Todd said:

>It's important to be clear about what the effect is supposed to
>be.  Is it that decreasing DHA consumption *caused* brain size
>gradually to decrease?  If a certain level of DHA intake is a
>necessary condition for achieving a certain brain size, then I
>think we'd expect to see a comparable correlation between DHA
>intake and brain size in contemporary human populations.  Is
>there such a correlation?

That I don't know about, but it looks like it would be a good way to test
the hypothesis, for sure.

>It's interesting that some of the
>largest brains on earth are the marine mammals, dolphins and
>whales.  And they surely eat a lot of DHA.  But sharks also eat a
>lot of DHA, but their brains are only about 35g in weight.  So
>that tells us that eating a lot of DHA is at least not a
>sufficient condition for having a large brain.  The largest
>brains are found in Asian elephants, at over 7,000g.  They
>probably eat a lot of LNA, but I doubt they get that much DHA.

In the case of elephants, note that the outsize brain is accompanied by an
even more outsize body, proportionally speaking. As noted in comments to
Amadeus, encephalization quotient (EQ) is the comparative criterion for
this across species rather than absolute size. EQ for elephants is far less
than for humans or dolphins. From a metabolic perspective, an elephant
brain is a pretty small output from its metabolism for us to be looking at,
given their hulk.

Of relevance to the example of sharks who crunch down on plenty of DHA.
Scientists who side with the expensive tissue hypothesis as well as a DHA
connection usually take pains to make clear they don't consider DHA intake
to be sufficient by itself for brain size or EQ. What they do say is they
consider dietary quality or DQ a necessary or limiting factor or
precondition.

What else they say might be necessary is where no one seems to have
definitive answers yet. One common suggestion, of course, proposes that an
increase in complexity of interpersonal/social behavior/competition along
with increasing intake of animal tissues/DHA/energy may be the unique
combination that drove human brain expansion. A combination that might be
characteristic to humans and not much else. I don't know about the validity
of that claim, but that is what one see's discussed.

While chimps for example have relatively complex social behavior, for
whatever reason they didn't move into the large amounts of animal foods
like humans did. Sharks the opposite situation. This idea of necessary
cofactors that are not sufficient in isolation explains the lack of brain
size in sharks and chimps, at least, each of whom had one precondition but
not the other.

>It looks to me as if our ancestors increased their consumption of
>fish between 38,000 and 10,000 years ago, even as their brains
>started to get smaller.  I have a problem attributing that to DHA
>scarcity.  After agriculture, it *could* be DHA scarcity, or it
>could be some other factor, such as general malnutrition.

That makes sense, and is worth more thought. Maybe similar to the
hypothesis that social complexity + DQ may both be twin cofactors necessary
to explain brain size increase but can't do so by themselves, there may be
some additional cofactor needed to explain the recent decrease. One
possibility I've seen mentioned might be that increased grain consumption
has greatly displaced fruits/veggies. (No surprise there, doh.) F/V are of
course (doh again) a cornucopia of micronutrients and nutritional
co-factors (not just a source of carbohydrate or energy replaceable by
grains/starch). One wonders if a lack of these could affect overall growth.

Perhaps something along such lines that's a more overarching variable,
anyway (general malnutrition as you mentioned) could be necessary to
explain the reduced modern body size in addition to the reduced brain
growth, which have both decreased somewhat in tandem. What about increased
rates of infection which accompanied greatly increased population density
after agriculture. Also common to post-agricultural populations, and
mentioned frequently in discussing health problems/stature decreases in
neolithic times. We are only now in maybe the last 50-100 years via
hygienic measures getting beyond these issues for large segments of
populations. (Then again, west nile may get us yet! :-) )

>The only way that starch consumption could contribute to increase
>in brain size (The Wrangham theory is that tuber consumption
>increased during the earlier time when brains were still
>increasing in size) is by being calorically dense, permitting
>smaller volume of food to be eaten, thus decreasing metabolic
>requirements of the gut and allowing the brain a greater share of
>energy.  The Expensive Tissue theory has the gut and brain in
>competition for energy.

Just an aside here: Haven't more recent proponents of the Expensive Tissue
Hypothesis (ETH) begun to include nutrient density besides just energy
density? In its original form of considering energy density only, the
hypothesis was overly simplified and incomplete, it seems to me. There is a
lot more to dietary quality (DQ) than just energy. Bioavailability of
nutrients, at least ones the brain competes for, might conceivably impact
the ETH. Is there a possibility some of them might affect body size as
well, perhaps.

>And as you point out, starch consumption kept increasing
>with agriculture.  It doesn't seem likely (to me) that increasing
>starch consumption could explain both the increase and decrease
>in brain size.
>
>It's very puzzling.  What happened between 35,000 years ago and
>10,000 years ago that resulted in a 3% decrease in brain size?
>After 10,000 years ago we can hypothesize a number of factors,
>since a lot of things changed: DHA shortfall, om-6 overload
>(could this inhibit brain growth?), malnutrition.

That's something I hadn't thought of, which in some way should eventually
help us clarify things further. However, I mainly wanted to point out here
the significance of the brain size decrease evidence in general, because I
am similarly puzzled by something else. In this case, why the 8% decrease
in the face of starch skyrocketing isn't better known in the paleo
community, and publicized outside of it. Why should it be? Because the
tuber theory in some ways may prove to be high-carb's last stand in
scientific circles. And it is high-carb thinking is really what is keeping
much of the scientific community from giving paleo theory fair
consideration. Or so it seems to me...

Herb