INSIDE THE BRAIN – part 2: From Evolution to the Neurone
- Marcela Emilia Silva do Valle Pereira Ma Emilia
- há 18 horas
- 13 min de leitura
🧠 From Evolution to the Neurone
After understanding brain anatomy, its lobes, systems and structural divisions, the question that naturally arises is: how did this architecture become functionally capable of sustaining life?
The answer begins long before thought, memory or emotion.
It begins in the very evolutionary history of the nervous system.
The structure of the Central Nervous System, which sustains life, goes beyond the brain and extends through the spinal cord, protected by the vertebral column.
It is this anatomical continuity that allows the body not only to sustain bipedal posture, but also to ensure that information generated in the brain is sent to the other parts of the body and that signals coming from the body continuously return to the nervous system.
✨ It is precisely this bidirectional communication that makes possible the integration between movement, perception, visceral regulation and physiological balance.
All of this harmonisation necessary to maintain the homeostasis of the human body depends on a capacity acquired throughout evolution: a brain that is relatively larger and more complex than expected for body size, when compared to other species.
This process is known as encephalisation — a fundamental evolutionary step that contributed to making the human being a species with a high capacity for adaptation, learning, planning and behavioural flexibility.
But all of this work inside the brain is also a biologically demanding process.
It demands enormous energy consumption and generates continuous electrical activity from small cellular units in the billions: the neurones.
🧠 They are the true protagonists of this organ with a gelatinous texture.
It is the neurones that make the brain and body functionally integrated for the maintenance of homeostasis, whilst encephalisation was the decisive step to allow the encephalon to house the number of neurones necessary for the functional complexity of each species.
Nature made the animal body a machine profoundly dependent on the brain for life to exist.
More than that, the brain is a unique and non-transferable organ, and each characteristic acquired throughout evolution has been shaping this singular architecture, capable of sustaining not only survival, but the very experience of existing.
🔑And Evolution Created the Distinct Human Brain
Evolution is nothing more than the process of adaptation to the ecosystem in which a species is inserted, allowing its survival over time. And, from this logic, we can affirm that the human being continues to be a species in constant evolution, even if this process happens slowly on biological timescales.
It was approximately 6 to 7 million years ago that one of our first known ancestors lived, the Sahelanthropus tchadensis, represented by a skull found without the rest of the body, but whose morphology allowed the inference of characteristics of its ancestry and possible posture. Since then, different lineages arose, coexisted and disappeared, whilst the human lineage persisted.
Natural selection, environmental pressure, constant adaptation to new contexts, nomadism, the need to solve problems and growing social complexity were some of the factors that allowed human beings to stand out in relation to other animal species. Added to the female's reproductive age and the wait for the child's maturity, these trace two other points that precede the main theory of human persistence.
But, before all of these factors, the biological characteristic that most profoundly marked the persistence of the human species is recorded in the relative increase of the encephalon.
Having a large brain is not common in the animal kingdom.
The adult human brain weighs approximately 1,508 g, which is equivalent to approximately 2% of body weight and demands a great deal of energy — approximately 25% (~500 kcal/day) of daily energy at rest.
✨ This makes the brain a metabolically very costly organ.
And perhaps this cost helps to explain one of the most striking characteristics of the human species: prolonged development.
By possessing a brain that continues to mature for many years after birth, the human being presents a long childhood, greater parental dependence and an extended window for learning, language, social bonding and adaptation to the environment. And this prolonged dependence on care may also have been a decisive factor in the increase of longevity and in the complexification of human social relations.
However, absolute size is no guarantee of complexity. The central point lies in relative size and, above all, in neuronal density and organisation.
The human brain reigns over all other lives on the planet — with the exception of bacteria, viruses and parasites (which manage to mutate within a few days to survive) —, yet what one perceives is that the larger the body, the larger the brain, but the relative size of the brain decreases when its mass is compared to body mass.
What does this mean? The larger brain size in larger animals is in fact a necessity. The brain has to be larger to care for a larger mass, a larger body. It is precisely for this reason that primates with greater encephalisation tend to show better performance in tasks requiring behavioural flexibility, working memory, problem-solving and contextual adaptation, when exposed to adequate environmental opportunities.
