General Intelligence: What Is It And How Has It Evolved?

One of the most important debates when addressing the evolution of human intelligence is whether humans have developed a single general intelligence (og) or, on the contrary, an intelligence divided into a set of specializations.

Part of the literature attributes the first to humans and the second to non-human animals, but as always in science, not everything is so simple and there are studies that provide data against this idea.

Regarding this debate, Judith M. Burkart and her colleagues at the University of Zurich prepared, in 2017, a review in which they evaluated the presence of g in non-human animals and explored its implications on theories of the evolution of cognition.

What is the intelligence of humans and animals like?

In humans, we can understand intelligence by the ability to reason, plan, solve problems or think abstractly, among other abilities. In animals it has been defined rather by the ability to acquire knowledge of the physical or social environment and use it to solve new problems.

But What does it mean that a species has general intelligence? At an empirical level, we speak of general intelligence when individuals of the species score similarly in different types of cognitive tasks (such as causal reasoning or social learning tasks), giving rise to the famous g factor. Or, in other words, that there is a significant correlation between some scores and others.

It is what is known as positive manifold, and it is the great argument in favor of the presence of g in humans. Another is the correlation of g with brain size, gray matter volume and cortical thickness, as well as school and work success, among others. In summary, the presence of a general character intelligence in humans is represented by the g factor and finds support both in neurobiology and in characteristics of the lives of individuals.

The alternative or, perhaps complementary, vision of general intelligence is to speak of modular intelligence. An intelligence based on specialized modules for different cognitive abilities. The evolutionary basis behind this concept consists of considering these modules as cognitive adaptations to problems that have been repeated over a long period of time in the course of the evolution of a species.

Under this context, solutions to these problems would have been channeled through natural selection. An example would be if a species developed a strong spatial memory when it has historically needed to find food in large, complex territories. Therefore, according to this view, the human and animal minds can be considered a set of specializations that emerged to respond to specific problems in the environment.

In the past, a very strict concept of a modular mind was defended, with modules, or independent intelligences that process information with different “input channels.” This vision is totally incompatible with the presence of general intelligence in the same individual. However, Recently, many authors propose the compatibility of these modules with a “central system” for information processing and, in turn, with a general intelligence.

But if this central system has only been demonstrated in humans, the key question regarding the evolution of general intelligence would be how it has emerged, during the course of human evolution, above the previously existing modular system. To answer this question, it is necessary to investigate the cognitive characteristics of non-human animals.

General intelligence in non-human animals

The vast majority of studies that have tried to find g in non-human animals have been carried out primarily in rodents and primates, especially great apes. In rodents the presence of g appears to be quite robust, with studies examining up to 8 different tasks in mice and rats. As for non-human primates, the results have been rather mixed:

Some studies, mainly focused on chimpanzees, have found alternatives to the g factor to explain the intelligence of this species. An example is that of Esther Herrmann and collaborators who, applying similar intelligence tests on chimpanzees and human children, found that intelligence is organized differently in different species. The children’s performance was best explained through three different modules, or intelligences (spatial, physical and social). On the other hand, “chimpanzee intelligence” was better explained through two factors: one spatial and a second that grouped both physical and social tasks).

Later studies such as that of Herrmann and Call and Amici and collaborators found similar results (no presence of g) in chimpanzees and at the interspecific level, respectively.

On the contrary, other authors have defended the presence of general intelligence in chimpanzees after finding characteristics shared with humans. William D. Hopkins and collaborators at Georgia State University found that intelligence is largely heritable in chimpanzees. Furthermore, the g factor has been linked to larger brains and greater cortical thickness in this species, and Beran and Hopkins found a strong correlation between g and scores on self-control tasks.

Although the presence of g in great apes continues to be debated, These studies raise the possibility that general intelligence is not exclusive to the human species In favor of this idea, the majority of studies that have investigated the presence of general intelligence at the interspecific (or G) level find evidence in favor of it.

So how has general intelligence evolved?

The fact that a large part of the studies support the presence of general intelligence in rodents and primates leads us to consider that it has developed in some lineages above or, perhaps at the same time, that the specific adaptive capacities, theoretically easier to shaped by natural selection.

Here a component comes into play that has been directly correlated with general intelligence: brain size. Just as specific abilities (no matter how sophisticated they may be) have not required a large brain expansion, it seems that those species that have more general intelligence have needed a significant increase in brain tissue.

But, What are the conditions that have led these species to possess these capabilities? A proposal that tries to answer this question is the cognitive buffer hypothesis, which considers innovation and learning as two main drivers for developing general intelligence. Based on this idea, species whose environment often changes or becomes unpredictable would have required general intelligence to cope with unusual or changing ecological difficulties. Examples in favor of this theory would be the correlation between more innovative species and with a greater presence of G in primates, or the fact that a higher proportion of “colonizing success” has been found in species with more larger brains (including birds, mammals, amphibians, reptiles and fish).

If we believe this hypothesis, the logical thing would be to ask why not all species have ended up developing this intelligence that would allow them to adapt to all types of environments. Well, the answer lies in the great costs it has. The brain expansion that this type of adaptation requires entails an enormous energy cost (remember that, in humans, the brain can consume up to 20% of the energy required by the entire organism) which, in addition, also requires a slowdown in physical development and brain at the ontogenetic level.

Under these conditions, only species capable of providing special and long-term adult care for offspring would be able to afford such sacrifice. In this scenario, both the absence of constant predators that threaten the survival of adults and allomaternal care (care of the offspring by, in addition to the mother, other individuals in the group) that many species present, especially primates, would play an important role.

This explanation coincides with Michael Tomasello’s well-known hypothesis of social intelligence in giving importance to social learning and holding it responsible, to a large extent, for brain expansion and the high cognitive abilities of the human species.

Concluding, this review leads us to accept (or at least consider) the compatibility between specialized cognitive abilities and general intelligence. At this point, perhaps it would be more interesting and accurate to ask ourselves which skills emerged through specializations and which are the result of a subsequent adaptation thanks to the cognitive flexibility that accompanies general intelligence. In this direction, and as always in science, more comparative studies are necessary to understand when and why g evolved.