BRAIN EVOLUTION

Just the other day, I sent a letter to Dr. P. Thomas Schoenemann who is a professor of anthropology at the University of Pennsylvania. He is studying brain evolution and has published a review on the size and functional areas of the human brain (Annu. Rev. Anthropol. 2006. 35:379–406). In Prof. Schoenemann review, he outlined several criteria for assessing brain evolution focusing primarily on the hominid brain. In my letter, I described my thoughts on the fallacy of seeking clues to the origin of the human brain by focusing solely on hominid brain structure and lineage.

As I see it, the fallacy of most ideas and all research on the nature and function of the contemporary brain is where the science for those ideas and research begin. Invariably, the science of brain research begins with the emergence of cortical structure and function. The error in this beginning is that the brain is not just the cortex and it did not evolve from cortical structure or function.

It is incredibly naïve to think that the totality of the human brain only encompasses cortical structure when the cortex is entirely dependent on subcortical relays and functions. Dr. Michel Jouvet proved as much in his early 1960’s experiments with decerebrate cats. Jouvet showed that the cortex does not engage in any spontaneous activity when it is isolated from subcortical structure (Jouvet, M., & Jouvet, D.,1963: A study of the neurophysiological mechanisms of dreaming. Electroenceph Clin Neurophysiol., Suppl. 24.). Further still, the cortex is not the most primitive constituent of our central nervous systems (CNS), which in itself represent the totality of brain structure and function.

Clearly, the totality of brain structure and function involves a concert of neural processes and activity between the subcortical and cortical components of our CNS. The neural processes and activity these components produce enable the perceptual and behavioral abilities essential to normal life. Of these components, our brainstem is the most primitive. As such, it evolved before cortical structure. Rather than replace the primitive, the evidence in evolution suggests that nature builds upon the successes of the primitive to create modern versions that are more robust and adaptive. As such evidence suggests, the cortex—as a more recent neural development—likely evolved from the success of primitive neural structures. Therefore, if our goal is to determine how the cortex evolved to its current size and function, we must begin with the structure that came before—the brainstem.

As a primer to future entries, consider the shape of the thalamus, the similarity of its form to current cortical structure, and why it has relays for every sensory system of the body. Now consider if it is likely that the thalamus, a brainstem component, was a prototype for current cortical structure and function.