Updated: Mar 7, 2021
Here's how neuroscientist Suzana Herculano-Houzel and her study of primates and rodents changed the world of human intelligence.
At the hands of one gene, the birth of humankind came to exist.
Such a revelation was cultivated by Brazilian neuroscientist, Suzana Herculano-Houzel, who conducted experiments that revolutionized the way researchers understood organismal brains.
The postulation of nerve cell quantities correlating with one's intelligence remained certain for many scientists in the past.
But how did they come to this conclusion?
For years, many believed that one's brain size was proportionate to their body mass. This relationship was represented by a mathematical equation called allometric scaling. It enabled scientists to predict an organism's brain size if given its bodily information such as mass.
This equation fascinated Houzel, and though she did not begin researching to undermine it, she did end up doing so unintentionally. Houzel wanted to understand just how many nerve cells were in a chosen organism's brain and how accurate allometric scaling was for representing them.
She began her research by brainstorming an innovative method for accomplishing such a goal. The journey was difficult and tedious since the strategy used at the time involved slicing portions of the brain and individually counting the cells one by one. However, this would not cut it for Houzel, and so, she sought out her own mechanism.
To put the brain cells into one accessible pool, Houzel froze the brain in liquid nitrogen and blended it into a slushie-like liquid. Now, all of the cells were available for her observations. To identify how many cells there were, Houzel utilized the principles of cell structure. She mixed them in a DNA-binding dye that made each nucleus blue and then began counting them.
Wanting to locate only the nerve cells within the brains she was studying, Houzel incorporated yet another dye for nerve cell protein-binding that identified only nerve cell nuclei within her samples.
After countless hours of quantitative observation, the results were astonishing. Using such techniques, enabled Houzel to count the nerve cells of varying organisms and compare their data.
What she found was even more fascinating.
For many years, scientists believed the relative brain size of any mammal was the product of its body mass raised to two-thirds. After Houzel's experiment, however, they discovered that such an equation was not the most accurate.
To validate her new revelation, Houzel moved onto larger mammals: primates. She was able to get hold of 6 primate brains and count their nerve cells. Again, Houzel made an intriguing discovery.
Primate's nerve cells were relative to their body masses.
Essentially this meant that one monkey twice the size of another would have approximately twice the amount of nerve cells. As primates and humans evolved, it was no question that their brains became bigger. The most puzzling thing that came out of this observation was that a bigger brain did not always mean a smarter organism.
Houzel noted that even though a cow has a brain 100 times larger than a rat, it did not seem that much smarter than one.
So, why was this the case? The bigger the brain, the more nerve cells, right? And the more nerve cells there are, the more cells an organism's brain has to work with, making it ultimately smarter and more efficient, right?
Now, the second part of Houzel's research begins. How were primates and humans so much more intelligent than other large mammals that have equally sized brains?
The answer is white matter.
What is white matter? White matter refers to the axons that serve as the chemical-signaling pathways between nerve cells in the brain. The farther apart the nerve cells are, the thicker and more visible these axons become to send signals out quickly, despite the distance. As these pathways increase in visibility, their fat-coated exteriors exhibit a white-ish appearance, giving them their name white matter.
In Houzel's studies, she found that rodents and primates used the space between their nerve cells very differently from each other. In the brain of a large rodent called agouti, Houzel found that it had about 77 times as much white matter than a mouse's brain, even though its brain's size was only eight times larger. That meant that as a rodent's brain largened, its axons became substantially thicker to accommodate the longer distance between its spaced-out nerve cells. Consequently, this left less space for the actual nerve cells in charge of interpreting the chemical signals.
On the other hand, primate brains have a much more efficient method of utilizing their brain space that arguably contributes to their heightened intelligence. Over time, primates have evolved to categorize their nerve cells for particular functions. The most important ones that need to relay urgent information, such as the ones involved in recognizing a predator, are the only cells that develop enlarged axons. Cells receiving less important messages maintain smaller signaling pathways, and thus, create more space for additional nerve cells to develop.
As primates gained the ability to compartmentalize their neurons, their brains dedicated other cells to more comprehensive functions such as linguistic abilities. This allowed them to advance mentally and pave the way for future human intelligence.
Eventually, these compartments gained their scientific term: cortical areas.
Though it is evident that all mammals have these specialized sections within their brains, it is important to keep in mind the inefficient use of space that rodent brains continue to encompass. Despite capybaras being the largest known rodents and having equally large brains to match, it is important to note that their brains have only 40 cortical areas compared to humans' 360. This substantial increase in compartments is what allows primates and humans alike to develop such a broad range of capabilities to engender their collective intelligence.
The gene responsible for such a life-changing process remains unknown as researchers continue to explore it in the world of human evolution.
But ahead of all of this, the journey began with Suzana Herculano-Houzel.
A scientist who was willing to take the first step into the unknown for the discovery of humankind and beyond.