Suzana Herculano-Houzel created a revolution in our
knowledge of the mammalian brain and its evolution when she discovered a simple
technique that would, for the first time, allow measurement of the numbers of
various cells that make up brain material.
She presented results from studies using this technique in her book The Human Advantage: A New Understanding of How Our Brain Became Remarkable. A brief summary of
her findings can be found here.
Not surprisingly, the most interesting results from her
studies centered on what could be determined about the human brain. Humans are primates and are considered to be
one of what are often referred to as the great apes. When Herculano-Houzel evaluated her data on
the neuron cell count in the brains of various classes of mammals, she
discovered what she referred to as “the primate advantage.” Taking the number of neurons in a brain as a
proxy for the cognitive capacity of the brain, the primate advantage consisted
in the fact that primates had evolved the means to control neuron cell size so
that for a given brain size, a primate brain would contain more neurons than
the brains of other classes of mammals, whose neurons gained size as brain mass
increased. Primates then had greater
cognitive capacity per unit of brain mass, but not necessarily greater
cognitive capability. Capacity becomes
capability when the neurons are used to learn things that provide an advantage
to the possessor of the brain.
An example was provided to demonstrate the size of this
“primate advantage.”
“Once we knew the neuronal
scaling rules for rodent brains, in 2006, we could already do some rough
calculations. With the equations
relating the number of neurons in a rodent brain to the mass of the brain and
of the body, we could estimate that a rodent brain that had anything in the
order of 100 billion neurons, as the human brain was supposed to have, would
weigh more than thirty kilograms [66 pounds] and belong in a body that weighed
more than 80 tons. In other words: if we
were rodents, we would look like a blue whale, have to live in water, and carry
an impossibly large brain, one that would likely collapse under its own
weight.”
Herculano-Houzel eventually determined that the human
brain contained about 86 billion neurons.
She was always puzzled by the assumption that humans were
somehow exceptional to an extent that seemed inconsistent with the notion that
humans evolved in the same manner as other animals. The fact that humans were primates closely
related to gorillas and chimpanzees (the great apes) that were comparable in
size but only had about a third the brain mass of a human supported this notion
of human exceptionality. Once she had
data on primate species similar to that for rodents she could apply scaling
rules to the number of neurons versus brain size, and brain size versus body
size.
“According to the neuronal
scaling rules that apply to primates, we would expect a generic primate brain
with a total of 86 billion neurons to weigh about 1,240 grams (2.75 pounds) in
a body weighing about 66 kilograms (145 pounds). These numbers are just about right for us
humans, with our, on average, 1,500 gram (3.3 pound) brains and 70 kilogram
(155 pound) bodies. The conclusion
should come as no surprise to a biologist: we are that generic primate with 86
billion neurons in its brain. Our brain
is made in the image of other primate brains.”
If that explanation is correct, then the other great apes
must be the exceptions because they have much smaller brains than the scaling
rules suggest. Therein resides an
interesting tale of how humans, in fact, became exceptional. It was not the brain we were born with that
makes us exceptional, it is what we did and what we continue to do with that
brain that made us unique.
It is necessary then to explain why the other great apes
ended up with smaller brains in order to understand why we ended up
different. Consider that the amount of
energy required to keep a neuron functioning is believed to be about constant
and independent of neuron cell size, although it is a function of the brain
region.
“What our findings indicate,
however, is that within each of these
neuronal types, cerebellar or cortical, variations in cell size across species
are accompanied by neither an increased nor a decreased energy cost: neurons of
the same type still cost the same across species, regardless of their size,
with a fixed average energy budget per neuron across species.”
Nonprimate species have brains that increase with body
size, but their neurons also become larger as brain size increases. Consequently
they end up with many fewer neurons for a given brain size than primates and
avoid creating an excessive energy demand to support their brains.
On the other hand, primate brains, with their large
number of neurons in a relatively small size, consume a large amount of
energy. A human requiring a 2000 calorie
diet to maintain its body weight would be devoting 500 of those calories to
keeping its brain functioning. A large
brain with a lot of neutrons produces a large energy requirement that must be
satisfied. Herculano-Houzel provides an
argument to support the hypothesis that given the environment in which the
great apes evolved, their available food sources, and their physical
capabilities, it is not possible for gorillas and other great apes to greatly
exceed their current caloric intake and thus could not support a much larger
brain.
Field studies of apes in the wild tally the amount of
time they spend foraging for food. Since
apes tend to be on the lean side and generally do not gain excess weight, one
can assume that they forage until they attain their needed nutrition. What was discovered was that the bigger the
ape, the more food that could be gathered and consumed in a given time
period. However, as body weight
increased, the energy demand of the body increased faster with weight than the
ability to consume more calories. That
meant that there was, in principle, a maximum mass that an ape could attain. It also means that there was no extra energy
available to support larger brains.
These apes could have evolved to a smaller size and, perhaps, could have
acquired a larger brain as compensation, but natural selection seems to have
decided against that strategy for those apes in their particular
environments.
