All About the Brain (Part 2)

In Part 1 of the series, I discussed some of the basic structures and functions of the brain. Obviously there was much in that article that I did not cover, but for this next article I’d like to discuss how the brain might have developed through evolutionary processes. I’m going to use some of the things we learned in Part 1 to make my argument here. But let’s first talk just a little bit about evolution in general.

The Evolutionary Apex

Survival of the fittestOne of the most common phrases used to describe evolution is this: “survival of the fittest.” Sometimes this is construed in a way which suggests that it is always the biggest, meanest, most ruthless animals who survive, and the weak ones that die off. But this is a fairly limited idea of evolution. “Fittest” does not always mean “most ferocious.” In very real terms, evolution functions based on reproductive success: The more likely an animal is to create more offspring, the more “fit” they are. But that can be expanded to include other traits that help an animal to survive (since, after all, survival is a prerequisite to reproduction). And organisms have different ways of surviving. While a toad might develop bright colours to signify to predators that it is poisonous, some birds develop extravagant feathers (think of the peacock) or complex behaviours (the bowerbird being one example) to attract mates. While some organisms, like eagles, live generally solitary lives, others live in complex societal arrangements (from chimpanzees to the lowly ant). All this goes to show that evolution can select for different traits in different organisms, so there is no “one method” to achieving fitness.

Animal brains

Why does this matter? Well, it’s important to keep in mind that brains require enormous amounts of energy to operate. And the more complex the brain, the more energy it takes. So, it may not be efficient or useful for all animals to develop the most complex brain possible. If an earthworm can get by with a few nerves, mostly controlling movement, then they probably have no need for brains that can solve calculus equations. But other animals may need more complex brains to deal with more complex behaviours, environments, or social structures. So the first thing that is important to know is that there is no “best” brain. The evolutionary development of the brain has not reached a “pinnacle” in the brain of humans. Certainly ours is the most complex of any species, but such a brain is only beneficial because it is housed in a creature that can actually use it. The brain we have is only “best” for us, but it is not a goal of evolution to produce the most complex structure in every circumstance. Please keep this in mind as we explore how evolution has shaped brains over time.

Different Strokes for Different Folks

Sea spongeLet’s take a look at the wide variety of brains that are out there by walking through the evolutionary timeline and seeing the development of the brain. We’ve already looked a little bit at human brains, of course. But are there organisms that don’t have brains at all? Well of course, there are bacteria and fungi and plants. But there are animals that don’t have brains, either. For instance, sponges (yes, they’re animals) do not have any nervous system whatsoever. They do have some capacity for movement, but this seems to be done through chemical signals and (in some types of sponges) electrical impulses. Sponges don’t have any specialized neurons that control movement. Later in time came jellyfish, who have no central nervous system (like a brain or spinal cord), but have a “nerve net” that allows for movement and rudimentary detection of stimuli. This detection will tell a jellyfish that something has touched it, but doesn’t tell them where the thing is or where it was touched.

Earthworm nervous system

Earthworm nervous system

After jellyfish, we move on to molluscs and worms, in which we start to see the precursors of a true “brain”. These organisms have nerves that help them to move, but they have clusters of neurons located in the head region that can coordinate more complex movement. It is in these clusters of neurons that we find the beginnings of the brain. At the far end of the mollusc spectrum are the cephalopods (like octopuses and squid), which are generally regarded as the most intelligent of invertebrates. The octopus, for example, has a fairly complex nervous system, and shows evidence of memory and problem-solving skills, but even still, two-thirds of its neurons are found in its arms.

As the mobility of organisms increased, so too did the need for more complex brains to handle more complex functions. Brains needed to cope with the increasing demands of a more varied environment. After all, though a sponge may spend its whole life in a small area, flying insects can travel through different habitats and climates even in their short lifetimes. In addition to the need to deal with change, mobile organisms also need to have better senses to interpret the world around them. Eyes and/or antennae, for example, become a necessity. So, with insects and crustaceans, brains become more complex. Different parts are needed to control all those legs, and perhaps the compound eyes, the wings, and so on. Then, of course, vertebrates (including fish, amphibians, reptiles, birds, and mammals) arrived on the scene, and the demands grew even more. Predator-prey relationships in particular tend to create a need for more complex cognitive capabilities. After all, learning where the lions come from and what to listen for to know if they are around is important for survival. From vertebrates onward, the general pattern is towards increasing complexity, organization, and specialization of the nervous system. Different functions are needed. For instance, the squirrel has developed amazing spatial memory to remember where it has hidden its acorns. Also, the addition of social structures adds immense complexity to the situation. Animals who live in herds have dominance behaviours, hierarchy, and needs for communication. The process has its twists and turns, with some animals developing a solitary lifestyle (like tigers) and others living in social hierarchies (like lions). But the general trend is clear: As the lives of animals became more complex, they needed more complex hardware to support them. And eventually, very late in the game, humans arrived. We’ll talk a little more about them soon.

