Re: There's something fishy about human brain evolution

On Tue, 21 Feb 2006 16:43:23 +0100, "Marc Verhaegen"
<fa204466@xxxxxxxxx> wrote:

http://www.eurekalert.org/pub_releases/2006-02/nsae-tsf021706.php
Public release date: 18-Feb-2006
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Contact: Stephen Cunnane
stephen.cunnane@xxxxxxxxxxxxxx
819-821-1170
Natural Sciences and Engineering Research Council

There's something fishy about human brain evolution
Forget the textbook story about tool use and language sparking the dramatic
evolutionary growth of the human brain. Instead, imagine ancient hominid
children chasing frogs. Not for fun, but for food.
According to Dr. Stephen Cunnane it was a rich and secure shore-based diet
that fuelled and provided the essential nutrients to make our brains what
they are today. Controversially, according to Dr. Cunnane our initial brain
boost didn't happen by adaptation, but by exaptation, or chance.

"Anthropologists and evolutionary biologists usually point to things like
the rise of language and tool making to explain the massive expansion of
early hominid brains. But this is a Catch-22. Something had to start the
process of brain expansion and I think it was early humans eating clams,
frogs, bird eggs and fish from shoreline environments. This is what created
the necessary physiological conditions for explosive brain growth," says Dr.
Cunnane, a metabolic physiologist at the University of Sherbrooke in
Sherbrooke, Quebec.

The evolutionary growth in hominid brain size remains a mystery and a major
point of contention among anthropologists. Our brains weigh roughly twice as
much as our similarly sized earliest human relative, Homo habilis two
million years ago. The big question is which came first - the bigger brain
or the social, linguistic and tool-making skills we associate with it?

But, Dr. Cunnane argues that most anthropologists are ignorant or dismissive
of the key missing link to help answer this question: the metabolic
constraints that are critical for healthy human brain development today, and
for its evolution.

Human brains aren't just comparatively big, they're hungry. The average
newborn's brain consumes an amazing 75-per cent of an infant's daily energy
needs. According to Dr. Cunnane, to fuel this neural demand, human babies
are born with a built-in energy reservoir - that cute baby fat. Human
infants are the only primate babies born with excess fat. It accounts for
about 14 per cent of their birth weight, similar to that of their brains.

It's this baby fat, says Dr. Cunnane, that provided the physiological
winning conditions for hominids' evolutionary brain expansion. And how were
hominid babies able to pack on the extra pounds? According to Cunnane their
moms were dining on shoreline delicacies like clams and catfish.

"The shores gave us food security and higher nutrient density. My hypothesis
is that to permit the brain to start to increase in size, the fittest early
humans were those with the fattest infants," says Dr. Cunnane, author of the
book Survival of the Fattest, published in 2005.

Unlike the prehistoric savannahs or forests, argues Dr. Cunnane, ancient
shoreline environments provided a year-round, accessible and rich food
supply. Such an environment was found in the wetlands and river and lake
shorelines that dominated east Africa's prehistoric Rift Valley in which
early humans evolved.

Dr. Cunnane points to the table scrap fossil evidence collected by his
symposium co-organizer Dr. Kathy Stewart from the Canadian Museum of Nature,
in Ottawa. Her study of fossil material excavated from numerous Homo habilis
sites in eastern Africa revealed a bevy of chewed fish bones, particularly
catfish.

More than just filling the larder, shorelines provided essential brain
boosting nutrients and minerals that launched Homo sapiens brains past their
primate peers, says Dr. Cunnane, the Canada Research Chair in Brain
Metabolism and Aging.

Brain development and function requires ample supplies of a particular
polyunsaturated fatty acid: docosahexaenoic acid (DHA). DHA is critical to
proper neuron function. Human baby fat provides both an energy source for
the rapidly growing infant grey matter, and also, says Dr. Cunnane, a
greater concentration of DHA per pound than at any other time in life.

Aquatic foods are also rich in iodine, a key brain nutrient. Iodine is
present in much lower amounts from terrestrial food sources such as mammals
and plants.

It was this combination of abundant shoreline food and the "brain selective
nutrients" that sparked the growth of the human brain, he says.

"Initially there wasn't selection for a larger brain," argues Dr. Cunnane.
"The genetic possibility was there, but it remained silent until it was
catalyzed by this shore-based diet."

Dr. Cunnane acknowledges that for the past 20 years he's been swimming
upstream when it comes to convincing anthropologists of his position,
especially that initial hominid brain expansion happened by chance rather
than adaptation.

But, he says, the evidence of the importance of key shoreline nutrients to
brain development is still with us - painfully so. Iodine deficiency is the
world's leading nutrient deficiency. It affects more than a 1.5 billion
people, mostly in inland areas, and causes sub-optimal brain function.
Iodine is legally required to be added to salt in more than 100 countries.

Says Dr. Cunnane: "We've created an artificial shore-based food supply in
our salt."


Is there no shortage of mammals who live on the shoreline eating DHA
rich foods and getting a lot of iodine who don't have particularly
large brains? Are there no primates that live on the shoreline but
never developed particularly large brains? And what about all those
invertebrates and pre-tetrapod vertebrates living their whole lives in
an aquatic environment getting lots of DHA and iodine but never
developing enormous brains? Is the sequestering of DHA in baby fat a
result of the large human brain or a cause?

And why is iodine now considered "brain" food when it really is only
involved in thyroxine production. True, thyroxine is a developmental
hormone necessary for proper brain development, but it has far wider
and more significant implications than that one aspect.


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