The Evolution of the Human Brain: An Introduction to Brain-Body Communications
by: mike morse, 2/7/2021
Understanding the biological workings of our brains as a basis of our behaviors was what originally drew me to the field of neuroscience. I’ve always been an observer- I tend to watch people from a distance, recognizing behavioral patterns, looking for connections between others. My last year of high school I took my first classes that really dove deep into the anatomy and physiology of the brain, specifically memorizing different brain regions and functions. For a while now, I’ve been interested in trying to understand the evolution of the human species through the lens of our brains. This concept seemed a bit overwhelming to tackle at first; however, after taking a big enough step back to look at the larger picture, I’ve realized that it’s not actually as complex a topic as I may have originally thought. Before I get deep into the brain details, I think it’s necessary to review some basic “tree of life” vocabulary and background, aka biological taxonomy, that most of us probably haven’t heard since elementary school.
Early identification of species dates back to early signs of communication between people; around 1500 BCE, Egyptian medical drawings of species were used to warn each other about poisonous plants and animals. Aristotle further differentiated species classification by noting differences in beings by their attributes (i.e. how many legs they had, if they had fur or feathers, etc). Classification of life remained mainly focused on the two groups of plants and animals until the Renaissance era, and early modern taxonomy began during the 1500’s; large works published at the time were mostly focused on further naming plant species. This was known as the “pre-Linnaean era”, or rather work done before the infamous Carl Linnaeus, a Swedish botanist (also known for his encouragement of State control in the economy) who many taxonomists consider the father of modern taxonomy. Today, modern species naming uses the following breakdown:
(note: the “Domain” level is now beneath an additional “Empire” level)
To understand how the human brain developed from our more recent relatives, I find it important to review a few of the first steps in the tree of life here. In modern classifications, we have two empires of life: prokaryotes (organisms without an enclosed nucleus, i.e. bacteria), and eukaryotes (organisms with cells containing an enclosed nucleus, i.e. plants, animals, etc). Within the eukaryotic empire, we have five kingdoms: protozoa, chromista, plantae, fungi, and animalia. These kingdoms all differentiated in their cellular structure, specifically related to what energy they take in and how they process that energy. For example, plants can photosynthesize sunlight through structures unique to plant cells that animal cells lack. Within the animalia kingdom, we have nine phylums: four land-living and four water-living invertebrates, and chordata (vertebrates, including mammals and humans), from which the human lineage differentiates due to advanced bone structures. Within the chordata phylum, we see five classes: fish, amphibians, reptiles, birds, and mammals; the mammalian evolutionary specialization occurred through self-regulated body temperature and oxygen and the method of reproduction. When following the mammalian line down to primates (and eventually humans), we see specifically that the sheer number of cells in the brain increases as we approach the modern human; however, the cause of this increased cell count is not thoroughly understood. Most mammals do not have as many brain cells as humans, and they also lack as complicated of a nervous system.
Now that we have explored the evolution that led up to the differentiation of the mammalian nervous system, I want to briefly go over the actual development of the brain within an individual growing human. In embryonic development, the first sign of neural development occurs when a neural plate, an outer tissue layer in the head area, forms and eventually folds in on itself to make three vesicles, or air pockets that fill with fluid, which together compose the neural tube (see below).
The three primary vesicles of the neural tube later develop into the three main brain areas: the forebrain (or cerebrum), the midbrain (including the thalamus and hypothalamus), and the hindbrain (aka old brain, containing the cerebellum). As I discussed in my most recent thought piece, the cerebral cortex is often associated with ‘higher level’ processing, or executive functioning, a term preferred by many psychologists. Some primates and other species (such as whales) have been found to have a developed cerebral cortex as well; however, it takes up a slightly smaller percentage of their total brain compared to humans. The midbrain contains areas associated with emotional regulation and processing, as well as the thalamus, which many neuroscientists believe to be the sensory processing center of the brain. Finally, the hindbrain contains the cerebellum as well as the transitional area between the spinal cord and the base of the brain, or brainstem. This area received the name ‘old brain’ as it is associated with our most basic ‘animalistic’ (or innate) regulatory processes, and most other vertebrate species also have variations of these hindbrain areas. As these three vesicles develop within the embryo, the neural cells eventually specialize and migrate throughout the brain and body to build our giant, interconnected nervous system.
