What part of the brain evolved first?

No organ in our body causes more fascination than the brain. One often wonders what part or structure of the brain evolved first. One parsimonious answer is that the part that evolved first is probably the one that we share with our fellow reptiles. 

How the species evolved was the subject of one of the most important books in Western culture: The Origin of Species by means of Natural Selection, published in 1859 by the British naturalist Charles Darwin. How we became what we are now is thus a central issue in contemporary science. 

Depending on the evolutionary level you are looking at, the answer about what part of the brain evolved first is different: 

  • Considering all living beings, what appeared first were cells that lived in primitive seas and had the capacity of sensing the environment and responding accordingly. But this was not a brain as we currently conceive it. In any case, those early cells, which were already unicellular living beings, are the precursors of the neurons, the sophisticated brain’s cells. 
  • When neurons first grouped did that in a sort of diffuse network (called a nerve net) in an organism, as seen in jellyfish and sea anemones. 
  • When thinking in multicellular organisms such as the lancelet or amphioxi, the part of the brain that evolved first was a small region in the head that contains the foundations of future structures in the human brain: the diencephalic forebrain, the midbrain, and a hindbrain
  • Considering the human brain as shown below, one can assert that the so-called reptilian brain is the oldest set of structures or the part of the brain that evolved first.      

Let us briefly review the human brain anatomy to set the basis for understanding how it evolved.  

In the introduction above, I said that the first brain may be said to be a single cell living in ancient seas and that those cells had the same properties of the future neurons indeed, the bricks of future brains. Hence, let us briefly describe what is a neuron?

What is a neuron?

Neurons are the nerve cells. They have similarities with any other cell, but they are unique in various important senses.   

When you were born, your brain had 100 billion neurons approximately, but along your life, many neurons die and do not reproduce. That cellular death is necessary for the proper functioning of your brain. 

Neurons have a body and several branches; one type of branches is called “dendrites” and the other type is called “the axon”.  Dendrites and the axon have various lengths. A short axon maybe 0,1 mm long or 2-3 meters long; think for example in the length of axons traveling up and down inside the neck of a giraffe.  

Neurons are like a small brain in themselves. They receive electrical information from other neurons in their dendrites, in a special type of electrochemical connections called synapses

After a sort of algebraic calculation in the neuron body, the electrical information travels along the axon and reaches other neurons, muscles, or glands.  

This basic structure of neurons is essentially similar in all animals on our planet, even though there are many types of neurons, regarding their size, location, and number of branches

Evolutionary anatomy of the human brain

The brain reptile figure uses old-fashioned terminology with which contemporary scientists do not agree, but it still useful to have a big picture of evolution. Keep in mind that modern humans are 200.000-300.000 years old, and many other creatures were formed millions of years before us.

You can observe that the lower part of the brain is named “reptilian”, meaning that these structures were already present in reptiles, such as Hylonomus lyelli, the earliest known reptile, and the first animal conceivably to have completely adapted to life on land, around 315 million years ago. Since we, humans, are a more recent species, one may assert in a sense, that this set of “reptilian“ structures is the among the oldest parts of the human brain.  

The so-called reptilian brain is said to deal with basic bodily functions such as breathing, feeding, cardiovascular and respiratory system regulation, sleep, and many others. The term “basic” is confusing because these functions are integrated with many others at complex levels but in any case, it has the virtue of reminding the early origin of those structures.      

Scientists think that as early reptiles continued evolving, new brain structures appeared or developed. Around 200 million years ago the first mammalian developed and then birds arose from dinosaurs during the Jurassic, around 165–150 million years ago. 

How the reptilian brain evolved into the brain of mammals and birds

The primal reptile linage diverged into many branches, but two of them eventually led to what became the early mammals and birds.  

Importantly, early mammals and birds followed different paths in their brain evolution based on their common reptilian brain. Mammals gradually developed layered neural structures in the forebrain called the cerebral cortex. In birds, the forebrain structures became packed in nuclei. 

This is a remarkable example of the called divergent evolution, where two different types of structures, eventually lead to the development of highly intelligent and skillful creatures.   

The notorious differences in brain structure and design between mammals and birds supports the notion that evolution does not have a predetermined plan.   