Feedback loopI’ve framed this very brief summary in two ways that are likely a bit misleading. First off, I’ve framed it in terms of necessity: “Since animals did this function, they needed more complex brains to handle it.” But please keep in mind that this is a continual feedback process throughout thousands and millions of years. The animals who ventured out into new territory needed bigger brains to handle it, but the animals with the bigger brains could venture out and be successful. The environmental needs shaped the development of the brain, but the development of the brain also shaped the possibilities for the animals as well. Over thousands of generations, the path of any given species is filled with twists and turns. But the important thing to remember is that the functions shaped the development, and the development shaped the functions.

The second possibly misleading thing I’ve done is frame it in terms of an upward process. Like I mentioned earlier, there is no overall “best” brain. What I’ve been doing is sketching out the evolutionary timeline (very briefly, of course) and showing the general trend in brain development. But each species has its own unique history, and not all of them have led to greater brain complexity. Looking at it in broad strokes, however, the general trend is that increased complexity of the organism (whether in physical capabilities, social structure, or environmental adaptability) leads to increased complexity of the organism’s brain. And through the evolutionary timeline, we can see a clear development of the brain from none at all, to simple nerve nets, to localized nerve structures, to simple brains, to more complex brains. For those who examine it, the development of the brain is laid out in the grand scope of the evolutionary timeline.

The Continuity of Learning

Now that we’ve sketched out in broad terms how the physical structure of the brain evolved, how do we understand how cognitive processes evolved? Such a topic is clearly a complex one, but there are at least two fundamental processes which have run like thin threads throughout the long, winding history of the brain. These processes, identified by psychologists, are known as classical conditioning and operant conditioning. They are two different forms of learning that have been around for millenia, in even very simple organisms.1 You may recall that I have already outlined a form of “learning” at the neural level: long-term potentiation. In fact, this is related to these forms of learning, and it helps to make classical and operant conditioning possible. I can’t get into the details about how it does so, but hopefully an outline of these two forms of learning will be illustrative of how cognitive processes from even simple organisms can be scaled up in size and complexity to get to the amazing functions we see in the human brain today.

Classical Conditioning

One of the iconic experiments in psychology was done by Ivan Pavlov. You may be familiar with the basics of it already: Pavlov, in the process of studying the digestive system of dogs, found that after a while, the dogs began salivating not only to the presence of food, but also to the lab technician that fed them. Before the food even arrived, the dogs had made a connection between the technician and the upcoming food. As a result of this finding, Pavlov started studying it directly. He paired the ringing of a bell (among many other things) with the arrival of food, and found that over time, the dogs would salivate merely at the ringing of the bell.

Now, this might not sound very amazing to you. However, what is surprising is the impressive generalizability of the result. Pavlov tried different types of stimuli: electric shocks, ringing bells, visual cues. Each one produced similar results. After Pavlov’s work was published, other researchers tried with different stimuli, different items (instead of food), and different animals, and found the same thing. And in addition, the effects could be replicated with people as well! So what is going on here that produced such an impressively generalizable effect?

Pavlov's dog

Well, the process is actually quite simple. You need a stimulus that already produces a response in the organism (reflexes work really well). This stimulus is known as the unconditioned stimulus (UCS), and the response is known as the unconditioned response (UCR). In the example with Pavlov’s dogs, the UCS was the food, and the UCR was the salivation response to the food. Now, you need another stimulus that is initially neutral, like the ringing of a bell. When you repeatedly pair this neutral stimulus with the UCS, it can turn into the conditioned stimulus (CS) which leads to the conditioned response (CR), salivation at the sound of the bell. That’s all there is to it. You ring the bell and present the food at the same time, and after a little while, you can get the dog to salivate just at the sound of the bell.

This simple learning procedure is present in nearly every organism you can think of, and it works in the exact same way. Now, sure, the UCS and UCR are going to be different for different animals (it’s difficult to get a slug to salivate since it doesn’t have any salivary glands). But if you can find an organism that is capable of sensing its environment and reacting in some way, chances are you can classically condition it. It is a basic cognitive process that has been with us for millenia.

Operant Conditioning

The second form of learning is called operant (or behavioural) conditioning. This process was studied by people like Edward Thorndike and B.F. Skinner, the latter of whom is known for his work on pigeons and rats. It can also be found in very simple organisms, but it does take a little bit more complex nervous system in order to operate.

