The intelligence that characterizes our species has allowed us to perform incredible feats never before seen in the animal world: building civilizations, using language, creating very broad social networks, having consciousness and even being able to (almost) read minds.

However, there are reasons to think that having a privileged brain has cost us dearly .

The price of a great brain

From the point of view of biology, intelligence has a price. And it is also a price that in certain situations could be very expensive. The use of technology and the exploitation of knowledge handed down by past generations may mean that we have forgotten this, and yet, since Darwin included us in the evolutionary tree and as science unravels the relationship between the brain and our behaviour, the frontier that separates us from the rest of animals has been breaking down. Through its rubble, a new problem can be glimpsed.

Homo sapiens, as life forms subject to natural selection, have characteristics that can be useful, useless or harmful depending on the context. Could it be that intelligence, our main feature as human beings, is just one more characteristic? Is it possible that language, memory, the ability to plan… are only strategies that have been developed in our organism as a result of natural selection?

The answer to both questions is “yes”. Greater intelligence is based on drastic anatomical changes ; our cognitive capacity is not a gift granted by the spirits, but is explained, at least in part, by drastic changes at the neuroanatomical level compared to our ancestors.

This idea, which was so costly to admit in Darwin’s time, implies that even the use of our brain, a set of organs that seems so clearly advantageous to us in every way, can sometimes be a burden.

Of course, one could argue at length about whether the cognitive advances available to us have caused more fortune or more pain. But, going to the simple and immediate, the main drawback of having a brain like ours is, in biological terms, its very high energy consumption .

Energy consumption in the brain

Over the past few million years, the evolutionary line from the extinction of our last common ancestor with the chimpanzees to the emergence of our species has been characterized, among other things, by seeing our ancestors’ brains grow larger and larger. With the appearance of the genus Homo, a little more than 2 million years ago, this size of the brain in proportion to the body rose sharply, and since then this set of organs has been enlarged over the millennia.

The result was that the number of neurons, glia and brain structures inside our heads rose dramatically and were “freed” from having to engage in such routine tasks as controlling muscles or maintaining vital signs. This allowed them to process the information already processed by other groups of neurons, making it possible for the first time for primate thought to have the “layers” of sufficient complexity to allow the appearance of abstract ideas , the use of language, the creation of long-term strategies, and, in short, everything that we associate with the intellectual virtues of our species.

However, biological evolution is not something that in itself costs the price of these physical modifications in our nervous system.

To be able to preserve a functional brain, resources are needed, that is to say, energy… and it turns out that the brain is a very expensive energy organ: although it accounts for about 2% of the total body weight, it consumes more or less 20% of the energy used in the resting state. In other apes contemporary to us the size of the brain in comparison to the rest of the body is smaller and, of course, so is its consumption: on average, about 8% of energy during rest. The energy factor is one of the main drawbacks related to the brain expansion needed to reach an intelligence similar to ours.

Who paid for the brain expansion?

The energy needed to develop and maintain these new brains had to come from somewhere. The hard part is knowing what changes in our body paid for that brain expansion.

Until recently, one of the explanations for what this compensation process consisted of was that of Leslie Aiello and Peter Wheeler.

The expensive tissue hypothesis

According to Aiello and Wheeler’s “expensive tissue” hypothesis ,the higher energy demand produced by a larger brain had to be compensated also by a shortening of the gastrointestinal tract, another part of our organism that is also very expensive energetically. Both the brain and the intestine competed over an evolutionary period for insufficient resources, so one had to grow at the expense of the other.

To maintain a more complex brain machinery, our bipedal ancestors could not rely on the few vegetarian bites available in the savannah; they needed rather a diet that included a significant amount of meat, a very high-protein food. At the same time, ceasing to depend on plants for food allowed the digestive system to be shortened , with the consequent saving of energy. Furthermore, it is quite possible that the habit of hunting regularly was both a cause and a consequence of an improvement in general intelligence and the management of its corresponding energy consumption.

In short, according to this hypothesis the appearance in nature of an encephalon like ours would be an example of clear trade-off: the gain of one quality entails the loss of at least another quality. Natural selection is not impressed by the appearance of a brain like ours. Its reaction is rather: “so you have chosen to play the intelligence card… well, let’s see how you do from now on”.

However, Aiello and Wheeler’s hypothesis has lost its popularity over time, because the data on which it was based were not reliable . It is currently considered that there is little evidence that the increase in the brain was compensated for as clearly as the reduction in size of certain organs and that much of the loss of available energy was cushioned by the development of bipedalism. However, this change alone did not have to fully compensate for the sacrifice of resources in maintaining an expensive brain.

For some researchers, a portion of the cuts that were made for this purpose are reflected in the diminished strength of our ancestors and ourselves .

The Weakest Primate

Although an adult chimpanzee rarely exceeds 170 cm in height and 80 kg, it is well known that no member of our species would be able to win a hand-to-hand fight with these animals. The weakest of these apes would be able to grab the average Homo sapien by the ankle and scrub the ground with it.

This is referred to, for example, in the documentary Project Nim, which tells the story of a group of people who tried to raise a chimpanzee as if it were a human baby; the difficulties in educating the ape were compounded by the danger of its outbursts of anger, which could end in serious injury with alarming ease.

This fact is not accidental, and has nothing to do with that simplistic view of nature according to which wild beasts are characterized by their strength. It is quite possible that this humiliating difference in the strength of each species is due to the development that our brain has undergone throughout its biological evolution .

In addition, it seems that our brain has had to develop new ways of managing energy. In research published a couple of years ago in PLoS ONE, it was found that the metabolites used in various areas of our brain (i.e. the molecules used by our body to intervene in the extraction of energy from other substances) have evolved at a much faster rate than those of other primate species. On the other hand, in the same research it was observed that, eliminating the factor of the difference in size between species, ours is half as strong as that of the rest of the non-extinct apes that were studied.

Increased brain energy consumption

As we do not have the same body strength as other large organisms, this higher consumption at the head level has to be constantly compensated by intelligent ways of finding energy resources using the whole body.

We are therefore at a dead end in evolution: we cannot stop looking for new ways to face the changing challenges of our environment if we do not want to perish. Paradoxically, we depend on the ability to plan and imagine that we are provided with the same organ that has stolen our strength .

  • You may be interested in: “Theories of Human Intelligence”

Bibliographic references:

  • Aiello, L. C., Wheeler, P. (1995). The expensive tissue hypothesis: the brain and digestive system in human and primate evolution. Current Anthropology , 36, pp. 199 – 221.
  • Arsuaga, J. L. and Martínez, I. (1998). The chosen species: the long march of human evolution . Madrid: Ediciones Planeta.
  • Bozek, K., Wei, Y., Yan, Z., Liu, X., Xiong, J., Sugimoto, M. et al. (2014). Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes Parallells Human Cognitive and Physical Uniqueness. Plos Biology , 12(5), e1001871.