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How does the evolution process affect the size of the human brain?

wallpapers News 2020-11-02

The human brain is the most complex machine that has ever existed. Often it can solve very complicated tasks within milliseconds. By now, there is no existing computer system, which can beat this result.

A typical example, which can prove the power of our brains, is the pattern recognition process. Evolution has shaped the brains of mammals in various ways. It happens because they must operate in a variety of environments that require different behavioral skills.

Despite the geometric and behavioral differences in mammalian brains, they retained plenty of the common features. Let’s take a look at how the hominin brains have evolved. Whether they are different, or maybe some of their common features have prevailed?

Image credit: M. Osial


The center of decision making processes in our body, the brain, is perhaps the most fascinating organ. This spongy-like structure full of blood vessels and water has almost 100 billion neurons. The Internet, currently the largest computer network, has only millions of processing units, making it much smaller than the brain. The brain can solve complex tasks within milliseconds, which is much faster than any modern computer vision system.

Due to the complexity of the brain, we still do not fully understand how it works, and there are many things left to discover [1, 2]. “Consider the human brain,” says physicist Sir Roger Penrose, “If you look at the entire physical cosmos, our brains are a tiny, tiny part of it. But they’re the most perfectly organized part. Compared to the complexity of a brain, a galaxy is just an inert lump”. The brain controls all our organs and makes them function as one organism.

Like all living organisms, humans evolved, and so did our brains – striving for progress and knowledge, with a constant desire to change the world. The current technology and the high living standard is the result of brain evolution. As Carl Sagan put it, “Curiosity and the urge to solve problems are the emotional hallmarks of our species” [3].

Brains’ size

The extinction of dinosaurs started the era of mammals. One mammal, named “Australopithecus”, appeared about 2 million years ago. What made him different from the other species was the brain features. Australopitecus’s brain had a volume barely equal to 450 cm³ (almost half of the liter), yet it could probably use stone tools. Probably, its complexity and size in comparison to the overall body mass differed it from other mammals.

Considering that its brain size was still very similar to living apes’ brain size, it was extraordinary. This fact alone already suggests that brain evolution is a much more complicated process than just This fact alone already suggests that brain evolution is a much more complicated process than just our brains’ growth. However, there is still an enormous gap between Australopithecus and today’s Homo sapiens, which emerged because of complicated brain evolution.

During evolution, the brains of individual animal species have increased. This trend is particularly evident in mammals [4]. Brain size is usually highly correlated with body mass. However, humans are a special case among primates. The human brain is 3 times larger than that of its mammalian weight counterpart [5], and the cortex is 3.4 times larger [6].

The brain’s cognitive barrier is related to the processing and integration of a large amount of sensory information and the ability to make comparisons between them. It means that its functional capacity is inherently limited by the architecture and time of signal processing [7]. Let’s take a look at how this translates into brain size.

Image credit: M. Osial

Bigger means better?

The rate of brain evolution profoundly depends on the ways that the brain regions are connected. Thus, neural connections influence both cognitive and behavioral abilities. Following the human brain’s evolution might be a bit tricky – soft tissues such as the brain are not usually preserved. We can compare only the skulls that are left with current ones.

Instinctively, we might think that bigger brains result in higher intelligence, but it does not work that simply. However, the brain/skull’s size is a useful tool in tracing the human brain’s evolution. So what about the size? Let’s think about whales’ and humans’ brains. The larger brains should be more advantageous than the small ones. Why? Bigger brains have more neurons, which means that some neurons are further away from each other.

To communicate rapidly and efficiently between these neurons, the brain needs to be more modular and orderly. Larger brains require more local connections because long dendrites take up too much space, and as a result, it is more probable that the brain will have a more organized structure [8].

If comparing sizes of brains does not work, what about a bigger brain-to-body mass ratio? Even though it may sound like a great idea, it is not accurate. Compared to humans, small birds have a higher brain-to-body ratio, just like ants [9], yet it would be irrational – and factually incorrect – to say that ants or birds are more intelligent than humans.

Nevertheless, as mentioned earlier, the brain’s immense size is not everything. There are many other ways for a brain to evolve and be more complex other than just becoming bigger. In fact, our brains have been getting smaller for 10 000 – 20 000 years [10], which is at least partially linked to human’s decreasing body size. Smaller bodies can easily function with smaller nervous systems and, as a result, use fewer resources – after all, the nervous system needs a lot of energy.

Back then, in the time where our brains decreased, a genetic mutation arose that enabled human speech development. It contributed to the increase in human intelligence. A region of the brain in the left temporal plane, which is much larger in the left hemisphere. This area is responsible for language skills. At this moment of evolution, the human brain becomes asymmetric, both functionally and anatomically [11].

However, it is worth noticing that domesticated or captivated animals tend to have smaller brains [12] because they do not need as much information to survive. Can the same mechanism be applied to the human brain? Science does not give us answers yet, but that is still a possibility.


It is known that human evolution started 7-8 million years ago in the African savannah regions when its position changed from upright to bipedalism. Its main driving force was getting food more and more efficiently, while the nervous system uses disproportionally high levels of energy.

Changes in the human brain itself also played a large part in this process. They are strictly related to the better quality of food. Advancements, in its structure, are related to cognitive and behavioral development. An increase in body size has always accompanied the increase in brain size. Brain size varies significantly in healthy adults.

However, it is not closely correlated with intelligence. From the anatomical point of view, it can be related to the number of cortical neurons and physiologically to nerve pathways’ conduction velocity. Certainly, the evolution of language has conditioned humans to think consciously and thus distinguishes them from other animals.

This article is a joint work of Zuzanna Kozłowska (Faculty of Chemistry, University of Warsaw), Ewa Klejman (Faculty of Chemistry, University of Warsaw), Agnieszka Pregowska (Institute of Fundamental Technology Research, Polish Academy of Sciences),  Magdalena Osial (Faculty of Chemistry, University of Warsaw),  as a part of the Science Embassy project. Figures – M. Osial.

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