Paleoneurobiology
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Paleoneurobiology is the study of
History
Comparative anatomy began its emergence in the latter part of the 19th century. Two main views of life sprung forth; rationalism and transcendentalism. These formed the basis for the thought of scientists in this period. Georges Cuvier and Étienne Geoffroy St. Hilaire were leaders in the new field of comparative anatomy. Cuvier believed in the ability to create a functional morphology based simply on empirical evidence. He stressed function of the organ must coincide with its form. Geoffroy, in contrast, put a heavy emphasis on intuition as a method of understanding. His thought was based on two principles: the principle of connections and the principle of unity of plan. Geoffroy was one of the first to look for homologies in organs across species, though he believed that this was evidence of a universal plan rather than descent with modification.[4]
The late part of the 19th century in comparative anatomy was heavily influenced by the work of Charles Darwin in the On the Origin of Species in 1859. This work completely changed the views of comparative anatomists. Within eight years of Darwin's release of the Origin of Species, his views on descent from a common ancestor were widely accepted. This led to a shift in trying to understand how different parts of the brain evolved.[4] The next major innovation that helped to bring about paleoneurobiology was the microscope. Although the microscope was invented in the 17th century, it was only used in biology in the beginning in the late 19th century. The techniques of observing brain cells under a microscope took a long time to refine. In 1873, with this tool in hand, Camillo Golgi began to cellularly detail the brain and employ techniques to perfect axonal microscoping. Ludwig Edinger took advantage of this and came up with a new branch of anatomy called comparative neuroanatomy. Edinger held that vertebrates evolved in a linear progressive series. He also thought that changes in the brain were based on a series of additions and differentiations and that the most highly, complex brains were those that were the most encephalized.[5] The period of 1885-1935 was an explosion of ideas in comparative neuroanatomy. This era culminated in the publication of "The Comparative Anatomy of the Nervous System" by Arienns, Kappers, Huber, and Cosby. This paper influenced Tilly Edinger and she later became the founder of paleoneurobiology.[4]
Tilly Edinger
Ottilie "Tilly" Edinger was born in
While still in Germany, Edinger began studying extant species from a paleoneurobiological perspective by making inferences about evolutionary brain development in seacows using stratigraphic and comparative anatomical evidence. Edinger continued her research in Nazi Germany until the night of November 9, 1938, when thousands of Jews were killed or imprisoned in what became known as Kristallnacht. Although a visa was not immediately available for immigration to the United States, with the help of friends and colleagues who valued her work, Edinger was able to immigrate to London where she translated German medical texts into English. Eventually her visa quota number was called and she was able to immigrate to the United States where she took on a position as a research fellow at Harvard's Museum of Comparative Zoology.[6]
Her contributions to the field of paleoneurobiology include determining the extent to which endocasts reflect the anatomy of ancient brains, the adequacy of comparative anatomy to interpret brain evolution, the ability of brain endocasts to predict the lifestyles of extinct organisms, and if brain size has increased over geological time; topics which are still being explored today. In her later years, Edinger corresponded with the next generation of paleoneurobiologists, which insured that the work from her 50-year career continued into the future. The pinnacle accomplishment of her career was the compilation of an annotated bibliography of paleoneurobiological papers published between 1804 and 1966. The bibliography, Paleoneurology 1804–1966, was completed and published by colleagues posthumously in 1975 due to the untimely death of Edinger from injuries sustained during a traffic accident in 1967.[6]
Conflict between Holloway and Falk
Paleoneurobiologists
Brain endocasts
More recently,
Radiographic technique such as computed tomographic imaging, or
Methods of research
Paleoneurobiology revolves around the analysis of endocasts. Much of this analysis is focused on interpreting sulcal patterns, which is difficult because traces are often hardly recognizable, and there are no clear landmarks to use as reference points. Furthermore, the only clear reference plane is the sagittal plane one, which is marked by distinct cerebral asymmetries. Since the obtaining of clear data from fossil details is usually very difficult, much debate arises over interpretations. Experience is often an important factor in endocast analysis.[1] Therefore, a large portion of the field of paleoneurobiology arises out of developing more detailed procedures that increase the resolution and the reliability of interpretations.
