Re: Shrinking brains in evolution

dkomo wrote:
Graham Jones wrote:
"dkomo" <dkomo871@xxxxxxxxxxx> wrote in message
news:f2a7og$96j$1@xxxxxxxxxxxxxxxxxxxxxx

In _Evolving Brains_, John Allman mentions that early modern humans had
brains that averaged 1450 grams, whereas the average for contemporary
humans is about 1300 grams. He opines that this is a result of the
"domestication" of humans. The development of agriculture and
domestication of animals as sources of food and clothing have served as
major buffers against environmental variability.

[...]

I don't buy this domestication/taming idea. Pets are weird, but humans were
never anything's pets. It seems clear that a decrease in brain size
coincided with the development of agriculture, but I am far from convinced
that agriculture made life easier, or that the "settler's" life style
requires less intelligence.

I prefer the following explanation (although I don't understand all the
acronyms ;-)).

"While human cranial capacity tripled over the 2.5 million years after H.
habilis first appeared, this trend has recently reversed. Since peaking
among Cro Magnons and other humans living during the Late Paleolithic,
cranial capacity has fallen off about 11%. (Ruff, 1997) This diminution has
paralleled a decrease in consumption of animal foods and, consequently, a
diminished dietary intake of preformed long chain PUFA, the building blocks
necessary for formation of brain tissue. These PUFA can be synthesized by
humans from their 18 carbon precursors, LA and ALA, but the process appears
too slow to supply the amounts of AA, DTA and DHA needed for optimal brain
growth during fetal development and infancy. (Woods, 1996; Salem, 1996) DHA
deficiency is especially important, first because its concentration in brain
tissue exceeds those of AA and DTA and second, because our current w-6
excess tends to promote formation of long chain w-6 AA and DTA while
inhibiting synthesis of w-3 DHA. Association is not necessarily causation,
but the fluctuations which have occurred in brain size over evolutionary
time, both increases and decreases, correlate well with dietary intake of
the preformed long chain PUFA necessary for formation of brain tissue so it
is tempting to speculate on a causal relationship."

I don't buy this explanation. If PUFA is so necessary for formation of
brain tissue and brain size is correlated with dietary intake of animal
foods, how does this explain why "human cranial capacity tripled over
the 2.5 million years after H. habilis first appeared." Did the later
hominids eat three times more animals than the earlier ones? It doesn't
seem like a plausible explanation.

Crawford, and Marsh had much the same idea:

The Driving Force: Food, Evolution, and the Future -
by Michael Crawford, and David Marsh:

''Seafood is an excellent source of unsaturated
fatty acids,particularly the omega-3s. Crawford
and Marsh theorize that our brains developed along
with our increased body size because we had the
abundant nutrients, including minerals, vitamins,
and both families of fatty acids, that are
available on the seashores. They point out that
all savannah animals that increased in size lost
out in relative brain size. They speculate that
our ancient relative, Australopithecus, with a
large pelvis and small brain, "having become
land-locked, became a degenerate side-shoot which,
like the savannah species, followed the common
path of increasing body size outpacing the growth
of the brain." Thus it may be that we humans owe
our pre-eminent brainpower to the sea, with its
abundance of omega-3 fatty acids. The old notion
that fish are brain food may not be far-fetched
after all.''

- http://www.nutrition4health.org/NOHAnews/NNF91CrawfordFDFE.html

The idea is not so much that nutrient availability
provided the only source of selection for larger
brains, but rather that it rearranged the costs
of constructing brain tissue - allowing existing
selection pressures more room for take off.
--
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