Is intelligence a gift of evolution?

The brain: evolution of the brain

More than half a billion years ago, nature made an ingenious invention: It created neurons. Cells that can receive, process and transmit stimuli. In doing so, she laid the foundation for the most complex structure in the universe - our brain

It took evolution more than 650 million years to restore the initially very simple nervous systems in the animal world -

for example in jellyfish and sea anemones - to develop into the human brain.

If you allow yourself to humanize nature for a moment, then in the course of evolution it proceeded like a somewhat eccentric builder who gradually turned a garden shed into a villa in the course of his life: Hardly anything was thrown away, only rarely a wall torn down, instead repeatedly on and rebuilt. New room suites were created, while old little rooms were still used and the cellar remained almost unchanged. Little by little a magnificent building took shape, which is suitable for many purposes.

Ancient form of signal traffic

In terms of materials and technology, too, nature stuck to the tried and tested: the human brain in all its complexity is ultimately based on those building blocks - nerve cells - and means of communication - electrical and chemical signals - that can already be found in simple living beings.

Even a simple creature like the intestinal bacterium Escherichia coli is able to react sensibly to stimuli in its environment. Special receptor molecules in the cell wall help him to perceive food sources or toxins.

When these receptors are stimulated, they generate chemical signals. They cause the unicellular organism to move in the most favorable direction with its propeller-like flagella - for example towards the food or away from danger. Nature has retained this age-old form of signal traffic on its way to the human brain.

More complex organisms, which, in contrast to the coli bacteria, consist of many cells, do not come to their decisions so easily.

Rather, they need an entity that brings together the information from different parts of the body, derives a result from it and controls the reaction. Otherwise every part of the body might strive in a different direction - provided the organism can move at all and does not remain in one place like a plant for its entire life.

As a consequence, evolution introduced an innovation in the course of development between sponges and jellyfish: nerve cells (neurons). They were formed from cells in the outer skin layer that were directly exposed to the environment and specialized in receiving, processing and transmitting stimuli.

A sponge that neither hunts nor can flee from enemies does not need any signal lines - consequently it has no neurons. The mobile, predatory jellyfish, on the other hand, are among the oldest organisms still in existence today that have a simple nervous system. It consists of a network of interconnected neurons that runs through the entire body.

But a cluster of such cells, worthy of the name brain, is not yet found in jellyfish.

Nature only tried this construction with worms. In contrast to radially symmetrical animals such as jellyfish or starfish, the front and back of them can already be distinguished - and that meant a huge leap in the evolution of the brain. If an animal prefers to go in one direction, i.e. forwards, it makes sense if a large part of its nerves and sensory cells are concentrated at the front end. After all, this part is usually the first to encounter the promises and dangers of a new environment.

The brain increases in volume

The flatworms are among the simplest creatures in which this blueprint can be observed: a head sits in front and the brain rests in it. Over time, the head became more pronounced and the brain kicked in

Volume too. Gradually, it became more and more efficient - not because fundamentally new building blocks were added, but because the number of neurons and their interconnections increased.

This development was caused by mutations - changes in the genetic make-up that proved to be beneficial for the organism. Genetic changes played a key role in this, in which important genes were passed on twice to the next generation. The gene copy could now mutate in turn without jeopardizing the organism's viability.

Thanks to genes like this, additional neurons grew, which could then be used for new tasks.

Of course, the back of the worm couldn't do without nerve cells entirely, after all, this too had to report signals from its environment to the brain. That is why a nerve cord runs lengthways through his body - like our spinal cord.

Somewhat more developed animals such as annelids and the later insects have bodies that are divided into segments. Each section has two nerve nodes (ganglia) that control the respective segment like mini-brains. The ganglia are linked to form a rope ladder-like structure that leads into the head. This is where its larger counterpart, the actual brain, sits and coordinates the concert of nerve cells like a conductor.

Even if insect brains only consist of just under a million neurons, they enable their carriers to behave extremely complexly.

The male of the scorpion fly, for example, not only gives the courted female an edible gift, but also measures the size of the gift according to the expected fertility of the partner.

And ants live in states organized according to the division of labor, which sometimes wage real wars against rival peoples.

But there are limits to the capabilities of the insect brain. They are similar to computers on which only certain software runs - they can hardly adapt to changing environmental conditions.

Vertebrate brains adapt to their environment

The vertebrate brains developed in a fundamentally different way than the insect brains, which were unbeatably efficient, but remained relatively inflexible, with the same requirements: They are more dynamic and designed for individual development and change and can therefore do better in an environment that is no longer exactly the same as that of their parents claim.

Their circuit diagram, i.e. the pattern of connections between the nerve cells, is largely determined by external influences during the development of the embryo and in the early phases of life.

For example, a canary does not hatch out of the egg with a firmly inscribed melody of its advertising song in its head, but learns its song by listening to adult males. Likewise, after a few painful collisions with a window pane, he is able to understand that an invisible obstacle is blocking his path.

A fly, on the other hand, tries again and again to get through the glass until it is finally exhausted.

