by Jamie Hale
College students are taught that the human brain
consists of 100 billion neurons. This
claim can be found in numerous textbooks.
College instructors often promote the 100 billion neuron claim. This claim is also promoted by widespread
media sources. If you have read much about the brain or engaged in dialogue
regarding the human brain there is a good chance you have encountered this
statement: seemingly, this is general neuroscience- basic stuff. When I was a graduate student this number
was accepted without question. What is
the original source for this number?
Another claim that is often made regarding brain
science is that there is "ten times more glia (glia often referred to as
neuron support cells) than neurons in the human brain." Is there an original source for this
number? Is there evidence for 1 trillion
glia in the human brain?
Herculano-Houzel addresses both of these topics in
her book The Human Advantage: A New
Understanding of How Our Brain Became Remarkable (2016). She provides evidence to
refute the 100 billion neuron and the 1 trillion glia claim. Her research has
led to a change in teaching neuroscience and has driven pop and scholarly
publications to make changes.
The
Human Advantage: In Review
Suzana Herculano-Houzel, is the author of The Human Advantage. She is a former associate professor and head
of the Laboratory of Comparative Anatomy at the Federal University of Rio de
Janeiro. She is the author of six books
on the neuroscience of everyday life. She
is a former writer and presenter of the TV series Neurologica. Currently,
she resides at Vanderbilt University.
How do humans have such tremendous cognitive
abilities? Herculano-Houzel argues that
human are remarkable, but they are not special in light of evolution. Human brains follow the rules of primate
evolution. Primates have an advantage
over other mammals regarding brain structure;
primates brains have evolved in a way that allows neurons to be added to
the brain, without the large increases in average cell sizes seen in other
mammals. Primate brains have evolved
differently than those brains of other animals.
As an example, cows and chimpanzees have brains that are similar in
mass, but the chimpanzee can be expected to have at least twice as many neurons
as a cow. Human brains are scaled up
primate brains. Contrary to the popular
claim that the human brain is larger than can be expected for body type (expressed
as encephalization quotient), the author argues the number of brain neurons as
a function of body mass is what
can be expected for a non-great ape primate.
Neuroscientists, in the past, thought
that the human brain is large relative to the size of the body that contains
it, when directly comparing to brain and body size of great apes. If our body is smaller, then our brain should
be smaller, and yet it is three time larger in terms of mass. However, Herculano-Houzel's data show that
when great apes are excluded humans show the same relationship between their
body mass and number of brain neurons as that of other primates. In the first decade of the twenty-first
century, systematic comparisons relative to the encephalization quotient, started
being made of cognitive abilities among nonhuman primates, and of self control
abilities among birds and mammals. The
general finding was "simple absolute brain size was a much better
correlate of cognitive capabilities than the encephalization quotient. It was back to square one. If the human brain is not the largest, then
how can it be the most capable of them all?" (Hercualno-Houzel, 2016, pp.
16-17.). The human brain is just what
can be expected for a primate brain that has evolved to adapt to human
conditions. The primary mechanism
responsible for human cognitive abilities is the number of neurons in the
cerebral cortex. The human brain has
more neurons in the cerebral cortex (16 billion) than any other animal, even
when the animal (African elephant) has 257 billion brain neurons.
What is the original source for the - 100 billion
neurons in the human brain assertion? Herculano-Houzel
asked senior neuroscientists and no one was able to point her to the original
source. After an extensive search through the scientific literature she wasn't
able to find a single source supporting the 100 billion neuron claim. According to Herculano-Houzel, Eric Kandel
(Nobel Laureate), co-author of Principles
of Neural Science, couldn't
provide an original source for the claim, even though the claim was made in Principles of Neural Science (a book
Kandel co-authored). When asked about
the claim, Kandel responded saying he wasn't responsible for the chapter containing
the 100 billion neuron claim.
Is there evidence for 1 trillion glia in the human
brain? Herculano- Houzel reports she
couldn't locate any research to support
the claim- 1 trillion glia in the human brain.
Both of these claims (100 billion neurons and ten times more glia) are
often taken as fact. Accepting
information as fact, even though it is not
supported by evidence is problematic; specifically problematic as it is
odds with a central tenet of science; scientific data is based on evidence. A paper published by Herculano-Houzel and
colleagues titled "Equal Numbers of Neuronal and Non-Neuronal Cells Make
the Human Brain an Isometrically Scaled-Up Primate Brain," which is now a
heavily cited paper, was rejected by high ranking journals including Nature, Proceedings of the National Academy of Sciences of the U.S.A., Neuron and the Journal of Neuroscience. The paper was eventually published in the Journal of Comparative Neurology.
Houzel developed a method called the "isotropic
fractionator" that allowed her to create what she calls brain soup? The method allows dissolving only cell
membranes, but not nuclear membranes (each neuron consists of one nuclear
membrane), therefore producing brain soup with free-floating nuclei. These nuclei are relatively easy to count by
sampling tiny amounts of the soup. All
the nuclei from all of the cells are stained blue, collected and counted. In the
book she provides a description of what went into developing the
technique. The first attempts to use the
method led to the destruction of some of the nuclei. Early attempts involved
testing the preparation after a few hours of fixation. In order for the all of
the nuclei to remain intact longer preparation times were required. It was
finally established that after
approximately two weeks of fixation the nuclei would all stay in place during
testing. Other researchers have used
this method. Christopher von Bartheld,
from the University of from the University of Reno, and Jon Kaas, from
Vanderbilt University have shown this method to be faster, more reliable and
easier to apply than stereology, which was commonly used in the past.
Results, after using the isotropic fractionator,
indicate the human brain has an average of 86 billion neurons and 85 billion
non-neuronal cells (glia and endothelia- cells composing blood vessels) For people who like to point out that
"86 is close to 100" and who claim the 100 billion is reasonable as
an order-of-magnitude estimate, Houzel asserts, an entire baboon brain contains
11 billion neurons. Fourteen billion is
not a small number of neurons.
The author concludes that the human brain is
remarkable due to the number of neurons in the cerebral cortex (approximately
16 billion) and secondly it is remarkable thanks to cooking, which allowed
humans to escape the energetic limitations of a raw food diet, that limits
other animals to less cortical neurons.
Chapter 11 provides detailed information on how cooking contributed to
the human brain.
The book appeals to a large audience. Even though sections of the book might be
difficult for some to read, with the appropriate effort the information is
accessible for most people. The author
points out that some of her earlier work was met with resistant. It shouldn't be surprising that some may have
a problem accepting views that challenge what they thought to be neuroscience
fact for so many years. I highly
recommend this book. Herculano-Houzel is a major player in neuroscience.
Title: The human advantage: a new understanding of
how our brain became remarkable / Suzana Herculano-Houzel.
Description: Cambride, MA: The MIT Press, 2016
ISBN 9780262034258
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