👶The Birth of the Brain
Brain development depends on opportunities that are given to it, and in the animal kingdom most learning is practically born with the infant and is taught in the first months of life, as the mother "abandons" the offspring.
Different from this "wild life" process, the brain of a newborn human being is merely raw material ready to be shaped. That is, it is born knowing practically nothing, not even how to feed itself. And for this reason, this brain can transform into anything, depending on the information it receives from the external environment.
When we speak of the formation of the human brain, the appearance of the nervous system occurs in the third week of life, and it is in the fourth week that the three primitive encephalic vesicles (prosencephalon, mesencephalon and rhombencephalon) differentiate to give rise to the brain in the foetus. And in the course of embryonic formation, approximately in the fifth month of gestation, depending on the internal conditions of the infant's nervous system and interactions with the external environment, the first synaptic connections begin.
These synaptic connections correspond to the environment in which the infant is inserted, how it must behave there, and also learning from what comes from the external environment. This means that the infant, from this moment on, begins to acquire knowledge of what to expect from the external environment — such as the voice of whoever is speaking to it, the assimilation of sound waves, and even taste.
However, an interesting fact about this intrauterine development is that far more cells and synaptic connections are developed than necessary, which is why when the infant is born there is a large quantity of neuronal death, or apoptosis. Why does this happen? It is called programmed apoptosis — it is believed to occur because these neurones fail to establish adequate connections after birth and to achieve an optimisation of circuits.
Another interesting fact is that with the evolution and environmental pressure that made the human being bipedal, there was a narrowing of the pelvis, in addition to an increase in the skull. The theory holds that because of this narrowing, for birth to still be possible, children began to be born neurologically immature (with a smaller and less developed brain).
For this reason, the first years of life are the most important in an individual's life, as they are going through an entire process of assimilating information that comes from the environment around them to create their personality, individual characteristics and abilities. This process of transformation and acquisition of information is uninterrupted, continues throughout life, yet this first phase generates a question of self-organisation of biological and physiological capacities.
🧠The So-Called Encephalisation
Like all other animal species, the human species underwent the process of evolution primarily through natural environmental pressure. But even so, throughout this entire process, something very characteristic of the human being stood out: the increase of the skull and, consequently, of the brain.
Alongside this evolution, the position shifted to bipedalism, where the arms became shorter than the legs, freeing the hands for the use of tools, the pelvis took on a basin shape and the feet were consequently modified for balance — resulting in lower energy expenditure, the ability to travel long distances more easily and less solar radiation on the body.
There was also the replacement of fur by fat accumulation and consequently skin, the elongation of the throat, reduction of dentition and shape of the jaw — all of this concurrent with the use of fire and omnivorous diet.
🧬 During the era of Homo Habilis and Erectus, the capacity to store energy developed. With this, the human being discovered they were capable of hunting as they could run for longer than other animals.
✨ Curious fact: the human being is the only animal capable of running for so long — like a marathon — without succumbing.
There was also the use of tools and the cooking of food, which contributes to the reduction of digestion time and also allows greater caloric intake.
🧠 To give an idea of the energetic cost: for an elephant to have the same equivalent number of neurones as a human, it would need to spend 18 hours a day eating. (Herculano-Houzel, in-person lecture)
With all of these improvements made possible by brain evolution, leisure time then arose and with this the human being was able to go even further. During the era of modern Homo Sapiens, they learnt to adapt the environment to themselves — instead of only hunting, also farming, using animals for transport, developing language, writing, navigation, transformation of substances, and so on.
And finally, how much brain one has to maintain the body and how much is left over, if any, to develop other functions — this is what is called encephalisation. The proportion of the relative increase of the encephalon in relation to the animal's body size.
And it was through the encephalisation quotient (EQ) that the scientific community was able to explain the cerebral difference between humans and other species.
But reducing encephalisation to the idea of a "larger brain" would be oversimplifying a profoundly biological phenomenon.
What truly matters is not merely the volume, but what this growth permitted in terms of sensory integration, scenario prediction, behavioural flexibility, learning and adaptation to the environment.
🧬 In evolutionary terms, encephalising was a response to real environmental problems: predators, social organisation, the search for food, parental care, spatial navigation and risk anticipation. The greater the need to interpret complex contexts, the greater the selective pressure for more sophisticated circuits.
