Adaptive evolution of the primate nervous system has taken place over milleniaresulting in the current brain size and circuitry of the modern human nervous system. This neural evolution has lead to an increase in cognitive capacity such as primitive tool use by chimpanzees and the development of language. Primates have faced a wide range of different evolutionary pressures dating back over the past 65 million years, the time when the earliest identified primate lived on earth (National Geographic 2013). Adaptive evolution, or positive selection, occurs when an allele increases in frequency because it is beneficial to reproduction or survival caused by increased fitness (Swanson, 2003).
With advances in genetic sequencing and protein biochemistry, researchers can study nervous system evolution through expression, function, and regulation of various genes targeted for their role in nervous system development (Kaas, 2006). While analysis of genes or gene families is a remarkable tool becoming more popular as technology advances and more genomes are sequenced, other methods exist allowing evolutionary biologists to study the evolution of the nervous system. Researchers can compare the brains of extant mammals, primates, and humans to infer the commonalities in structure and neural organization imply common ancestry (Kaas, 2006). A third resource for studying the evolution of the nervous system is the fossil record. While brain tissue degrades over the years, bone surrounding nervous system tissue of common ancestors remains intact and provides a window into the past of brain size, shape, and exterior features (Kaas, 2006). A final technique involves making predictions and conclusions based on the scaling of different brain areas relative to absolute brain volume (Kaas, 2006). As we progress through evolutionary time, absolute brain size tends to increase. However, the increase in size is not uniform across all brain structures. Studying areas of the brain that increase and areas that remain the same size allow for identification of potential selective pressures experienced by an organism during their lifetime.
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A major challenge in studying the evolution of the complex human nervous system is where to begin the analysis. If we start 200 million years ago, we observe from the fossil record that mammals had developed large olfactory bulbs and olfactory cortex resulting from the selective advantage of having a superior sense of smell for finding food (Kaas, 2006). Additionally, mammals have evolved a thicker neocortex than their mammalian ancestors which had much thinner cortical tissue levels (Kaas, 2006). The earliest primates were small, often nocturnal, and lived in small tree branches capitalizing on the rich food sources such as fruits, leaves, insects and small vertebrates. Their ability to flourish stemmed from their increased visual and sensorimotor capacities, the first neocortical areas to evolve significantly in primate nervous system evolution (Kaas, 2006). As we move forward in time, old world monkeys and apes developed a somatosensory cortex with four differentiated areas in the anterior parietal corte (Kaas, 2006). They also evolved differentiated cortical areas in the visual and somatosensory regions in the posterior parietal cortex (Kaas, 2006). The apes that lead to the rise of chimpanzees, bonobos and humans display some of the most remarkable structural differences directly leading to the humanâ€™s increase in cognitive capacity. Lateralization, or assymetrical enlargement of different sides of cerebral hemisphere accelerated among this lineage in the temporal lobe (Kaas, 2006). This provides evidence that the primary anatomical platform for language production and processing evolved before language existed (Kaas, 2006). Also, chimpanzees display different organization in the primary visual cortex than humans suggesting that humans have restructured visual areas from V1 to other regions in the brain (Kaas, 2006). As we move into modern humans and their close relatives, such as the hominins, we see increased hemispheric lateralization attributed to right-handedness and language development (Kaas, 2006). Research also indicates a general trend of increasing relative brain size, or the size of the brain relative to body mass, throughout primate evolution(Kaas, 2006). However, absolute brain size is now stabilizing because as brain tissue size increases, the ability to transmit signals over long distances without structural reformation becomes problematic (Schenker, 2006). The distance the signal needs to travel is directly related to the time of signal transmission and as brain size increases primates and other mammals have adapted similar structural adaptations to account for this (Schenker, 2006).
