A revolution has occurred in our understanding of how our
brain and those of other animals have evolved.
This revolution has produced a profound change in how we should consider
ourselves in relation to other species, and, in particular, to how we should
compare ourselves to the other great apes.
While areas in science where great amounts of money are involved, in
terms of products to be produced or in the quest for research dollars, tend to
be argued in the mainstream media, others that aren’t associated with a large revenue
stream can rattle around in obscure (to most of us) journals for many years
before they are revealed to the general public.
That has been the case with the research started by Suzana
Herculano-Houzel and summarized in her fascinating new book The Human Advantage: A New Understanding of How Our Brain Became Remarkable. Her work began in
2003, was first published in 2005, and after 10 years of additional research is
now being presented to the rest of us in her book.
The author, a native of Brazil,
had an exemplary academic career with a stint at Case Western Reserve
University in Cleveland before obtaining a Ph.D. in neurophysiology at the Max
Planck Institute for Brain Research in Frankfurt. For personal reasons, she returned to Brazil
and worked at the Museum of Life in Rio for three years.
“For
the first three years back in Rio I designed hands-on activities for the
children who visited the museum, created a website, and wrote my first book on
Neuroscience for the general public, which landed me a job at her alma mater.”
The year before she initiated
her research she was named an associate professor at the Federal University of
Rio de Janeiro whose specialty was assumed to be science communication. However, she was told she could perform
research as well if she wished.
Herculano-Houzel had always been troubled by the quandary
that arose when true-believers in evolution reasoned that humans had to be
produced by evolution, yet they continually evaluated humans in a manner that
placed them beyond the bounds of what could be explained by evolution. If we are the end point of evolution of the
great apes, how could we have evolved to have a brain that was about three
times larger in mass? Why are we so
unique? Her great contribution was to
demonstrate that, in terms of evolution, we are not unique.
It was believed that cognitive abilities in a species
would be related to the number of neurons contained within the brain. If that was the case, then how can one
discuss the relative brainpower of a given species if this number was not
available? This is the problem Herculano-Houzel
wanted to solve. Prior attempts at
measuring neuron numbers utilized a technique known as stereology in which thin
slices of a brain would be sampled in depth at a few locations and the results
applied to the entire section. She
needed a method both simpler and more powerful for what she intended.
“The highly heterogeneous
distribution of neurons across different structures of the brain, however,
makes stereology impractical for determining numbers of cells in whole
brains. Neuronal densities vary by
factors of up to 1,000 across brainstem structures and the cerebellum. Even within a single structure, like the
cerebellum, different layers have neurons packed in widely varying
densities….[Stereology] would be prohibitively expensive even for a well
equipped lab. And it was even more so
for myself, with no lab and no funding.”
Her solution to the problem required some trial and
error, but no exotic new equipment or materials. She merely produced what she gleefully refers
to as brain soup. Sections of brain
material to be analyzed where sloshed around in a detergent solution that
dissolved the cell boundaries but left the cell nuclei intact. This soup had to be homogenized so that
samples of the fluid could be used to visually count the number of nuclei. This approach was aided by applying chemicals
that would attach to the nuclei and produce a blue color. This provided a total cell count. To get a separate count of the number of
neurons there was a known molecule that would attach itself to neuron nuclei
and produce a red color.
“Counting 500 nuclei (which took
around 15 minutes at the microscope) was enough to determine the percentage of
neurons with a certainty of 0.2 percent.
Applying the percentage of neurons to the total number of cells in the
structure of origin yielded an estimate of the total number of neurons in
it. By subtraction, I had the total
number of other cells, presumably mostly glial cells, in the tissue. Summing the results for the various brain
structures—and I started with entire brains subdivided simply into cerebral
cortex, cerebellum, and rest of the brain—yielded for the first time, direct
estimates of the total number of neurons and other cells in the whole rat
brain. And the entire process took less
than a day.”
“Luckily for us, however, there
were a few stereological estimates available in the literature for the rat
cereberal cortex and cerebellum—and our estimates matched them.”
This picture will help with the nomenclature.
The rather small cerebellum is clearly indicated, as is
the brainstem. The rest of the visible parts
make up the cerebral cortex, which is often divided into sections that have
more to do with physical structure than function.
