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Evolutionary neuroscience is the scientific study of the evolution of nervous systems. Evolutionary neuroscientists investigate the evolution and natural history of nervous system structure, functions and emergent properties. The field draws on concepts and findings from both neuroscience and evolutionary biology. Historically, most empirical work has been in the area of comparative neuroanatomy, and modern studies often make use of phylogenetic comparative methods. Selective breeding and experimental evolution approaches are also being used more frequently.
Conceptually and theoretically, the field is related to fields as diverse as cognitive genomics, neurogenetics, developmental neuroscience, neuroethology, comparative psychology, evo-devo, behavioral neuroscience, cognitive neuroscience, behavioral ecology, biological anthropology and sociobiology.
Evolutionary neuroscientists examine changes in genes, anatomy, physiology, and behavior to study the evolution of changes in the brain. They study a multitude of processes including the evolution of vocal, visual, auditory, taste, and learning systems as well as language evolution and development. In addition, evolutionary neuroscientists study the evolution of specific areas or structures in the brain such as the amygdala , forebrain and cerebellum as well as the motor or visual cortex.
Studies of the brain began during ancient Egyptian times but studies in the field of evolutionary neuroscience began after the publication of Darwin's On the Origin of Species in 1859. At that time, brain evolution was largely viewed at the time in relation to the incorrect scala naturae. Phylogeny and the evolution of the brain were still viewed as linear. During the early 20th century, there were several prevailing theories about evolution. Darwinism was based on the principles of natural selection and variation, Lamarckism was based on the passing down of acquired traits, Orthogenesis was based on the assumption that tendency towards perfection steers evolution, and Saltationism argued that discontinuous variation creates new species. Darwin's became the most accepted and allowed for people to starting thinking about the way animals and their brains evolve.
The 1936 book The Comparative Anatomy of the Nervous System of Vertebrates Including Man by the Dutch neurologist C.U. Ariëns Kappers (first published in German in 1921) was a landmark publication in the field. Following the Evolutionary Synthesis, the study of comparative neuroanatomy was conducted with an evolutionary view, and modern studies incorporate developmental genetics. It is now accepted that phylogenetic changes occur independently between species over time and can not be linear. It is also believed that an increase with brain size correlates with an increase in neural centers and behavior complexity.
Over time, there are several arguments that would come to define the history of evolutionary neuroscience. The first is the argument between Etienne Geoffro St. Hilaire and George Cuvier over the topic of "common plan versus diversity". Geoffrey argued that all animals are built based on a single plan or archetype and he stressed the importance of homologies between organisms, while Cuvier believed that the structure of organs was determined by their function and that knowledge of the function of one organ could help discover the functions of other organs. He argued that there were at least four different archetypes. After Darwin, the idea of evolution was more accepted and Geoffrey's idea of homologous structures was more accepted. The second major argument is that of the Scala Naturae (scale of nature) versus the phylogenetic bush. The Scala Naturae, later also called the phylogenetic scale, was based on the premise that phylogenies are linear or like a scale while the phylogenetic bush argument was based on the idea that phylogenies were nonlinear and resembled a bush more than a scale. Today it is accepted that phylogenies are nonlinear. A third major argument dealt with the size of the brain and whether relative size or absolute size was more relevant in determining function. In the late 18th century, it was determined that brain to body ratio reduces as body size increases. However more recently, there is more focus on absolute brain size as this scales with internal structures and functions, with the degree of structural complexity, and with the amount of white matter in the brain, all suggesting that absolute size is much better predictor of brain function. Finally, a fourth argument is that of natural selection (Darwinism) versus developmental constraints (concerted evolution). It is now accepted that the evolution of development is what causes adult species to show differences and evolutionary neuroscientists maintain that many aspects of brain function and structure are conserved across species.
Throughout history, we see how evolutionary neuroscience has been dependent on developments in biological theory and techniques. The field of evolutionary neuroscience has been shaped by the development of new techniques that allow for the discovery and examination of parts of the nervous system. In 1873, Camillo Golgi devised the silver nitrate method which allowed for the description of the brain at the cellular level as opposed to simply the gross level. Santiago Ramon and Pedro Ramon used this method to analyze numerous parts of brains, broadening the field of comparative neuroanatomy. In the second half of the 19th century, new techniques allowed scientists to identify neuronal cell groups and fiber bundles in brains. In 1885, Vittorio Marchi discovered a staining technique that let scientists see induced axonal degeneration in myelinated axons, in 1950, the "original Nauta procedure" allowed for more accurate identification of degenerating fibers, and in the 1970s, there were several discoveries of multiple molecular tracers which would be used for experiments even today. In the last 20 years, cladistics has also become a useful tool for looking at variation in the brain.
Darwin's theory allowed for people to start thinking about the way animals and their brains evolve.
The cerebral cortex of reptiles resembles that of mammals, although simplified. Although the evolution and function of the human cerebral cortex is still shrouded in mystery, we know that it is the most dramatically changed part of the brain during recent evolution.
Research about how visual perception has developed in evolution is today best understood through studying present-day primates, since the organization of the brain cannot be ascertained only by analyzing fossilized skulls.
The organization of human auditory cortex is divided into core, belt and parabelt. This closely resembles that of present-day primates.
Evidence of a rich cognitive life in primate relatives of humans are extensive, and a wide range of specific behaviors in line with Darwinian theory are well documented. However, until recently, research has disregarded nonhuman primates in the context of evolutionary linguistics, primarily because unlike vocal learning birds, our closest relatives seem to lack imitative abilities. Evolutionary speaking, there is great evidence suggesting a genetical groundwork for the concept of languages has been in place for millions of years, as with many other capabilities and behaviors observed today.
While evolutionary linguists agree on the fact that volitional control over vocalizing and expressing language is a quite recent leap in the history of the human race, that is not to say auditory perception is a recent development as well. Research has shown substantial evidence of well defined neural pathways linking cortices to organize auditory perception in the brain. Thus, the issue lies in our abilities to imitate sounds.
Beyond the fact that primates may be poorly equipped to learn sounds, studies have shown them to learn and use gestures far better. Visual cues and motoric pathways developed millions of years earlier in our evolution, which seems to be one reason for an earlier ability to understand and use gestures.