This is an excerpt from the book series Philosophy for Heroes: Act.
What evolutionary steps contributed to the development of the conscious experience and process of decision-making humans possess today?
Attention Attention is the brain’s process of limiting alternative thought patterns, then increasing the most dominant thought pattern’s strength. It is like a simple majority rule: the most successful thought pattern gets all the resources while other thought patterns are suppressed. While we can jump back and forth between different thoughts, we cannot have two dominant thought patterns at the same time.
Attention refers to the ability to select between competing or even contradicting sense data. For example, you hear something on your left, see something moving on your right, you are hungry, and tired; which sense data gets your attention first? The brain parts involved that help us make such a decision underwent half a billion years of evolution and can be traced back to simple multi-cellular organisms [Kaas, 2017, p. 547–554]. Figure 5.2 shows the evolutionary timeline of primates with different species branching off. As we are looking only at the evolution of attention, our focus will be a small selection of species rather than a comprehensive discussion.
Over this long history, the brain became a collection of different functions, layered on top of each other, with many systems having overlapping responsibilities. If a neural pathway helped one of your ancestors to avoid danger, that pathway survived, even if that meant that the architecture became somewhat chaotic (“complex”).
Nature does not care about an easy-to-understand architecture; it cares only about what works and what does not.
Sometimes, the organization of a certain function into clearly distinct brain parts had an evolutionary advantage (for example, separating the neocortex from the rest of the brain). In other cases, the most efficient layout was having one function directly beside the other (for example, the different brain regions within the necortex). Yet, for our understanding of the brain, it is sufficient to look at it as a system of separate parts interacting with each other. We just have to keep in mind that the functions of brain parts usually blend into those of neighboring brain parts, and that there are many more interactions and connections between brain parts than listed here.
Major structural components and properties of the brain include:
Allocortex The allocortex is part of the cerebral cortex (the neocortex is the other part) and consists of the olfactory system and the hippocampus.
Neocortex The neocortex is the newest part of the mammalian brain and consists of the cerebral hemispheres. Its main tasks are focus, language, long-term planning, and modelling of the world. It can generate strategies that involve detours if goal-directed behavior is not successful (for example, going around a fence instead of trying to get through it).
Cerebrum The cerebrum includes the neocortex (the cerebral hemispheres), and the allocortex (the hippocampus, the basal ganglia, and the olfactory bulb).
Cerebral cortex The cerebral cortex is the outer layer of the cerebrum. It contains most of the neurons of the brain.
Gyrus A gyrus is a fold or ridge in the cerebral cortex.
Sulcus A sulcus is a groove in the cerebral cortex.
Nerve Nets in Hydras
Looking at our tree of animal ancestors (see Figure 5.3) in regard to brain development, sponges were the first to settle into an evolutionary niche (more than 600 million years ago). They are sea animals that are mostly immobile and simply filter oxygen and nutrients from the ocean water. Although they have a primitive way of pushing water, when it is toxic or otherwise polluted, out of their bodies, they lack any form of nervous system as we understand it. Their cells communicate directly with each other using calcium signalling. Each cell contains a concentration of calcium that can be released if it receives calcium from neighboring cells. This way, it creates a calcium wave propagating throughout the organism. You can imagine it like having many square containers grouped together and filled to the brink with water. When you take one container and pour it into its neighboring containers, all the containers will overflow.
The first animals with some semblance of a brain were Hydras. They branched off our evolutionary tree more than 580 million years ago. They are small (around 10 millimeters in length) animals that usually attach themselves to the surface of an object in their environment and can slowly move over it or detach themselves and float in the water. They have a basic nervous system that allows them to use their tentacles to attack prey. If another animal (mostly tiny planktonic crustaceans like Daphnia or Cyclops up to five millimeters in length) touches a tentacle, the nerve cells activate the tentacle to take that animal into the Hydra’s mouth. There is no central nervous system that organizes this activation. Instead, nerve cells are spread throughout the body of the Hydra in a nerve net. This enables the Hydra to respond to its environment without being able to detect where this original stimulus came from. Any signal leads to the same reaction—for example, all muscles contract at the same time.
If the human body had a nerve net instead of a nervous system and brain, we would not be able to figure out where we were touched, only that we felt something and as a result had to come up with a general response to this touch. Such a general response is comparable to our hormonal system: for example, the adrenaline released in a situation of danger does not cause specific actions but prepares the whole body for a possible injury or energy exertion. Another example would be the regulation of body temperature which, again, is a general response to certain conditions instead of a specific movement.
