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