Evolution of the mammal’s brain until reaching the human brain

  • Early mammals were small animals and had small brains. These brains had extensive areas for olfaction and a small cerebral cortex, formed by a single layer of pyramidal cells. This contrasts with present-day mammals, whose cerebral cortex has six layers of neurons.  These early mammals probably laid eggs.
  • Successive classes of mammals were marsupials like current kangaroos and placental mammals, from which our primate branch emerged later on.  The first placental mammals probably appeared 125 million years ago.
  • A critical issue in mammals evolution is that in placental animals, the fetus could spend more time inside the uterus. This extra time allowed additional brain development before birth and led to further development of the frontal lobe, which is the place where several typically-human cognitive abilities are located.   Spending more time in the uterus also allowed the development of the corpus callosum, which is a structure that communicates both brain hemispheres, as shown below. Such an interhemispheric communication was also important for the origin of human intelligence. 

  • The progressive growth of the mammalian cortex inside a rigid skull forced the development of brain fissures, to accommodate the ever-growing brain. Hence, a considerable extension of the cortex could be accommodated in a relatively small volume. The more fissures a brain has, the more intelligent its holder will be.      
  • The early primates emerged approximately 82 million years ago, and about 6 million years the line that finally led to humans diverged from that of the chimpanzee. 
  • There are distinctive features in the primate’s brain, that illustrate how far evolution traveled from the reptile to the human brain.
  • One of these distinctive features is that the density of neuron packages and the size of the neuron has remained pretty stable throughout primate evolution. Since brains concurrently enlarged, that density stability meant to have additional space for an increasing number of neurons. And the more neurons you have, the more intelligent you are.   
  • In the last 2 million years, the brains of our ancestors increased their size from  400 – 600 cm3 to 1200 – 1600 cm3 in modern humans.
  • The human brain has a larger prefrontal area, which is the zone related to attention, planning, and undertaking initiatives. Besides, the posterior parietal cortex also increased in size.
  • And lastly, the human cortex has around 200 functional areas, which represents a 10-fold increase in active areas compared to early mammals.   

How fast did the prehuman and human brain grow? 

 You may argue that any creature is prehuman in evolutionary terms. But for our purposes when I say prehuman, I refer to primates, that stated above, appeared on earth around 80 million years ago. 

The term Hominidae refers to a taxonomic family including extant great apes and humans.  

There are some critical dates to be remembered for our discussion about how the human brain evolved from the reptilian mode to the present. 

  • The last common ancestor of humans and other great apes lives approximately 15 million years ago. 
  •  The oldest fossil of the hominoid clade dates 6 million years.   
  • The more recent and direct ancestors of modern humans are usually named as belonging to the genus Australopithecus and Homo, which appeared around 3 and 2.5 million years ago respectively.
  • The Homo sapiens, which is the species to whom we modern humans belong to, emerged around 170.000 years ago in South-Mid Africa. 

Brain size evolution during the last 6 million years, as calculated from skull fossils (Smithsonian)

From 6–2 million years ago

During this period, early humans started walking upright. The brain size slightly increased, for example, the brain of Australopithecus

From 2 million–800,000 years ago

Early humans disseminated around the globe and found many new environments on several continents. In consequence, both, body and brain size increased, for example, the brain of Homo erectus.

From 800,000–200,000 years ago

Brain size evolved rapidly during this period until reaching the size of current humans. This was probably related to dramatic climate changes that occurred at that time and forced evolution, by better surviving those with advanced cognitive skills. 

What happened in the brain after our reptilian origin: the wonderful human brain

After fishes and amphibia appeared on earth, the time came for reptiles. A reptile-like creature was thus a distant ancestor of mammals. 

Compared to us, its brain was simpler, but do not be fooled. That animal lived, reproduced, fought for its life besides breathing and having a pumping heart. It is thus misleading to say that such a reptilian brain dealt with “basic” functions. 

Early mammals were close to reptiles, and as stated above, they laid eggs. 

A step forward and marsupials appeared on earth. It necessarily implied a notorious change in their brain, and even more when placental mammals set their place. In these last species, a complex hormonal system was located in the midbrain or diencephalon. 

While early mammals already had a basic brain cortex, it is incomparable to the extensive and complex cortex of later mammals such as apes, dogs, elephants, that allowed the distinctive human intelligence.

References (in addition to linked text above)

Baptista T, Aldana E, Angeles F, Delgado H. Teaching evolution to psychiatrists, other medical specialists, and medical students: a pilot study. Canadian Journal of Medical Education. 2012; 3 (2): el27-el37.  

Baptista T, Aldana. Arthur Schopenhauer and the current conception of the origin of species: What did the philosopher anticipate? Sage Open; 2019. Enero-Marzo, 1-15.