🧠 In the human being, this process reached an extraordinary level. The encephalon becomes energetically costly, consuming an enormous proportion of glucose and oxygen, but in return it offers a singular adaptive advantage:
✨ The capacity to transform experience into an internal model of the world.
The discovery by Suzana Herculano-Houzel that the human brain has approximately 86 billion neurones and 85 billion non-neuronal cells (that is, 1 to 1 — the same as expected in a primate brain), with the cortex corresponding to 82% of brain mass, yet housing only 19% of the neurones found throughout the brain. And the cerebellum, which corresponds to only 10% of brain mass, houses 69 billion neurones (72% of all neurones present in the brain) — demonstrates that body size is irrelevant. The number of neurones in the brain that cares for the body is so small that it makes little difference whether the body is slightly smaller or slightly larger.
Therefore, the theory of encephalisation is accepted, but with caution — since the great difference of the human being in relation to other animals lies in using their biological capacities to develop technology — any object, method and knowledge that allows the solving of problems rapidly — and thus more time and opportunities are gained to try to solve other problems.
In the end, it was the development of technology, the brain's ability to solve problems and the cooking of food that allowed the human species to develop the brain. With problems solved and nutritional needs met, there is free time for human beings to occupy themselves with things beyond survival.
⚡ The Neurone: The Electricity That Conducts Us
But if the theory of encephalisation tells the story of the growth of the encephalon, it is the neurone that explains how this structure becomes alive.
Richard F. Thompson describes the neurone as "the most interesting cell in all of biology. It is born before the birth of its host, lives alongside the host's life, never divides to form another neurone, and dies with its host".
The neurone, then, is the cellular unit of an individual nerve cell specialised in the reception, integration, propagation and transmission of information between and with other neurones. And this cell is considered essential — in fact, the most important cell in the human body from a functional standpoint.
The neuronal cell has variable forms and sizes, with a cell body that houses the nucleus and intracellular organelles, with the cell's extensions being either axons or dendrites, and in their majority possessing the myelin sheath.
The cell body of neurones, also called perikaryon or soma, has quite varied forms and are classified morphologically by this characteristic. The shape of the cell body is determinant for the type of connection and specific processing of the neurone in the central nervous system. Amongst the various forms, the following stand out:
Fusiform (or Von Economo): spindle-shaped and allow rapid communication between distant areas of the brain, being associated with social cognition and emotional processing.
Pyriform (or Purkinje cells): pear-shaped and found in greater numbers in the cerebellum, being essential for balance, motor coordination and fine adjustment of movements.
Stellate: star-shaped body, functioning primarily as interneurones, receiving and processing local signals in the cerebral cortex.
Pyramidal: triangular body, being the largest in the cerebral cortex and crucial for thought, learning, memory and motor control.
Alongside the shape of the cell body, the organisation of the neurone is not random. Each part exists to solve a specific biological problem.
Dendrites come from the Greek word dendron (tree) and can also take the most varied and complex forms, as they are fibres that extend from the cell body and whose final design resembles tree branches of varied forms and sizes. Furthermore, a large part of them are constituted by dendritic spines — an extension of the dendrite itself formed from a synapse with the terminal of an axon from another neurone.
The dendrites themselves are the extension of the neuronal cell body to cover the largest possible area for signal reception (synapse) of that neurone, as they are the part specialised in receiving signals from other cells.
✨ And it is at this moment that neuroscience gains beauty.
Once the information is received by the dendrite, it may or may not be transmitted to the cell body (soma), which in turn integrates this information, evaluating the intensity, frequency and relevance of the stimulus. When the sum of these signals reaches a firing threshold, the action potential begins — a transient electrical alteration of the membrane that travels along the axon to the synaptic terminals. Should this threshold not be reached, the signal dissipates and no message is sent.
It is worth noting, however, that neuronal communication goes beyond this classic pathway. Transmissions of information are not exclusively carried out by the axon: they can occur through chemical synapses — dendro-dendritic (from dendrite to dendrite or dendritic spine), somato-dendritic (from the cell body to a dendrite or dendritic spine) or axo-axonic (from axon to axon). It is these chemical synapses that utilise neurotransmitters — glutamate, GABA, dopamine, serotonin, acetylcholine and noradrenaline — not merely "substances", but specialised chemical languages, capable of modulating excitation, inhibition, reward, attention, mood, memory and learning.