As people age and develop, their bodies continue to create new nerve cells, a process called neurogenesis; in fact, the average adult has 100 billion brain cells. Scientists have even discovered that brain cells are continually regrown throughout our lives, even as adults, unlike some other types of cells. Our closest primate relatives, gorillas and chimpanzees, have about a third of the number of brain cells as we do, and the ratio between body size and number of neurons is astronomically higher for humans than any other species (the only species recognized as having more brain cells than humans is elephants, but we must also think of the overall size of an elephant compared to people). However, because the evolution of humans took place over so many millions of years, and we are limited by fossil evidence to make inferences based on skulls rather than live tissue, scientists can only theorize as to what caused humans to have such rapid brain cell growth. This phenomenon is particularly interesting as the change in size of the human body as a whole has proportionally grown significantly slower than that of the brain. One neuroscientist, Suzana Herculano-Houzel, explains her theory of cooked food being the cause of this exponential brain cell growth. In her Ted Talk, she theorizes that when early humans first learned to cook food over fire, they began the process of speeding up energy consumption and distribution. She suggests that when these people’s bodies didn’t need as much time to process raw food, the excess energy reserves allowed for expedited brain cell growth instead, and gave them time to engage in activities other than resting and digesting. This theory makes sense to me as we think about the mass early human migrations that occurred millions of years ago, since people needed the time to move, manage survival, and communicate; this also allows for a theory as to the rise of human socialization.
Now that we have such highly evolved and specialized brains, neuroresearchers have spent significant time looking into the ways in which our body communicates with itself. In fact, due to the numerous cells and connections within the brain, scientists have come up with the term ‘connectome’ (derived from ‘genome’) to attempt to map the connections of the brain and nervous systems. Nerve cells communicate with each other by transmitting electrical signals known as ‘action potentials’, neuroelectrical signals that are stimulated by a sensory input and transmit information through molecular movement within the cells. Action potentials work using an ‘all-or-nothing’ approach, meaning that there is no partial nerve cell activation- either a sensory input initiates a response or it doesn’t. When an action potential fires down a nerve cell, it results in the release of electrochemical molecules known as ‘neurotransmitters’, which communicate with other cells to either induce or inhibit a reaction. It is important to note that nerve cells connect directly with blood vessels and muscles, which is how these body systems receive processed information from our brain.
Neurotransmitters were first discovered in the early 20th century when Spanish scientist Santiago Ramón y Cajal speculated that the previously understood electrical connections in our brain also had a chemical component. Currently, neuroscientists recognize over 50 different neurotransmitters, all different types of molecules that relay unique information throughout our body. I believe that we have eight neurotransmitters that everyone should recognize and understand: serotonin, dopamine, acetylcholine, GABA, norepinephrine (aka noradrenaline), epinephrine (aka adrenaline), glutamate, and histamine. The chart below shows these essential neurotransmitters, although it substitutes histamine (related to regulating our allergies, inflammation, and immune system) with endorphins (which are technically hormones instead of neurotransmitters; however, they function similarly and are also important to our overall well-being).
These tiny, electrochemical molecules are released into the synapse, or the space between two nerve cells, to either stimulate or inhibit action potentials from initiating in the next cell. Interestingly enough, if we look at when and where these different neurotransmitters are synthesized, we can learn interesting information about how we as humans function. For example, while many neurotransmitters exist in the brain (ready to be released into synapses), 90% of serotonin is synthesized in our intestines, suggesting that our food intake may play a significant role in serotonin creation, though not necessarily in its distribution and activation throughout our body. Different neurotransmitters exist in all neurons in our body, whether to communicate to another nerve cell, muscle cell, blood vessel, or something else. Understanding neurotransmitters as the primary unconscious form of communication within our bodies may open a door to understanding the basis of biological problems better, as issues with our body’s communication mechanisms, rather than as innate, unmodifiable physical conditions.
Additionally, many psychologists and neuroscientists recognize that human brains have high levels of plasticity, meaning intercellular connections can break and rebuild throughout our lifetimes. While there are many incorrect assumptions that once our brain cells die they can never be reborn (think sports injuries for example), this is not the full truth. In developing, young brains, neuroplasticity tends to initiate changes more easily; however, our brains can still learn and reorganize throughout our entire lives. One example of how brain plasticity and rewiring works can be seen in people who have limited or weakened senses (for example people who are deaf or blind). Deafness and blindness are not known to have a singular cause, and cannot be viewed as a completely disabling state. In fact, many people who experience deafness or blindness find that their bodies are able to adapt and they can strengthen other senses instead. This process of our brain cells rewiring and rebuilding is not a conscious process; however, it is initiated through learning and repetitive practice (which can include mindful training).
When we make any attempts to comprehend the ‘human condition’, as philosophers and common people alike want to understand, I find it irresponsible to ignore the biological and evolutionary background that connects humans with all other life forms. By accepting the truth that all life is connected in one way or another, we can open ourselves up to learning and understanding more about our own selves while feeling connected to the world around us. Forgetting our roots in the interconnected web of all living things has led us down paths of selfishness and destruction; the collective goals of sufficiency and equity have become clouded with individualist ideals of power and control. I want to challenge the idea that any one life is more worthy than another, and further encourage active, collective action towards undoing harms of the past. Overall, I believe it is important to understand the complexity of our brains, its communication systems, and its abilities as a basis of tackling bigger psychological and social structures that impact both our personal and societal barriers to mental health liberation.
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