Classification of Signals in Arthropods
A basic form of attention appeared at the time the arthropods (insects, spiders, crabs, etc.) split off the evolutionary tree around 550 million years ago. With this new form of attention, instead of treating all sensory input as equal, the information is pre-processed and can thus be amplified and classified. Imagine noticing something suddenly moving in the grass—it immediately draws your attention. Once you see it emerging from the grass, you classify it as a particular concept, for example, a snake.
At its core, classification is about filtering information we do not need. No longer would every signal cause a reaction. Instead, the organism was able to focus on specific signals and react to those. Most multi-layered nervous systems (including our own) support this kind of filtering. By comparing several images on your retina for changes, your visual system can make out which moving part belongs to which previously seen part. For example, if your visual system identifies a dog and then the same dog in subsequent images, you perceive any changes in those images as movements of the dog. If you closed your eyes every second, you would perceive the dog “jumping” from place to place. You would have to use your short-term memory as a workaround and remember where the dog was earlier to decide whether or not he had moved.
Visual pre-processing is done partly by the retina of our eyes, detecting edges and changes, and compressing the data-stream toward the rest of the visual system. To understand what is happening, imagine looking at a picture of a palm tree in front of a white background. The brain perceives the detailed raw image, then the visual system extracts the edges of objects to identify them (see Figure 5.5). This way, the brain can determine that there is the shape of a palm tree. This is the opposite of what happens when drawing a picture: we start out with the palm tree in mind, then draw the edges and contours and then finally fill them in with details.
Beyond being just a one-way street of information (classifying the image data to abstract information), classification systems can also help you to direct attention. When we see things that are new or unusual, our brain allocates resources to finding out what they are. This could play out by turning our head, refocusing our eyes, looking at things from a different perspective, going closer, or asking others about the new or unusual things.
The Olfactory System in Lancelets
The olfactory system was probably one of the earliest sense organs that evolved in animals, as detecting molecules is closely connected to a lifeform’s search for nutrients. While not directly related to us (they diverged from our ancestors around 535 million years ago), we share some of our olfactory-related genes with lancelets (see Figure 5.6). They can be seen as predecessors of fish with similar organs but in more primitive form. For example, their gill-slits are used for feeding but not for respiration. Likewise, their circulatory system transports nutrients but not oxygen. While they have no centralized olfactory system (we humans do), their olfactory receptors are studded along their flanks to detect possible sources of nutrition in their aquatic environment.
Molecules connect to the olfactory system over the peripheral olfactory system. In aquatic animals, this happens directly via contact with the water. In land animals with lungs, this happens by having the airborne molecules dissolve into mucus on top of the olfactory receptor cells. If the molecule binds to the receptor cell, a nerve signal is created and transmitted to the brain. A peculiarity of our sense of smell is that it is the only sense that can bypass the thalamus and send signals directly to the neocortex.
While our sense of smell might seem to play little to no role in our modern hectic life, it actually has a significant impact. In combination with our sweat, our sense of smell can communicate emotions. Usually, we think of emotions being contagious by way of our sense of sight or sense of hearing—we tend to laugh when we see or hear someone else laugh. But studies have shown that emotions are also contagious via our sense of smell. This works even when the smell is separate from the person (for example, on his clothes). So, even without words or gestures, people can communicate their distress to others nearby Mujica-Parodi et al. . The evolutionary advantage of this mechanism makes sense, especially when it comes to fear. Putting yourself into a heightened state of alertness when detecting fear in other people can increase your chances of survival.
The olfactory system also supports mate selection by detecting pheromones which contain the MHC complex. In Philosophy for Heroes: Continuum , we discussed the relevancy of the MHC complex in the immune system’s ability to differentiate self from other. In mate selection, a similar system is used to find a partner that is genetically not too similar but also not too different. The evolutionary advantage is to have a compatible partner with increased resistance to infectious diseases by providing a variability in the MHC complex [Ejsmond et al., 2014]. At the same time, it reduces the chance for children to inherit genetic diseases. Similar to the immune system, the olfactory system probably becomes accustomed to the MHC complex of relatives in early childhood. If this contact does not happen, there is no biochemical obstacle to falling in love with close relatives [Potts and Wakeland, 1993].
