>While acknowledging that correlation alone is not causation, at the same
>time I think we also have to acknowledge that there seem to be no other
>worthy hypotheses as yet to explain the dietary basis that could have
>supported the dramatic increase in brain size during human evolution. At
>least I'm not aware of any alternative hypotheses other than the recent
>Wrangham cooked-tuber-starch-theory paper that appears in the Dec. 1999
>issue of Current Anthropology ("The raw and the stolen: cooking and the
>ecology of human origins"), which turns out to be weak and implausible at
>best.

The book "The Ascent of Mind" by  William H Calvin discusses the expensive
tissue hypothesis and compares it to several other possible explanations.
Now, I'll grant you this is a pop anthropology book and not peer reviewed
primary research, and furthermore my recollections of the book may be
somewhat inaccurate as I read it several years ago and can't seem to
find a copy now (it is out of print).   However, as I recall, Calvin
believes that the expensive tissue hypothesis did not adequately explain
encephalization in hominids because it did not explain the _speed_ with
which we developed bigger brains (relative to other mammals throughout
history).  He proposes a synthesis of neoteny and enlargement stimulated
by the ebb and flow of ice ages.

I may have some details wrong here, but as I recall the basic idea is
that the onset of an ice age would mix the human genome by concentrating
humans in equatorial warmer regions.  Then, when the ice age ends,
humans enter a "boom time" economy as they are able to exploit the new
environment revealed by the receding glaciers.  This "boom time" rewards
neoteny (and multiple offspring), as there are fewer selective pressures
so whatever human trait causes the fastest reproduction dominates.
Neoteny is self-limiting when cranial size exceeds hip-size in child
bearing females, and so we see enlargement of the species to compensate.
In this way, encephalization is seen as a side-effect of neoteny inspired
by a relaxation of selective pressure during receding ice ages.

This occurs only at the edges of the receding glaciers, causing a
divergence in the hominid genome.  Then, at the onset of the next ice
age, hominids "migrate" south again and reintegrate with the hominids
that did not live right near the edge of the glaciers.  This reinforces
and concentrates the neotenized hominid genome during ice age onset
as well as ice age recession.

In addition to large brains, humans exhibit many other traits
of neotenized, enlarged apes: less body hair, smoother features,
underdeveloped ears, etc.  Furthermore, historically encephalization in
humans occurs precisely at the same time as the onset of the ice ages.
Calvin further cites examples of neoteny/enlargement in other animals,
so we see that this is not an uncommon phenomenon.  Calvin also describes
some early human inventions and shows how they would have helped hominids
to exploit the kind of environment revealed by receding glaciers.

Tom Billings has mentioned to me in email that the expensive tissue
hypothesis is considered to be the dominant hypothesis in anthropology,
and I do not dispute this but I would love to read (especially on
beyondveg.com!) a more in-depth coverage of the different theories and
their relative strengths and weaknesses.  I see paleodiet researchers
as potentially biased in this, since the expensive tissue hypothesis
supports the paleo diet, whereas neoteny/enlargement does not.

-Bruce