Overall brain volume
Statistical analysis of brain endocasts gives information on the increases in overall brain volume ("endocranial volume"). Because endocasts are not exact replicas, or exact casts, of a once-living brain, computer algorithms and
Brain volume is prominent in the scientific literature for discussing taxonomic identification, behavioral complexity, intelligence, and dissimilar rates of evolution. In modern humans, cranial capacity can vary by as much as 1000 cc, without any correlation to behavior. This degree of variation is almost equivalent to the total increase in volume from australopithecine fossils to modern humans, and brings into question the validity of relying on cranial capacity as a measurement of sophistication.[12]
Many paleoneurobiologists measure cranial capacity via the submersion method, in which displacement of water in a beaker is taken as the volume of the endocast. Scientists who believe that this method is not accurate enough will use a similar procedure in which a beaker with a spout is filled until it is full. The water displaced by the endocast is then weighed to determine the endocast volume. Although both of these techniques are significantly more precise than previous methods, scientists are optimistic that more advanced techniques such as computed tomography will provide greater accuracy of volume measurements.[7]
Morphometric analysis
Convolution pattern and cerebral organization
Convolutions, the individual
Asymmetry
The degree of asymmetry between right and left hemispheres is a point of interest to most paleoneurobiologists because it could be linked to handedness or language development of the specimen. Asymmetries occur due to hemispherical specialization and are observed in both a qualitative and quantitative manner. The unevenness of the hemispheres, known as a petalia, is characterized by a lobe that is wider and/or protruding beyond the contralateral lobe. For example, a right-handed person typically has larger left occipital lobe and right frontal lobes than the contralateral lobes. Petalias also occur due to specialization in the communication centers of the frontal cortex of the brain in modern humans. Petalias in the occipital lobe are easier to detect than those in the frontal lobe.[7] Certain asymmetries have been documented on Homo erectus specimens such as the Homo redolfensis specimen from 1.8 million years ago that resemble the same asymmetries from modern humans.[4] Some gorillas have shown strong petalias, but they are not found in combination with other petalias as is almost always the case in humans. Scientists use the presence of petalias to show sophistication, but they are not a definitive indicator of evolution toward a more human brain.[7]
Meningeal patterns
Although the meninges have no link to behavior, they are still studied within the realm of paleoneurobiology due to the high degree of conservation of meningeal patterns within a species which may serve as a way to determine
Endocranial vasculature
Because meningeal blood vessels comprise part of the outermost layer of the brain, they often leave vascular grooves in the cranial cavity that are captured in endocasts. Endocranial vasculature originates around the foramina in the skull and in a living body would supply blood to the calvaria and dura mater. The vasculature is so well preserved in some fossils that terminal branches of the circulatory system can be observed. Analysis of cranial vasculature concentrates on the anterior meningeal system of the frontal region, the middle meningeal system of the parieto-temporal and part of the anterior occipital region, and the cerebellar fossa system of the cerebellar region. In the course of hominid evolution, the middle meningeal system has undergone the most change. Although cranial vasculature has been exhaustively studied in the last century, there has been no consensus on an identification scheme for the branches and patterns of the vascular system resulting from little overlap of results between studies. As such, endocranial vasculature is better suited for inferring the amount of blood delivered to different parts of the brain.[14]
Relative lobe size
It is impossible to determine accurate location of the central or precentral sulci from an endocast. Still it can provide a rough idea of lobe sizes.[4]
Significance
The study of paleoneurobiology allows researchers to examine the
Limitations
While paleoneurology is useful in the study of brain evolution, certain limitations to the information this study provides do exist. The limited scale and completeness of the fossil record inhibits the ability of paleoneurobiology to accurately document the course of brain evolution.[15] Further, fossil preservation is necessary to ensure accuracy of the endocasts studied.[16] Weathering, erosion, and overall gradual disfiguration may alter the naturally recovered endocasts or endocasts created from existing fossils.[17] The morphology of the brain can also be difficult to both quantify and describe, further complicating the observations made from the study of endocasts.[16] Additionally, paleoneurobiology provides very little insight into the actual anatomy within the brains of species studied; the study of endocasts is limited to the external anatomy only. The relationship among endocranial traits remains elusive. Comparative paleoeneurology reveals mostly only differences in endocranial size among related species, such as Gorilla gorilla. Since there is no proven direct relationship between brain size and intelligence, only inferences can be made regarding the developing behavior of ancient relatives of the genus Homo.
These limitations of paleoneurobiology are currently being dealt with by the development of more advanced tools to refine the study of endocasts.
Studies of interest
Brain shape, intelligence, and cognitive performance
Recent studies by Emiliano Bruner, Manuel Martin-Loechesb, Miguel Burgaletac, and Roberto Colomc have investigated the connection between midsagittal brain shape and mental speed. This study incorporated human subjects' cognitive testing in relationship to extinct humans. They used 2D from 102 MRI-scanned young adult human for comparison. Such correlations are small, suggesting that the influence of midsagittal brain geometry on individual cognitive performance is negligible but still provides useful information of evolutionary traits of the brain. Areas associated with the parietal cortex appear to be involved in relationships between brain geometry and mental speed.[18]
Degenerative diseases and functional disorder
Scientist J. Ghika believes use of paleoneurobiology is the best way to analyze several neurodegeneration leading to diseases such as
See also
References
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