The Evolution of the Brain

Our brains are fascinating, complex organs. At any given moment, neurons are sending electrical signals back and forth to help you think, breathe, move, and contemplate your own consciousness. So let’s take a moment and think about why we have a brain. Where did they come from? Who had the first brain? Let’s answer all of these questions (as best we can) as we explore the evolution of the brain. 

It’s important to remember, as you continue to learn about the brain, that not all answers may be definite. The ways in which we can trace the earliest brains back to the source include archeology and the unearthing of fossils. Fossils can show us bone structures and other parts of the body that suggest a brain but are not as great at preserving brain tissue. 

When Were Brains First Developed? 

Through ever-advancing technological means, including the study of fossils and embryos, the best guess we have for the earliest brains can be traced back to over 500 million years ago. Since then, brains have evolved very differently among different species.

For right now, we are going to cover a few steps on the evolutionary train, including:

  • Nerve nets
  • The photoreptilian brain
  • The photomammalian brain
  • Primate and early hominid brains 

Nerve Nets

The brain sits at the top of our nervous systems. As neuroscientists looked for the first brains, they looked for early nervous systems. These systems came in the form of “nerve nets,” and they first appeared in jellyfish, or Cnidaria. Jellyfish first appeared on the Earth over 580 million years ago. 

The nerve net of a jellyfish does not allow them to see the beautiful fish swimming around them or ponder on what might be above the surface. Their nerve nets simply take in, and respond to, touch. Did you know that we don’t need our brains to respond to touch? The reflex that we experience when a doctor takes a small mallet to our knee takes place without the brain. 

Nerve nets have worked out just fine for jellyfish – they have been able to survive as a species for a very long time! But from jellyfish came other species, and from those species came other species. All these species developed their own nerve nets, and later, something close to brains. 

Why did this happen? Some believe it’s because simply being able to respond to touch was not enough for some species. To survive, some animals needed to seek out prey. This required the evolution of muscles and other nerve cells that were more advanced than what the jellyfish could do. 

Another development in the earliest central nervous systems took place when animals with bilateral symmetry started to show up on the Earth. Bilateral symmetry is the presence of two alike sides: the left and the right. Humans are symmetrical in that way, while jellyfish, sponges, and earlier creatures are not. 

Animals with bilateral symmetry often have a central structure at the top of their bodies: a head. Humans, birds, and even flatworms have this area. For the earliest flatworm, this was the area where the earliest central nervous system was headquartered. (Flatworms likely appeared on the earth over 550 million years ago.) Some experts do not refer to the center of their central nervous system as a “brain” but merely a collection of ganglia cells that transport messages throughout nerve cells in the body. Either way, their central nervous systems are a long way away from what humans or even vertebrates have now. 

The Protoreptilian Brain 

Brains today are more than just a bundle of ganglia. We do, however, have a section of the brain called the basal ganglia. In humans, the basal ganglia play a role in motor control, eye movements, emotion, and learning. This structure is not as complex as other vertebrates. The “basal ganglia” only controls basic motor movements or maintains homeostasis in many species. 

For some, the basal ganglia is basically the whole brain! The reptilian complex, or the lizard brain, is simply a less evolved version of the basal ganglia. Some believe that this brain was first present in fish over 500 million years ago.

The Photomammalian Brain

Around 250 million years later, another part of the brain is said to have been formed. This version of the brain, known as the paleomammalian brain, includes the limbic system. If you study neuroscience, you may know that the limbic system has multiple parts: the hippocampus, amygdala, and hypothalamus. 

The limbic system handles emotions. And yes, that means that mammals experience emotions. The paleomammalian brain is the first instance of animals reacting to situations with good or not-so-good feelings. In some species, like humans, this part of the brain also influences the decisions that we make. This could be why it’s easy to train a cat or a dog through conditioning. The positive experiences that they associate with treats or praise register in their limbic system. Just like humans, their decisions are then made to invoke those good feelings. 

Another interesting thing about the limbic system in humans is that it develops before other, more “rational” parts of the brain develop. Always wondered why teenagers are so moody and impulsive? You can thank their limbic system and underdeveloped frontal cortex. 

Primate and Early Hominid Brains

The cerebrum, or the largest part of the brain, has undergone many changes since it first appeared in animals 200 years ago. And not all of these changes have applied to all species! Different brains have evolved to fit the needs of different species as they have survived through millions of years of existence. 

But let’s fast forward to primates. Primates, which first appeared around 55 million years ago, had distinct brains because their brains were big. They had more neurons in their heads that could take in sights, smells, and touch, and make decisions about that sensory information. This made them pretty advanced in terms of the vertebrates and other species that came before them.

We share a lot of similarities with primates and their brains, but other distinctions make us the powerful, rational, and chatty creatures that we are. One of these distinctions is that while primates have big brains, we have more voluminous brains. (Humans have brains three times as large as primates.) Neurons are packed more densely in human brains than in primate brains, giving us more ability to think, perceive, and store memories. This is possible because of gyrification. Gyrification is the process of forming the folds that show up in our brains, primarily the cerebral cortex at the front of the brain. 

The folds in our brain allow for more neurons to pack in and have paved the way for different sections of the brain. Many experts believe that without gyrification, we would not have been able to develop areas like Broca’s area or Wernicke’s area, which are crucial for speech processing. Human brains also use these specialized areas to communicate information with each other. We still have a limbic system and basal ganglia like more primitive areas. But due to communication with more advanced areas of the brain, we can do a lot more with the information these areas take in. 

Why are humans so advanced? Well, we can thank evolution. We can also thank a series of evolutionary steps that made our brains not the biggest in the animal kingdom, but one that is densely packed and able to perform a lot of functions.

Theodore T.

Theodore is a professional psychology educator with over 10 years of experience creating educational content on the internet. PracticalPsychology started as a helpful collection of psychological articles to help other students, which has expanded to a Youtube channel with over 2,000,000 subscribers and an online website with 500+ posts.