“….the lineages that remained on
all fours (and gave rise to modern apes) seemed to have invested any additional
kilocalories [what we refer to as calories are technically kilocalories]
amassed per day from longer times foraging and feeding into growing larger
bodies. For knuckle-walking species that
are, for anatomical reasons, not very mobile, becoming as large as they could
afford must have been advantageous, earning larger animals higher social status
and thus greater access to food, among other privileges.”
Opting for a larger brain can be a risky evolutionary
decision. The brain always gets the
energy it needs—and it always needs about the same amount no matter what the
animal is doing—or the animal dies.
“Because an individual brain
always uses the same amount of energy, no matter whether the rest of the body
is starving, having too many neurons is clearly a liability when a species
lives close to the limit of its caloric intake possibilities.”
Humans then had to have done something to escape from
this brain size dilemma. The starting
point of a new direction was probably becoming bipedal with increased mobility
and access to different and more extensive feeding zones, and new classes of
survivability issues.
“….for our newly bipedal and suddenly
highly mobile australopithecine ancestor, who some 4 million years ago diverged
away from the lineage that would give rise to the modern chimpanzee and bonobo,
investing the additional kilocalories it amassed per day in a greater number of
brain neurons housed in a leaner, lighter body must have proved a much better
investment strategy.”
Knowledge of how brain size must have increased over time
from archeological evidence suggests that pre-humans adapted physically to the
new environment and its challenges by growing slightly larger and increasing
brain size as well. However, the
increase in brain size was small until about 1.5 million years ago, when it
began to rapidly increase. The
hypothesis is that even though pre-humans learned not only to gather food but
also to hunt it, it still faced an energy constraint due to the time required
to obtain and consume food.
If one needed a dramatic increase in nourishment one
could either figure out a way to acquire much more food, or one could figure
out a way to obtain more nourishment from the food supply on hand. The archeological evidence suggests that
around 1.5 million years ago, when brain growth really began to rise rapidly,
humans also began to cook their food.
Richard Wrangham’s book Catching Fire: How Cooking Made Us Human took note of this occurrence and
formulated a “cooking hypothesis.”
“In
a nutshell, the cooking hypothesis proposes that it was the invention of
cooking by our direct ancestors and the resulting availability of cooked food
that offered the larger caloric intake that allowed the brain of Homo to increase in size so rapidly in
evolution. The circumstantial evidence
of the drastic reduction in tooth and cranial bone mass, expected for a species
that no longer had to use much effort to chew, was all there, along with the
fossil record that put the use of fire with transforming of foods between 1.0
and 1.5 million years ago. What Wrangham
did not have then was an indication that cooking, or some other way to increase
caloric input from food, was not simply a bonus, but rather an essential requirement
for their brains to become any larger.”
Why was a diet of raw
foods so limiting for primates?
“….cooking
presupposes the use of heat to denature proteins, break carbohydrate chains,
and otherwise modify the macromolecules of food, turning foodstuffs into
smaller, softer, more easily chewable and enzymatically digestible versions of
their former selves. Cooking with heat
breaks down the collagen fibers that make meat tough and softens the hard walls
of plant cells, exposing their stores of starch and fat. Cooked foods yield 100 percent of their
caloric content to the digestive system because they are turned into mush
inside the mouth, then digested completely by enzymes in the stomach and small
intestine, where, once converted into amino acids, simple sugars, fatty acids,
and glycerol, they are quickly absorbed into the blood stream. In contrast, the same foods may yield as
little as 33 percent of the energy in their chemical bonds when eaten raw
because these harder foods are swallowed while still in pieces, and thus are
broken down and digested only partially.
Only the surface of the raw food crumbs is exposed to digestive enzymes
in the stomach and small intestine; most of the unbroken starch finally gets
digested in the large intestine by bacteria that keep the energy for
themselves.”
Cooking available foods
increased the obtained caloric content of those foods by a factor of up to three. That is clearly a revolutionary occurrence,
but the additional energy did not immediately lead to a bigger brain. First, our ancestors would have to encounter
or create situations in which the increased cognitive capacity was needed in
order to obtain a survival advantage.
Natural selection would take over at that point.
The human advantage was
not so much its bigger brain, but the ways in which our ancestors put that
brain to use. Our brains at birth are
rather empty, useless things, but with a lot of potential. It is the fulfilling of that potential that
makes humans unique.
Now that we have learned
how critical cooking food has been to human development, it should come as no
surprise to learn that returning to eating only raw food has become a bit of a
movement in some areas. The terms
crudivore and crudivorisme (for those
comfortable with the French language) have arisen. The English version of wikipedia files it
under “raw foodism.”
Herculano-Houzel provides this comment on the topic.
“….despite the amenities of the
modern, technological world, obtaining enough kilocalories from raw foods remains so
difficult that the crudivore diet is
the ‘tried-and-true” way to lose weight—although not without its drawbacks: the
drastic weight loss that ensues, with a constant feeling of hunger, is often
accompanied by malnourishment to the point that women on the diet stop
menstruating.”
Interestingly, other animals
can still be smarter than humans on occasion.
Most animals that have been fed cooked food will gladly trade their raw
food for the new stuff.
The interested reader
might find these articles informative:
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