In terms of brain architecture, information travels not only from the olfactory system to the brain, but also in the opposite direction. If a particular faint smell wins the neural competition, resources are allocated to enhance our olfactory system’s sensitivity. This focus can improve the olfactory system’s efficiency by providing context information. In fact, the information from the millions of odor detectors in the olfactory system never even arrives at our neocortex. Instead, it is condensed into only 25 cells which are primed by the neocortex. If there is a strong smell, the sensitivity of the cells is reduced; if we want to pick up a faint smell, we can increase the sensitivity.
By combining the gustatory system (the basic tastes sensed by the tongue like salty, sour, bitter, umami, sweet, kokumi, calcium, and so on) with the smells detected by our nose, we can enhance our overall experience of food. Children learn to like or dislike certain types of food when observing what is safe for other people to eat [Elsaesser and Paysan, 2007]. While individual exceptions exist, if humans were genetically disposed to favor a particular food (like Koala bears prefer eucalyptus tree leaves) to the exclusion of other foods, our ancestors would have had a hard time spreading all over the globe.
If the olfactory system classifies something as inedible, it might initiate the gag reflex to protect the body from poisons. If we actually get food poisoning or an infection, the body reacts by increasing acetate levels in the blood. In the brain, this improves the ability to create memories. The evolutionary advantage of this pathway could be to better remember the situation that led to the food poisoning or infection and thus prevent it in the future.
All these properties are reflected in the architecture of the olfactory system. There are the following connections (Figure 5.7):
- Trigeminal nerve, vagus nerve (gagging reflex, face muscles, expression of disgust);
- Hippocampus (spatial memory);
- Amgydala, hypothalamus (emotional reaction, hormones, pheromone processing);
- Neocortex (processing of smells);
- Hippothalamus (pheromones, hormones);
- Olfactory bulb (sensory cells); and
- Nose (air flow).
The Optic Tectum in Fish
Controlling eye movements made it necessary for fish (they split from the evolutionary tree around 520 million years ago) to develop central processing, namely the optic tectum. In mammals, this organ is called the superior colliculus and most of the processing has moved to the visual cortex. It helps fish (and us) to track moving objects and is responsible for blinking as well as pupillary and head-turning reflexes. Relying on auditory information, the superior colliculus is also responsible for reflexively turning one’s eyes and head toward a sound source.
In the brain, the superior colliculus sits right behind the optic chiasm where the nerves from the left eye and right eye cross. If something or someone outside of your eyes’ focus moves, this part of the brain is responsible for bringing it to your attention. You might then turn your head and re-focus your eyes to get a better picture of the possible threat. Imagine you did not have this reflex to see anything moving in your environment. The risk of injury (say, from an oncoming tiger) would be much higher because of your longer reaction time.
Superior colliculus The superior colliculus or optic tectum (in non-mammals) helps the eyes to track objects, and controls blinking, pupillary, and head-turning reflexes.
The Thalamus in Fish
On the evolutionary timeline, the thalamus also first appears in fish. It combines and pre-processes different sources of sensory information into a coherent whole before relaying it to other parts of the brain:
- It combines the information from the left eye and right eye to build a three-dimensional representation of the environment.
- In humans, it translates the signals from the red, green, and blue cone cells in the retinas into colors. While further processing takes place in the neocortex, the first part responsible for this color encoding is the lateral geniculate nucleus (or LGN).
- Like the superior colliculus, the LGN also receives auditory information. The LGN changes the auditory information so that you perceive the sound as coming from a visual source. For example, when watching television, the LGN “moves” the perceived location of the source of sound from the speakers to the screen [McAlonan et al., 2006].
Thalamus The thalamus integrates different sensory information and relays the information to other brain parts. For example, it combines sense data from the retinas’ cones into colors, or calculates three-dimensional information from the two-dimensional images from both eyes.
Lateral geniculate nucleus The lateral geniculate nucleus (LGN) is part of the thalamus and relays information from the retinas (via the optic chiasm) to the visual cortex. It pre-processes some of the information, for example, combining red, green, and blue photoreceptor cells into colors.
The Basal Ganglia in Fish
For the evolutionary competition of neurons (that we have discussed in Philosophy for Heroes: Continuum ) to work, research points to an involvement of the basal ganglia [Redgrave et al., 1999]. The basal ganglia are thought to originate from the need to arbitrate between different courses of motor neuron activation. They identify the “best” among several possibly contradicting courses of action. Rules determine what “best” means in a particular context. For example, you can have the two competing thoughts (e.g., wanting to go left and to go right), but you cannot physically walk in two directions at once.
