On Wed, 28 Aug 2002, Herb Finkelstein wrote:

> Recall, however, that in the last 35000 years human brain size has in fact
> decreased by 11%. Most of that decrease (8% of the 11%) has come in just
> the last 10000 years. This is a significant decrease in a very short period
> of evolutionary time. Thus while people these days may seemingly appear to
> be maintaining brain size (the observation appears anecdotal to me, I'd
> like to see some support), it would be so only up to a point and only in
> the short run, since the historical evidence says otherwise.

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?  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.

Another thing to consider is this.  The "great leap forward"
about 38,000 years ago included an *increase* in man's
utilization of seafood, and the mesolithic period is defined by
the presence of more sophisticated tools for this purpose:
fishhooks, nets, etc.  And yet this is about the time when brain
size began to decline, and the decline accelerated after
agriculture.

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.

It is very difficult to determine cause and effect from
evolutionary history.

> What is noteworthy about the brain size decrease over the last 35000 years
> is that not only has it come during a time period when DHA consumption has
> decreased considerably, but when starch consumption has gone to an all-time
> high for humans due to the agricultural revolution of the last 10000 years.
> This evidence flies directly in the face of the tuber (starch) hypothesis
> for brain size evolution on both the DHA issue and the starch issue at the
> same time.

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.

Brain size increased steadily from Australopithecus until about
35,000 years ago, then started to decline, especially 10,000
years ago.  If Wrangham is right, tuber consumption also
increased during the time from Australopithecus to 38,000 years
ago.  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.

Todd Moody
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