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Brains have generally tended to increase in time throughout primate evolution until the problem of signal transmission over distance began to counteract the benefits of increased brain tissue size (Schenker, 2006). Larger brains contain larger ratios of white matter to gray matter, thus increasing the percentage of myelinated axons. Myelin is an electrically insulating material which forms the myelin sheath around the axons of white matter in the nervous system. Schenker and his team of researchers used magnetic resonance imaging of living humans and compared them to the magnetic resonance images of orangutans, an extant species of the great ape, to compare white matter composition in certain areas of the brain. Human brains had an increased portion of gyral white matter; that is the outermost portion of the neocortex, compared to core white matter (Schenker, 2006). Gyral white matter connects different surrounding cortical areas together which could contribute to increase human cognition (Schenker, 2006). Smaers et al reports through their allometric studies of the prefrontal cortex that apes display significant increases in white matter to gray matter ratios in the left prefrontal cortex. In addition, the human brain contains a much higher white to gray matter ratio which could suggest a human specialization in prefrontal white matter (Smaers et al, 2006).
As brain size began to stabilize later in primate evolution, sulci and gyri of the neocortex became more significant which increases cortical surface area. Gyral areas represent the outermost part of the brain of any primate cortex. The orbital gyral sector of the frontal lobe has been directly linked to the degree of complex social behavior along primate lineages (Schenker, 2006). Orangutans have the largest absolute brain size among the great apes, yet they have the smallest orbital sector (Schenker, 2006). As compared to other species of great apes, Orangutans do not have organized social groups and spend most of their time individually suggesting a direct behavioral link to neuroanatomical scaling in different species (Schenker, 2006).
The beginning of the use of simple and complex tools marks a period in evolutionary time of substantial increase in cognitive function. Using tools requires a higher degree of processing and planning compared to previously evolved cognitive processes such as improved hunting strategies and food retrieval. Neuroimaging studies have been performed comparing the active tool use in humans and macaques, an old world monkey (Johnson, Fray 2004). During a tool viewing and naming exercise performed on both macaque and humans, neuromaging showed activation of the left inferior frontal cortex. When macaques and humans were charged with identifying the action of the tools, a larger left middle frontal gyrus showed activation in both species (Johnson, Fray 2004). Members of the Pan species, which is closer to the human lineage, display an interesting dichotomy in tool use (Schenker, 2006). Both members of the Pan species, Chimpanzees utilize simple tools while bonobos do not reportedly use tools. The difference is attributed to the enlargement of the dorsal frontal cortex in chimpanzees (Schenker, 2006). This region of the frontal cortex contains motor cortices and prefrontal cortical areas related to perception, working memory, problem solving, and even language (Schenker, 2006). These differences in brain anatomy can be seen in the locomotor and cognitive behavior of each species. Bonobos exhibit more arboreal travel and hang from trees during feeding while chimpanzees spend time on the ground and utilize different objects as tools (Schenker, 2006). The difference in tool use among chimpanzees and bonobos marks a critical division in cognitive evolution along the primate lineage and is directly related to the difference in development of the dorsal frontal cortex in chimpanzees.
As can be noted from brain areas activated during different aspects of tool use, the left frontal cortex plays a critical role in cognitive activity. Apes and Humans have a larger left prefrontal cortex than Old and New World monkeys (Smaers, 2010). Traditionally, increase in intelligence and cognitive function has been attributed to a disproportionate increase in neocortex as a whole compared to the size of the rest of the brain. Smaers et. al allometrically compares the posterior neocortex, which includes the entire neocortex other than the frontal lobe, with the rest of brain volume and observes an isometric scaling over time. Therefore, the traditionally reported hyperscaling of the neocortex can be attributed to the increase in size of the prefrontal cortex which shows a significantly higher scaling coefficient in Smaers comparison analysis. In addition to the increase in size of the frontal cortex, there has been a lateralized movement towards an increase in size of the left prefrontal cortex in ape and human lineage (Smaers, 2010). This prefrontal lateralization could be a major contribution towards the difference in cognitive abilities between monkeys and apes by providing a structural foundation for action motivated behavior.
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