They can also occur through electrical synapses — via dendrite-dendrite, soma-soma or axon-dendrite — which makes them anatomically more flexible.
Beyond synaptic pathways, there are also non-synaptic mechanisms. In volume transmission, neuromodulators are released into the extracellular space — primarily from axonal varicosities and dendritic terminals — and diffuse through the extracellular fluid to affect neighbouring neurones without direct contact. In electrical field communication, the electrical activity generated by the membrane of the axon and dendrites creates fields that propagate through nerve tissue and influence the excitability of surrounding neurones.
But what are axons, you may ask?
In simple terms, the axon — normally a single one per neurone — is the neurone's data transmission cable. Whilst the dendrites and cell body receive information, the axon exclusively transmits this information to other neurones, muscles or glands.
It is through the axon that the electrical signal travels from the neurone's body to its next destination, with the transmission from this end being able to occur both electrically and chemically.
Like the dendrites, axons have a quite characteristic form — a long, cylindrical tube, like a wire. But it has some particularities:
It is generally longer and thinner than the dendrites
Its thickness tends to be uniform throughout its entire length (dendrites become thinner as they branch)
At the end, it branches into terminals called synaptic boutons, which resemble small rounded bulbs at the tips
When it possesses myelin along its length, it visually appears segmented — like a series of sausages lined up with small interruptions between them: the nodes of Ranvier
Without myelin, the electrical signal travels along the axon in a slow and continuous manner. With myelin, the signal "jumps" between the nodes of Ranvier — what is called saltatory conduction — making transmission much faster and more efficient.
⚡ The difference is impressive: Without myelin → 0.5 to 2 m/s With myelin → up to 120 m/s The same axon, sixty times faster.
🧠 In the end, every thought, emotion, movement or sensation exists because millions of neurones are firing in highly coordinated temporal patterns through their highly organised cell.
The human experience, seen up close, is a cellular phenomenon.
🌍 Conclusion
Everything that we are — every memory, every emotion, every decision, every word we say or think — depends on an incredibly organised, electrically active and chemically precise cellular architecture.
🧠 The brain is not a passive organ. It is a living story in constant change.
Every experience, every environment one inhabits, every connection formed — everything! — literally shapes the way our neurones organise and communicate.
✨ And perhaps this is the greatest lesson that neuroscience offers us: we are not merely products of our biology. We are also active agents of our own cerebral architecture.
In the next part, we will move beyond the cell and into the system — exploring how the brain maintains the balance of life through homeostasis and the sympathetic and parasympathetic systems.
🧠 Because understanding the brain is, at its core, understanding what makes us human.
📚 Scientific Foundations and Core Readings
Nolte, J. The Human Brain: An Introduction to Its Functional Anatomy. Translated edition. Elsevier, 2008.
Cosenza, R. M. Fundamentals of Neuroanatomy. 4th ed. Guanabara Koogan, 2021.
Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. Cognitive Neuroscience: The Biology of the Mind. 2nd ed. W. W. Norton, 2002.
Thompson, R. F. The Brain: A Neuroscience Primer. 3rd ed. Worth Publishers, 2000.
Herculano-Houzel, S. (2009). The human brain in numbers: a linearly scaled-up primate brain. Frontiers in Human Neuroscience, 3, 31. https://doi.org/10.3389/neuro.09.031.2009
OpenStax College. (2013). Embryonic-fetal development of the human brain [Figure 14.3]. In Anatomy & Physiology. OpenStax CNX. https://open.oregonstate.education/anatomy2e/chapter/embryonic-development-cns/
Herman, L. M. (2009). Absolute brain weights and encephalization quotients of chimpanzees, gorillas, orangutans, dolphins, and humans [Figure]. In LACUS Forum XXXIV: Speech and beyond. Houston, TX. Data courtesy of Lori Marino, Emory University. https://www.researchgate.net/figure/Absolute-brain-weights-top-and-encephalization-quotients-bottom-of-chimpanzees_fig1_299804372