The Evolutionary Development of the Neocortex and its Implications for Evolutionary Cognition

In the hustle and bustle of everyday life, it is understandable for one to overlook the privileged cognitive abilities that he or she bears as a human being in comparison to other species.  In particular, humans, as mammals and apes, are seen as cognitively more advanced than other life forms, such as reptiles, mollusks, and microbes because they can process the greatest amount of information of their social and physical surroundings. [1].  From research in evolutionary biology and neurobiology, it seems that the key adaptation in the nervous system that allowed early mammals, and eventually primates, to develop these more sophisticated processing skills was the neocortex.  Specifically, the expansion of surface area of the neocortex among primates has been argued to provide humans with advantageous abilities such as tool-making, language, and social organizing [2].  Using comparative physiology and developmental neurobiology of the neocortices of mammals, then, one can infer how the neocortex evolved in other mammalian clades. [3].
One way to understand neocortex evolution is to observe the changes that occurred in the brain over geological time scales with paleontology.  In the case of evolutionary neurobiology, however brain tissue will not preserve, and therefore sparse endocasts of extinct mammalian species are the best evidence of ancestral neocortices.  Therefore, another way of understanding the evolution of the neocortex is to compare its physiology (structure and function) from extant mammalian and non-mammalian clades [4].
For the purposes of understanding the information processing ability of the neocortex, its neurons have been grouped into ‘columns’ or ‘modules’ based on similar function [1]. The modular organization of the brain was also supported by conceptual prediction of axon size within evolving brains: since distance increases time for signaling, long axons in a larger brain would also need to be thicker, which leads to a prediction that larger brains are simply scaled up versions of smaller brains. [5] This assumption of the modular brain has been incorporated to the most recent theory of neocortex organization that posits a network of ‘cortical fields.’ Specifically, the neocortex is broken up into discrete parts, cortical fields, each of which provides a function for the nervous system and the organism as a whole. [4] The benefit of this model permits the evolutionary biologist to elucidate homologous parts within the neocortex that may reveal a more accurate explanation of how the brain adapted with characteristic cognitive abilities for a clade.  A network of cortical fields has been found common to all mammals, except marsupials and monotremes, that includes the sensory (visual, auditory, somatosensory) areas and a motor cortex.  By comparing this neocortical organization--the number and size of cortical fields in relation to the overall cortical sheet (neocortex surface area) size -- from one species to another, homology and specialization can be deduced to construct a phylogenetic model [6].
Although comparisons of cortical organization among extant species can reveal a snapshot of phylogeny, an understanding of how the phenotypic differences are created has prompted research into neocortex development.  By comparing the neocortex’s phenotype at various stages of development between different species, more keen similarities and differences in species may become apparent that were not apparent with the previous phylogeny. Not only does an organism’s development exhibit a variety of phenotypes (e.g. the maturation of a human from embryo to child to adult), but these developmental phenotypes reflect a combined influence of genetic and environmental factors. The environmental stimuli can be as varied as traditional factors like pH, light, humidity, and temperature, to more complex ones such as feeding behavior and loss of a sensory ability (e.g. blindness and deafness) [3; 4].  For instance, genes for molecular factors, such as beta-catenin, FGF2, and brain factor-1 (BF-1), produce developmental pathways for the six layers of neuronal cells of the neocortex [1;4]. Nevertheless, the cortical fields created by these genes are also subject to environmental perturbation, and may be changed accordingly. For instance, sensory areas, such as the large auditory cortex is proportionally larger in echolocating bats than in non-echolocating mammals.  These results suggest correlation, and perhaps causality between the large auditory cortex and echolocation [4].
Teasing out the specific genetic and environmental causality in the developmental origin of plastic phenotypes, such as neuroanatomical ones, becomes challenging because 1) the area of the brain may have already changed size for pre-existing genetic reasons, or 2) the brain changed size in development to account for the cognitive stimulus.  Regardless of the specific cause, the neural phenotype that was derived as a product of gene and environment will also be selected, if favorable for its environment, and perhaps made independent of its environment over successive generations if the phenotype is hard-wired into the genome via genetic assimilation [7]. Thus, development produces the phenotype, a product of both gene and environment, which natural selection will act upon to create the variation observed in phylogeny.
Although the cortical-field model of neocortex organization does not explicitly dictate causality of the brain on behavior, the hierarchy of the neocortex being foundational to cognition for an organism is evident in the research.  For instance, in one review, the authors insisted that the structure “contains a large number of functional parts … interconnected to produce various types of motor, perceptual, and cognitive behaviors…” [6; emphasis mine].  This assumption of placing one’s faith in the facts of the ‘matter’ of brain structure, rather than the ‘mind’ of cognitive behavior may simply be a prejudice characteristic of the science that seeks to find a material (e.g. physical) basis for scientific arguments.
While a pragmatic attitude that aligns with demonstrable evidence and faith in the hierarchy of ‘hard’ science would support this prejudice, other evidence, notably from developmental neurobiology, seems to suggest that one’s inquiry should also be approached from the perspective of the behavioral evidence as foundational.  For instance, developmental biology, especially research related to sex-determination, was quick to proclaim the merits of genetics in predicting phenotypes under controlled conditions with model organisms in the laboratory.  When these predictions were extended to organisms in the wild of their ‘natural’ environment, however, research found that the ecological conditions (notably temperature and social interaction) maintained a significant effect on the organism strong enough to overrule the genetic commitment on the sexual phenotype [8].  Since recent discoveries, such as those related to sex-determination, developmental biology has changed its stance to recognize that phenotypes, as Krubitzer (2007) acknowledges with neurological development, are products of both genes and the environment [4].  Perhaps the hierarchy of brain-based behavioral research in cognitive evolution is currently in a similar predicament to the gene- centered view of phenotypes earlier.
Maybe recognizing cognition as an effect of the neocortex is flawed.  Recent evidence supports this alternative perspective to understand evolutionary cognition.  In particular, efforts to determine a neurological basis for cognition across mammals overlook the concept that cognition may be divided into many different parts: there are many different kinds of intelligence [9].  In addition, growing evidence from research on avian species demonstrates that their cognitive abilities parallel those of primates even though they lack a neocortex. Even self-recognition, which was previously believed to be one of the last strongholds of characteristics that defined mammals, was recently found in magpies [10].  Efforts to understand the evolution of cognition, then, should be aimed at studying first the phylogeny of cognitive ability and combining this data with studies of cortical phylogeny to come to a ‘less-wrong’ picture of cognitive evolution.