Herculano-Houzel
was now off and running. She had a
credible technique. Now she needed specimen
brains—lots of them. Her studies were
restricted to mammalian brains which all have similar structures. One can postulate that mammals all derive
from a single ancestor and the various families of mammals (clades) broke off
from that original line and evolved according to its newly determined
evolutionary rules. Determining the
manner in which species in a clade added neurons to their brains as the species
increased in adult size would be of great interest.
After 10 years of research, Herculano-Houzel and her
collaborators had accumulated and published data for six different mammalian families. What they discovered was that for most
species the number of neurons did not scale proportionately with brain size,
whether in the cerebral cortex or the cerebellum. As the family evolved to larger animals with
larger brains, the size of the neuron cells increased and thus the number of neurons
increased but did not keep up with the gain in brain size. The primate family was the great exception to
that rule. The small primates studied
had somehow developed a mechanism that kept the size of neurons nearly constant
as larger species with bigger brains evolved.
Thus a primate brain would pack more neurons per volume than a brain of
a different family but of the same size.
This is referred to as “the primate advantage.”
Humans are primates.
The greatest enlightenment provided by these studies was placing the
human brain in context. Previously, it
was generally concluded that the human brain was exceptional in size because it
was so much larger than those of the other great apes. However, the scaling rules determined for
primates indicated that the human brain is exactly what it should be for a
primate of that size. The other great
apes were the outliers. They possessed
brains much smaller than they should have had.
What happened?
Herculano-Houzel attributes the hypothesis that can
explain this to Richard Wrangham who presented his thoughts in his book Catching Fire: How Cooking Made Us Human. Her results corroborate the hypothesis. It must be recognized that a brain requires
an exceptional amount of energy to keep it running. There appears to be a constant energy
requirement per neuron; a bigger brain, or just a rain with more neurons,
requires more energy. For humans, the
brain utilizes about 25% of the calories in a normal diet. For an ape to have developed three times the
brain size (as scaling suggested) that ape would have had to have a plentiful
supply of calories to support it.
Observational studies of the other apes in the wild suggest that food is
not plentiful for them; they work hard foraging for enough food to maintain
their weight. Given such a situation, it
seems that natural selection for these apes determined that a larger body and a
smaller brain was the more advantageous situation.
How did humans avoid this
evolutionary trap? First of all, they came
down from the trees and began to walk on their hind legs. This made them more mobile and put them in
new environments that might change the tradeoff between body size and brain
size. But the issue of energy had to be
solved. Foraging for food might or might
not have been more efficient on flat land.
What the historical record suggests is that the brain grew slowly in
pre-humans after they became bipeds until about 1.5 million years ago when
brain size began to grow dramatically.
This is also about the time that the historical record suggests the
pre-humans began to cook their food.
Why is cooking food so
important? The process of cooking makes
food much more easily digestible and allows the body to absorb about 100% of
the caloric content. Raw food, on the other
hand, provides only about a third of the potential caloric content. Cooking also saved the time and caloric
output required to chew raw food sufficiently that it could be swallowed.
Cooking then could have
relieved pre-humans of the energy constraint and allowed them to develop larger
brains, assuming that conditions favored a larger brain.
“And
once the energy afforded by cooking turns a larger number of neurons from being
a liability into being an asset, it becomes easy to envision a rapidly
ascending spiral where larger numbers of neurons are selected for, given the
cognitive advantage conferred to those individuals who have them and who now
also have the time available to use them to hunt in groups, navigate the
environment, look for better homing, hunting, and gathering grounds, and care
for the wellbeing of their group, protecting it and passing on knowledge about
where to find food and shelter.”
The human brain itself is not exceptional in an
evolutionary sense—it is basically just another primate brain, albeit a large
one.
“Those pieces of science writing
which hail the human brain as a wonder often forget to mention that it never, ever starts life as a wonder, but rather
as a 300-gram mass that doesn’t do much yet—although it certainly holds great
promise.”
The human advantage then is not so much our physical
brain as the knowledge that we have accumulated and have learned to impart to
our brains.
Many thanks go to Suzana Herculano-Houzel for providing us
such interesting insight on a fascinating topic.
The interested reader might find it curious to learn that
the human brain has actually begun to decrease in size over recent millennia: Change in Human Brain Size, NaturalSelection, and Evolution
No comments:
Post a Comment