The first vertebrates, which appeared about 500 million years ago, were similar to today's fish-like lampreys. They already had a skull capsule that protected the sensitive brain. Life at that time took place exclusively in the ocean; the oldest type of vertebrate brain can therefore be observed in lampreys and fish.

Despite all the external differences, the brains of fish and birds, rats and humans are basically designed in a similar way: the brain stem controls life-sustaining functions such as heartbeat and breathing, the cerebellum coordinates movements, among other things, and the forebrain serves demanding tasks such as planning, evaluating information and making decisions . However, many functions cannot be clearly assigned to one brain region, but are always fulfilled in the interplay of several structures.

While the brain stem changed relatively little in the course of evolution, the builder nature chose the forebrain as her favorite construction site. Here she was constantly expanding and adding until the new halls barely found space on the property.

The cerebral cortex is the most developed part of the brain

The progress towards more and more performance, willingness to learn and more complex skills is primarily due to the expansion of an outer layer of the forebrain, the cerebral cortex. The most recent part of the phylogenetic tree is called the neocortex and only exists in mammals. In humans, it makes up almost half of the volume of the brain.

In order for this expanding layer of neurons to fit into the skull, but only a few millimeters thick, it unfolded in such a way that the brain gradually took on its furrowed appearance in Homo sapiens walnut-like.

If the cerebral convolutions in the human head could be smoothed out, they would cover an area of ​​four A4 sheets of paper - four times the size of a chimpanzee. The rather smooth cortex of a rat, on the other hand, is only the size of a stamp.

The more developed the vertebrate's brain, the larger the areas of its cerebral cortex that can no longer be assigned to unambiguous functions such as seeing or hearing.

It is these associative areas that enable vertebrates to react flexibly. Instead of responding to a stimulus with a fixed behavior like an insect or a snail, the input in higher animals is processed and modulated over many intermediate stations; the reaction can therefore be different.

In certain phases of evolution these associative areas swell strongly, and their size is an essential difference between human and ape brains.

But the evolutionary history of the brain by no means followed a straight path from the “invention” of neurons in invertebrates to the human brain.

On the contrary: from the first “neuron building”, different buildings developed independently of one another, including some magnificent buildings, in order to retain the image of a house built by nature.

Octopuses, for example, are at the forefront of all invertebrates in terms of intelligence. The brain of an octopus is built completely differently from that of a vertebrate animal. But the intellectual abilities of octopuses can easily keep up with those of dogs.

Among the vertebrates, the brains of elephants and whales, but also those of some birds such as ravens, are among the masterpieces of brain architecture - in their complexity comparable to those of humans and great apes.

Climate change accelerates brain evolution

After the evolutionary paths of humans and chimpanzees parted about seven million years ago, the hominid brains grew slowly at first.

Only about two million years ago did its growth accelerate rapidly: while the organ of Homo habilis, who was then living, took up around 600 cubic centimeters, Homo sapiens 190,000 years ago increased it to around 1,400 cubic centimeters. This development made man what he is today.

It was possibly triggered by climate change 2.3 million years ago, which presented new challenges for early humans. They responded, according to one thesis, by using better tools, for example to find new sources of food.

For the production and use of these aids, increased mental abilities and increased dexterity of the hands were necessary. In such a phase of rapid environmental changes, the increase in intelligence therefore meant an advantage in the evolutionary selection process.

The emergence of language and the associated use in the daily struggle for survival probably promoted the development of large brains. The brain expansion may then accelerated through a feedback effect: improved tools and weapons made it possible to hunt big game, for example, so that the food supply expanded.

The increased amount of energy available to the human body allowed evolution to try out larger brains. As a result, people became more skillful and intelligent.

Big brains don't just bring benefits

However, a stately brain like that of humans not only brings its owner advantages in evolutionary competition, but is also a burden because of its high energy consumption.

In Homo sapiens it only takes up about two percent of the body volume, but uses 20 percent of the metabolic energy, in a newborn it is even two thirds. And the more lush an animal's brain is, the more time it needs to mature and develop its full potential.

For the parents, this means that they have to invest a lot of time and effort in their offspring, so their reproductive success remains numerically low. These factors could limit further evolutionary expansions of the brain in the future.

In fact, humans have lost brain mass in the past 35,000 years. Today's brain weighs an average of 1300 grams - 150 grams less than that of people in the Stone Age.

It is unclear whether the loss was a result of the decrease in existential hardships, for example through advances in agriculture, or whether it was related to the body weight of our ancestors, which was also falling over long periods of time.

As an architect, nature not only built new rooms and halls onto its brain complexes - it also uncompromisingly tore down unused rooms. For example, the dog's brain volume decreased in the course of its career as a companion to humans: the brain of a domestic dog the size of a wolf is a third smaller than that of its wild relative.

And natural history also knows examples of the fact that a brain that has been acquired can be lost again: The tapeworm, a descendant of the first flatworm with its nerve knot in the head, clings to the human intestine, so lives in a comfortable, safe ecosystem with rich Food supply. To afford a brain is senseless luxury for such a parasite.

As a result, it was completely dismantled.