Public release date: 18-Feb-2006
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Contact: Stephen Cunnane
stephen.cunnane@usherbrooke.ca
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."