Operant conditioning ratThe structure of operant conditioning is also quite simple to understand. To illustrate it, let’s imagine a rat in a special box. This box has a food dispenser and a lever. When pressed, the lever will dispense food. At first, when placed in the box, the rat will wander around randomly. However, it may happen to bump into the lever and release some food. Being a rat that is not accustomed to lever-pushing and food dispensing, however, it may not pick up right away that the cause (lever pushing) and effect (food) are connected. However, let’s say it bumps into the lever again, and more food comes out. Eventually the rat will learn that the two are connected, and this fact will be very clear to any observer: The rat will start pressing the lever over and over again to get more food!

Operant conditioningExperiments like these helped researchers discover more about this process. Fundamentally, it relies on reinforcement and punishments. That’s it. A reinforcer includes anything that increases the desired behaviour—it could be the reward of food, or it might be the removal of a negative stimulus like an electric shock. A punishment is anything that decreases the behaviour—which could be the start of a shock, or the removal of food. By using these reinforcers and punishers, we can get animals to string together sets of complex behaviours. For instance, Skinner was able to teach pigeons to do dances, identify shapes, and even play ping pong!

But what is the difference between classical and operant conditioning? Well, the major difference is that classical conditioning works only on pairing new stimuli to old behaviours. Operant conditioning, on the other hand, actually works to modify the behaviour to increase or decrease its frequency. Classical conditioning is passive association between events, but operant conditioning requires an active trial-and-error process on the part of the animal. However, they both work together to help organisms learn. These processes are even fundamental to the learning process of human beings as well (though we are better able to control them consciously and can also learn vicariously through observation of other people).2 In short, with these cognitive processes we see a very clear connection between simple organisms all the way up to extremely complex ones. The behaviours manifest in different ways, but the underlying processes are the same. It is clear that these learning processes have real evolutionary advantages, and so they have stayed with us to the present day.

The Human Advantage

So now that we have seen that humans share core cognitive functions with organisms at least as simple as sea slugs, what makes us different? Why have we developed planes and trains and automobiles, and other animals have not?3 What makes us different from our closest ancestors, the great apes? Well, I would like to argue that the differences between us and them are mostly quantitative in nature rather than real, qualitative differences. It is generally a matter of increased complexity in the processes that great apes already have.

Brain EvolutionRemember in Part 1 when I discussed the prefrontal cortex of the brain? I mentioned that this area is responsible for planning complex behaviours, decision-making and goal-setting. These are all components which make up the sense of “conscious control” that we have over our actions. Now, if you take a look at the graph to the right (click on it to see it in more detail), you can see that the brain size of humans (and human ancestors) has grown sharply—very sharply! The total capacity of it has tripled in size.4 Moreover, if you look at the shape of the brains, the biggest increase in size has occurred at the front of the brain, in the prefrontal cortex. An increase  in the size of the hardware allows for a dramatic increase in the software that we can run. So while apes can plan behaviours, make decisions, think creatively, control their bodily processes, and so on, we can do these to an increased degree. As one example, there are humans that have gone on hunger strikes in the name of an idea of some sort. In other words, they’ve been able to ignore their bodily urges and maintain a goal.5 That, right there, is the human advantage. But it is only a matter of degree, not a complete difference in the kind of things we can do.

I intend to discuss some of the more “human” traits in Part 3 of the series. I’ll be talking about consciousness, free will, and the mind/brain distinction. But this article has shown a clear development in the nervous system from simple to complex organisms, and a clear continuity in cognitive processes from the distant evolutionary past to the present day. Such a development shows that brains did not just appear out of nowhere. They came from precursors—undifferentiated cells that can send electrical impulses, to simple neurons, to more coordinated nerve systems, to concentrated clusters of nerves, to true brains, to more complex brains, and up and up until the amazing brain that we as human beings have. As the complexity of the physical system grows, so does the ability for more complex behaviour, and vice versa. The brain is clearly a product of slow development over time, and this development fits in perfectly with the independent evidence that has helped us to piece together our evolutionary lineage.


  1. For example, experiments on these types of learning have been done on sea slugs. There may have been successful experiments on even simpler organisms of which I’m not aware, but the studies on these slugs at least demonstrate how basic these forms of learning are to most forms of life. []
  2. B.F. Skinner and the school of behaviourism were convinced that animals, including humans, were nothing more than input-output processors. They were convinced that subjective thoughts and feelings did not matter, and that only stimulus-response behaviour modification was important. This view has since fallen out of favour, in recognition that cognitive states do actually make a difference on behaviour. So when I say that these processes are fundamental to human beings, I don’t mean to imply that they are the only fundamental processes. I simply mean that they are processes which play a large role in learning for virtually all organisms, including humans. []
  3. Well, the first reason is that sea slugs don’t have any arms! []
  4. Part of this is due to an increase in the size of the skull, but most of the change in capacity results from an increase in surface area by adding more ridges and grooves. []
  5. For a better understanding of how ideas can influence the behaviour of humans, Your Brain Is Almost Perfect by Read Montague ( offers an excellent introduction. []

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