Basal ganglia The basal ganglia are a part of the brain that, like a referee, arbitrate decisions by the neural committees. Also, like an orchestra conductor, they coordinate the sequence of entire motor programs. In both cases, they do not make decisions but merely provide rules and structure.
Beyond selecting individual motor actions, the basal ganglia seem to be involved in cognitive thought patterns. While contradicting thoughts can exist, different thought patterns cannot recruit the same cognitive resources at the same time. For example, imagining a unicorn leads to thought patterns recruiting parts of your visual cortex. Adding more elements to the scene requires more and more resources until you can no longer focus on all elements at the same time. The basal ganglia are also involved in coordinating entire motor programs. Imagine an orchestra without a conductor: sure, the musicians could play their respective parts, but at different speeds and starting at different points in time. The basal ganglia act like a conductor of an orchestra, synchronizing the different motor programs, activating them in the right sequence and with the right timing.
The Amygdala in Fish
Yet another brain part, the amygdala, first appeared in fish. Managing our attention with the basal ganglia is one thing, how we prioritize the signal is another. While we can make decisions based on the strength of the signal—turning our head to the loudest noise seems to be a good strategy—we also need to put the signal into context. For example, instead of always running away from a tiger, we might consider whether or not to take the risk and first pick some berries and only then run away—especially if we are very hungry. This demonstrates how the amygdala uses information from a number of sources to prioritize different courses of action.
Amygdala The amygdala is the brain’s value and emotion center. It helps with evaluating thought patterns of the basal ganglia depending on the context instead of the mere strength of the signal. It also connects the brain with the hippothalamus, providing a bridge to the hormonal system.
With its connection to the hormonal system, the amygdala also initiates fight, flight, freeze, or fawn responses when in distress:
- Attack the predator (“fight” response);
- Run away from the predator (“flight” response);
- Remain still (“freeze” response); or
- Display submissive behavior (“fawn” response, by humans and other social animals).
How the fight and flight responses can help in a threatening situation is self-explanatory. The “freeze” response can trick predators because many predators’ instincts depend on motion. If their prey is not moving, the predator’s hunting instinct is not activated and they will look elsewhere for food. For example, cats take great interest in a moving toy while they might ignore something that remains still. Similarly, the “fawn” response works if the attacker is from the same species and also a social animal. Showing submissive behavior communicates to the other party that you are not a threat, preventing possible injury for both parties.
Beyond helping with the immediate response (e.g., releasing adrenaline), it can also serve as an early basic form of memory. For the response to be effective, the hormonal changes caused by, for example, the flight response need to remain long after a predator has vanished from an animal’s view. It will cause it either to head home to a safe place or to be on alert when it returns to this location.
The information the amygdala is using is limited to immediately available sensory data. It activates emotional reactions based on mapping the input from the thalamus to emotional behavior. For example, the sight of fresh berries might evoke a positive emotional response while the sound of a rival might evoke a fight, flight, or fawn response. This response takes priority over any rational evaluation of the situation because it is quicker and possibly stronger than signals coming from the neocortex. The amygdala associates sense data coming through the thalamus with positive or negative events and, ultimately, emotions, for example (Tye et al. , Rogan et al. , McKernan and Shinnick-Gallagher ): Tiger ⇒⇒ fear; apple ⇒⇒ appetite; and sun ⇒⇒ happiness. What makes this mechanism so powerful is that it requires very little processing power while it can cover a wide range of sensations. The major limitation of behavior based on the amygdala is the limited range of reactions and the reliance on immediately available sense data. The amygdala cannot take into account abstract thinking or planning, or complex relationships between objects, animals, or people.
The Hippocampus in Fish
A predecessor of our hippocampus also developed around the time of the first fishes. The actual hippocampus is unique to mammals but there are theories that similar structures evolved from a common ancestor of reptiles and mammals around 520 million years ago. Its main task is to create a mental map of an animal’s environment to allow the animal to remember where possible food and water sources are located. It also helps with navigation, remembering paths the animal has taken, relating spatially to other animals or objects, [Danjo et al., 2018] and recognizing places for orientation. A good example for the use of the hippocampus is squirrels burying nuts as food stashes for the winter. Our current understanding is that this map is not a literal map but instead consists of points of orientation. While many people can construct a mental image of a map, we tend to orient ourselves by seeing something we know and then putting our goal in relation to the landmark. For example, when describing to another person the path to a location, we might say “Walk down the street until you get to the large tower, then turn right.”
