The evolution of intelligence: Do we know how to study it?
It is 2017. A 26-year-old me was exploring New York City for the very first time. As a biological anthropologist, I had a strong desire to visit the Bronx Zoo. On that hot summer afternoon at the beginning of September, I headed directly to the Congo Gorilla Forest exhibit. Despite my interest in the evolution of intelligence and complex behavior, that long-lasting curiosity that I was able to transform into a career, I have never seen these majestic animals live before. What I perceived that day changed my life forever. A large silverback gorilla looked directly at my eyes, establishing a connection that I felt thousands of times with other humans. After some seconds of deep contemplation, that gorilla started predicting my movements, anticipating and mimicking my reactions, and communicating outstandingly. I will never be able to directly ask that gorilla its emotions and thoughts, but I will do my best to understand the only thing that can scientifically explain the wide diversity of cognitive answers that exist in nature: our brains.
It is 1871. A 62-year-old Charles Darwin was publishing The Descent of Man, and Selection in Relation to Sex. This paradigmatic book fueled the young scientific revolution that aspired to study human origins in the light of the brand-new theory of evolution. A single line there shaped forever the way we study the evolution of intelligence: “The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind”. By stating that, Darwin challenged the core idea of intelligence as something exclusive to humans. After all, our scientific name as species, Homo sapiens, was conceived to reflect that uniqueness revisited by Darwin. A scientific movement was born to explain intelligent behavior across the animal kingdom. But, what do we understand by intelligence?
An understanding of intelligence and its comparison across species
There is currently no standard definition of intelligence. In a broad sense, intelligence can be described as “the ability to perceive or infer information, and to retain it as knowledge to be applied towards adaptive behaviors within an environment or context”.
There is a book that nurtured my perspective and inspired my current research goals, named On Intelligence. In there, Jeff Hawkins defines intelligence as the capacity to make predictions of the future based on memories of the past. This concept, in which hierarchical regions of the brain predict their future input sequences, is known as the memory-prediction framework. Since all organisms possess some degree of a memory system and behave predicting the reactions to their actions, I consider that understanding intelligence in these terms, retrieving memory to predict outcomes, is the way forward to construct an operational definition that permits comparing cognition across living organisms.
It is widely accepted in Biology that extant animals, those species that did not become extinct, were, and still are, successfully adapted to their specific ecological, social, and cognitive niches. The physical component that allows this adaptation is the brain. As the most specialized organic structure ever discover, the brain has precise regions and networks that encode different types of information to produce behavior and construct the mental representations of our external and internal realities.
Comparative neuroanatomy as the way to scientifically study the evolution of intelligence
Brains are the biological basis of intelligence, but they do not fossilize. So, how can we study its evolution? How can we measure human brain evolution on a very fragmented and incomplete record?
While fossil hominin skeletons and stone tools have revealed a good amount of information regarding diet, foraging, locomotion, and cultural practices, very limited evidence of changes in cognition has emerged from the fossil record. Measuring overall brain size is the inevitable result of a lack of measurable traits for extinct hominin brains. As a consequence, this traditional approach is limited to the only direct line of evidence that the paleontological record provides: the skull and its by-products, natural and virtual endocasts. The discipline of paleoneurology has therefore understandably focused on the changes in overall brain size over time. Nevertheless, this restricted focus is obscuring informative key aspects of brain evolution, primarily the variation of the relative size and connectivity of the precise brain regions and neural networks that mediate particular adaptive behaviors.
To meaningfully study the evolution of intelligence we need to compare functionally relevant brain areas and neural networks across species. Then, knowing the degree of genetic relatedness of those species in the phylogenetic tree, we can statistically model and infer the changes in evolutionary time. This approach is highly multidisciplinary, connecting neurobiology, neuroanatomy, and comparative phylogenetics. For instance, I am deeply interested in the evolution of the brain’s computational power, the optimization of behavior, and the different learning mechanisms. In consequence, I study the evolution of cortical folding, the relative degree of convolutedness of the cortex, as well as the cortico-striatal system, the key network that connects the cortex with the striatum, in all the different orders of mammals. I measure gyrification, along with the relative volumes and connectivity of the striatal subregions, in coronal slides of already prepared brains. With my results, I aspire to further expand our understanding of the evolution of the key neural components that allow specialized cognition in animals, and humans in particular, such as the prefrontal cortex, the cerebellum, and the hippocampus.
A way forward to study the evolution of intelligence
As an anthropologist, I am passionate to understand how the human brain works to then apply that knowledge to overcome the limits of our biological existence. The human brain did not develop in a vacuum, so we need to understand how the brain has evolved across the phylogenetic tree. Eons of evolution have shaped our brains into this unparalleled prediction machine that allowed our adaptation and survival as species. All the cognitive features we once thought of as only human, such as attention, consciousness, episodic and working memory, learning, problem-solving, self-awareness, and tool making, are present in other animals, paraphrasing Darwin, in different degrees.
Every single one of our thoughts, memories, and decisions are created in our brains. The human brain has been wired by millions of successful adaptations, exactly like the one from the silverback gorilla at the Bronx Zoo. Fortunately, there is a way to study this vast range of cognitive variation, and I am proud to be at the forefront of this research effort. The solution lies at the junction of neuroscience, the function and anatomy of the brain, and evolution, the diversity over time across genetically related species.