Works Cited

[1] Kandel, Eric, Schwartz, James, and Thomas Jessell, ed. Principles of Neural Science: 4th ed.  New York: McGraw-Hill, Health Professions Division, 2000.

[2] Aiello, Leslie and Peter Wheeler (1995) “The Extensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution.” Current Anthropology. 36:2.

[3] Krubitzer, Leah and Jon Kaas (2005) “The Evolution of the Neocortex in Mammals: How is Phenotypic Diversity Generated?” Current Opinion in Neurobiology. 15: 444-453.

[4] Krubitzer, Leah (2007) “The Magnificent Compromise: Cortical Field Evolution in Mammals,” Neuron. 56: 201-207.

[5] Kaas, Jon (2006) “Evolution of the Neocortex” Current Biology. 16:21:910-914.

[6] Krubitzer, Leah and Dianna Kahn (2003) “Nature Versus Nurture Revisited: An Old Idea with a New Twist,” Progress in Neurobiology. 70. 33-52.

[7] West-Eberhard, Mary J. Developmental Plasticity and Evolution. New York: Oxford University Press, 2003.

[8] Crews, David (2002) “Sex Determination: Where Environment and Genetics Meet.”  Evolution and Development. 5:1: 50-55.

[9] Rogers, Lesley J. “Increasing the Brain’s Capacity: Neocortex, New Neurons, and Hemispheric Specialization,” in Rogers, Lesley and Gisela Kaplan, ed., Comparative Vertebrate Cognition: Are Primates Superior to Non-Primates. New York: Kluwer Academic, 2004. pp. 289-318.

[10] Prior, Helmut, Schwarz, Ariane, and Onur Gunturkun (2008) “Mirror-Induced Behavior in the Magpie (Pica pica): Evidence of Self-Recognition,” PLoS Biology. 6:8: 1642-1650.