 years we will have, at most, the average brain-size of homo erectus. I tend to this theory as I consider that intelligence is linked to how much one uses it. In other words, that not only natural selection but, more importantly, epigenetics play a role in increasing intelligence.So, ie "use it or lose it". It frightens me to see more and more humans just pushing buttons or using calculators rather than using their brains etc.

[TylerDurden]

This is an attempt to provide an alternative explanation re the notion of  bigger brains  being linked to intelligence:-

http://news.discovery.com/human/evolution/brain-size-vision-humans-eyeball-polar-110726.htm

On the other hand, brain-size is, more or less,vaguely  linked to intelligence, judging from most other sources:-

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2668913/

The Eskimos have been cited as having IQ scores of c.91.
http://www.thebirdman.org/Index/Others/Others-RichardLynnOnFemaleAndAsianIQ.htm

or even higher:-
http://www.thephora.net/forum/archive/index.php/t-7666.html

 Of course, IQ scores really only evaluate education not real intelligence as such. I should know, as depending on my previous state of health and the kind of IQ test, I have scored anywhere from 113 IQ to 170 IQ on the numerous times I have tried them.

[TylerDurden]

Also it seems to be a myth that the frontal lobes are the seat of human intelligence:-

http://www.medicaldaily.com/frontal-lobe-size-brain-does-not-explain-human-intelligence-245843

Reminds me of the myth that we only use 10% of our brains. Anyway, even the brain-size/intelligence link, while proven via meta-studies, is rather tenuous.

[PaleoPhil]

Peter Frost posited that the development of larger brains at higher Eurasian latitudes might have been triggered by increased cognitive demands and increased need for specialization:

http://www.unz.com/pfrost/brain-size-and-latitude-why-correlation

http://www.unz.com/pfrost/brain-size-latitude-and-ambient-light/

Unfortunately, not much research is done on brain size and intelligence among past and present population groups because of moronic political correctness. 

Diet is still an interesting question, regarding what foods best fill the needs of the larger brains, and even if Frost's ideas are correct, it wouldn't necessarily rule out dietary triggering factors in other regions like Africa.

[TylerDurden]

One interesting point re the Inuit is that they practised 80% average mass infanticide of their children right up until recent times(1930s?). This might also explain the fact they had the largest humanoid skull-sizes(other than the Neanderthals). I mean 80% is even larger than the figures given for average mass infanticide in palaeolithic times, afaik.

[JeuneKoq]

Thanks for all the info guys.