Thalamic Inhibition in the Evolution of Human Intelligence: Evolutionary Pressure for Cortical Inhibition
Glendenning, K. K., Mankind Quarterly
One characteristic of a civilized society is the ability and propensity of its members to refrain from acting in a way that disrupts the harmony of the group. While this is not intelligence as measured by IQ, it is a form of social intelligence that will aid those having it in their ability to propagate. This characteristic is not unique to human societies but rather is true of most social species.
This report deals with one aspect of brain chemistry that may underlie this social intelligence, defined as the ability of a species to inhibit its thoughts as well as its behaviors. Selective pressure has unrelenting consequences on the mechanisms of brain function, and perhaps the first mechanism to be affected is the chemistry of the brain. Changes in brain chemistry and function can be explored by an investigation of species selected to represent the phylogenetic sequence leading to mankind. In an examination of nine species selected on this basis, we have found that one consequence of selective pressure is an increase in the potential for cortical inhibition. We suggest this may be a major component of the evolution of human intelligence.
Key words: intelligence, thalamus, inhibition, comparative psychology, evolution, mammals, receptor binding, GABA, muscimol binding, amniotic vertebrates.
By treating relative brain size as a measure of intelligence, it is possible to develop a coherent story about the probable history of intelligence as a biological phenomenon. Like other biological processes, intelligence must have evolved under the influence of natural selection. That is, a well known prime result of evolutionary pressure has been the selection for human intelligence.
The intelligence of humans has been shown to be correlated with an increase in size of the neocortex (eg., Ariens Kappers 1929; Diamond and Hall 1969). In this investigation we have extended this axiom one step further. Not only do humans have a larger cortex but we have found that humans also come equipped with an increased capability of inhibiting this expanded cortex.
To begin this investigation we began with the assumption that a look at our ancestral heritage gives us insight as to what the future may bring. In turn, an even broader spectrum of evolutionary analysis can lead to a similar insight and perhaps predictions about the path along which natural selection is taking us. That is, what lies ahead in the evolution of mankind is only a subpart of what has been happening to primates as they are a subpart of what has been happening to mammals and so on. By an examination of diverse species chosen to represent a phylogenetic series leading to man, this report shows one step in the consequences of selective pressure on the mechanisms of brain function. Our conclusion is that this step, an increase in the potential for inhibition of the cortex, is probably a major component of the evolution of human intelligence.
To draw this conclusion we must first examine the results of selective pressure on brain development. Secondary and tertiary synaptic levels in brain structure are relatively immune to changes in a species ecological niche. Natural selection first acts upon those parts of the brain most easily changed so they can revert with changing circumstances. For example, a species driven into a nocturnal habitat is subject to adaptive pressure on its visual system to adapt. The first noticeable gross anatomical changes are the receptors in the eyeball (Polyak 1957). Prior to these anatomical changes are changes in the chemistry of the brain. These are the most easily changed and therefore the first to be effected by selective pressure.
Yet the chemistry of the brain demonstrates a remarkable robustness throughout phylogeny. In this study we have looked at the constancy of an inhibitory neurotransmitter in the brain over a wide range of species. Particular attention has been paid to a part of the brain, the dorsal thalamus, through which all sensory information is channeled before it reaches the cortex. …