Hippocampus The brain’s hippocampus provides us with a mental map for navigation. It also builds temporal relationships between places, allowing us to determine, for example, which areas in our environment we have already foraged and in which areas the plants have regrown. The hippocampus and the olfactory system (sense of smell) make up the allocortex.
While earlier animals could drift and react to sensory inputs (evading predators and approaching food), once the food was out of sight, it was also out of mind. The hippocampus allowed animals to find more food by avoiding areas that they had already foraged and exploring areas they had not foraged. This requires mapping the environment based on odors (the sense of smell has direct connections to the hippocampus) and sights, as well as prioritizing those according to the time they should be visited [Murray et al., 2018]. This led to the evolution of the hippocampus to handle tasks in serial order with the right timing and in the right context. This stems from its ability to associate two memories with each other, which helps to find a path from one place to another [Samsonovich and Ascoli, 2005].
Did you know? The hippocampus’ function becomes most visible during dreams, when experiences retained during the day are played back for long-term memory backup in the neocortex. While we cannot ask animals whether or not they dream, some animals show rapid eye movements (REM) in their sleep, pointing to an activation of their hippocampus.
The Cerebellum in Sharks
More than 450 million years ago, sharks with a cerebellum emerged. This organ coordinates complex, time-critical behavior which includes movements, speech, and balance. When hunting, the shark might have had to outmaneuver its prey and then bite at the right moment. Similarly, its prey had to come up with movement strategies to navigate through the water to evade predators, locking predator and prey into an evolutionary race. Mammals face similar challenges of coordination when trying to jump from tree to tree, evade attacks by predators, or catch prey. Given that both the cerebellum and the basal ganglia are involved in coordinating motor programs, it is no surprise that they also form an integrated network to exchange information [Bostan and Strick, 2018].
Cerebellum The cerebellum is the brain part that helps with coordination of complex behavior. It provides a set of motor programs the brain can choose from repeatedly for similar actions (even in time-critical situations). With the help of the cerebellum we can, for example, walk or bicycle without having to consciously think of each movement.
In more general terms, the cerebellum is responsible for replaying movements. This becomes apparent when examining how the cerebellum learns new movements. Think about how each leg moves, as you did when you first learned to walk or to ride a bicycle: the programs to coordinate all your motor neurons were not yet transferred to your cerebellum (“learned”). Thus, they were not yet optimized and consequently, they were very slow. Until that optimization happened, walking or riding a bike had not become “second nature.” You had to take “baby steps” and focus on one step or motion at a time before making the next one.
In the (very rare) case of people born without a cerebellum, we see late walking development, reduced gait speed, unsteady gait, and a reduced ability to stand in darkness or when their eyes are closed [Yu et al., 2014]. This is explained by the fact that the cerebellum is connected to the inner ear, providing our sense of balance. In addition, people born without a cerebellum have late speech development, slurred and slowed speech, and a reduced control of pitch and loudness. This points to an additional role of the cerebellum in language fluency [Carta et al., 2019].
The Neocortex in Mammals
The neocortex, which is layered over the collicular control of attention (above the previously mentioned superior colliculus), developed more than 300 million years ago. Following the Permian-Triassic mass extinction event 252 million years ago (extinguishing 70% of land biodiversity), both the dinosaurs and mammals emerged. Given that dinosaurs still dominated the planet, mammals had to move into a niche and become nocturnal animals. According to the nocturnal bottleneck hypothesis, traits of growing fur, managing body temperature, and well-developed senses of smell, hearing, and touch helped our ancestors to stay active at night while evading predators during the day.
Nocturnal bottleneck hypothesis The nocturnal bottleneck hypothesis posits that many mammalian traits were adaptions to moving into a niche to become nocturnal animals and evade the dominant dinosaurs.
Functions of the neocortex include sensory perception, conceptualization, directed motor commands, spatial reasoning, communication, and long-term planning. The visual field is analyzed to create a mental representation of objects and their location. Instead of the raw sense data, this mental representation of the world is then used by the rest of the brain in, for example, coordination with motor control, language (reading this book), or face recognition. Similarly, other senses (hearing, touch, smell, etc.) are processed, and their signals are categorized and prioritized.